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Respiration

RESPIRATION

CONCEPT

Respiration is much more than just breathing; in fact, the term refers to two separate processes, only one of which is the intake and outflow of breath. At least cellular respiration, the process by which organisms convert food into chemical energy, requires oxygen; on the other hand, some forms of respiration are anaerobic, meaning that they require no oxygen. Such is the case, for instance, with some bacteria, such as those that convert ethyl alcohol to vinegar. Likewise, an anaerobic process can take place in human muscle tissue, producing lactic acidsomething so painful that it feels as though vinegar itself were being poured on an open sore.

HOW IT WORKS

Forms of Respiration

Respiration can be defined as the process by which an organism takes in oxygen and releases carbon dioxide, one in which the circulating medium of the organism (e.g., the blood) comes into contact with air or dissolved gases. Either way, this means more or less the same thing as breathing. In some cases, this meaning of the term is extended to the transfer of oxygen from the lungs to the bloodstream and, eventually, into cells or the release of carbon dioxide from cells into the bloodstream and thence to the lungs, from whence it is expelled to the environment. Sometimes a distinction is made between external respiration, or an exchange of gases with the external environment, and internal respiration, an exchange of gases between the body's cells and the blood, in which the blood itself "bathes" the cells with oxygen and receives carbon dioxide to transfer to the environment.

This is just one meaningalbeit a more familiar oneof the word respiration. Respiration also can mean cellular respiration, a series of chemical reactions within cells whereby food is "burned" in the presence of oxygen and converted into carbon dioxide and water. This type of respiration is the reverse of photosynthesis, the process by which plants convert dioxide and water, with the aid of solar energy, into complex organic compounds known as carbohydrates. (For more about carbohydrates and photosynthesis, see Carbohydrates.)

How Gases Move Through the Body

Later in this essay, we discuss some of the ways in which various life-forms breathe, but suffice it to say for the momenthardly a surprising revelation!that the human lungs and respiratory system are among the more complex mechanisms for breathing in the animal world. In humans and other animals with relatively complex breathing mechanisms (i.e., lungs or gills), oxygen passes through the breathing apparatus, is absorbed by the bloodstream, and then is converted into an unstable chemical compound (i.e., one that is broken down easily) and carried to cells. When the compound reaches a cell, it is broken down and releases its oxygen, which passes into the cell.

On the "return trip"that is, the reverse process, which we experience as exhalationcells release carbon dioxide into the bloodstream, where it is used to form another unstable chemical compound. This compound is carried by the bloodstream back to the gills or lungs, and, at the end of the journey, it breaks down and releases the carbon dioxide to the surrounding environment. Clearly, the one process is a mirror image of the other, with the principal difference being the fact that oxygen is the key chemical component in the intake process, while carbon dioxide plays the same role in the process of outflow.

HEMOGLOBIN AND OTHER COMPOUNDS.

In humans the compound used to transport oxygen is known by the name hemoglobin. Hemoglobin is an iron-containing protein in red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide from them. In the lungs, hemoglobin, known for its deep red color, reacts with oxygen to form oxyhemoglobin. Oxyhemoglobin travels through the bloodstream to cells, where it breaks down to form hemoglobin and oxygen, and the oxygen then passes into cells. On the return trip, hemoglobin combines with carbon dioxide to form carbaminohemoglobin, an unstable compound that, once again, breaks downonly this time it is carbon dioxide that it releases, in this case to the surrounding environment rather than to the cells.

In other species, compounds other than hemoglobin perform a similar function. For example, some types of annelids, or segmented worms, carry a green blood protein called chlorocruorin that functions in the same way as hemoglobin does in humans. And whereas hemoglobin is a molecule with an iron atom at the center, the blood of lobsters and other large crustaceans contains hemocyanin, in which copper occupies the central position. Whatever the substance, the compound it forms with oxygen and carbon dioxide must be unstable, so that it can break down easily to release oxygen to the cells or carbon dioxide to the environment.

Cellular Respiration

Both forms of respiration involve oxygen, but cellular respiration also involves a type of nutrientmaterials that supply energy, or the materials for forming new tissue. Among the key nutrients are carbohydrates, naturally occurring compounds that consist of carbon, hydrogen, and oxygen. Included in the carbohydrate group are sugars, starches, cellulose, and various other substances.

Glucose is a simple sugar produced in cells by the breakdown of more complex carbohydrates, including starch, cellulose, and such complex sugars as sucrose (cane or beet sugar) and fructose (fruit sugar). In cellular respiration, an organism oxidizes glucose (i.e., combines it with oxygen) so as to form the energy-rich compound known as adenosine triphosphate (ATP). ATP, critical to metabolism (the breakdown of nutrients to provide energy or form new material), is the compound used by cells to carry out most of their ordinary functions. Among those functions are the production of new cell parts and chemicals, the movement of compounds through cells and the body as a whole, and growth.

In cellular respiration, six molecules of glucose (C6H12O6) react with six molecules of oxygen (O2) to form six molecules of carbon dioxide (CO2), six molecules of water (H2O), and 36 molecules of ATP. This can be represented by the following chemical equation:

6C6H12O6 + 6 O2 6 CO2 + 6 H2O + 36 ATP

The process is much more complicated than this equation makes it appear: some two dozen separate chemical reactions are involved in the overall conversion of glucose to carbon dioxide, water, and ATP.

The Mechanics of Breathing

All animals have some mechanism for removing oxygen from the air and transmitting it into the bloodstream, and this same mechanism typically is used to expel carbon dioxide from the bloodstream into the surrounding environment. Types of animal respiration, in order of complexity, include direct diffusion, diffusion into blood, tracheal respiration, respiration with gills, and finally, respiration through lungs. Microbes, fungi, and plants all obtain the oxygen they use for cellular respiration directly from the environment, meaning that there are no intermediate organs or bodily chemicals, such as lungs or blood. More complex organisms, such as sponges, jellyfish, and terrestrial (land) flatworms, all of which have blood, also breathe through direct diffusion. The latter term describes an exchange of oxygen and carbon dioxide directly between an organism, or its bloodstream, and the surrounding environment.

More complex is the method of diffusion into blood whereby oxygen passes through a moist layer of cells on the body surface and then through capillary walls (capillaries are small blood vessels that form a network throughout the body) and into the bloodstream. Once oxygen is in the blood, it moves throughout the body to different tissues and cells. Among the organisms that rely on diffusion into blood are annelids, a group that includes earthworms, various marine worms, and leeches.

In tracheal respiration air moves through openings in the body surface called spiracles. It then passes into special breathing tubes called tracheae that extend into the body. The tracheae divide into many small branches that are in contact with muscles and organs. In small insects, air simply moves into the tracheae, while in large insects, body movements assist tracheal air movement. Insects and terrestrial arthropods (land-based organisms with external skeletons) use this method of respiration.

Much more complicated than tracheae, gills are specialized tissues with many infoldings. Each gill is covered by a thin layer of cells and filled with blood capillaries. These capillaries take up oxygen dissolved in water and expel carbon dioxide dissolved in blood. Fish and other aquatic animals use gills, as did the early ancestors of humans and other higher animals. A remnant of this chapter from humans' evolutionary history can be seen in the way that an embryo breathes in its mother's womb, not by drawing in oxygen through its lungs but through gill-like mechanisms that disappear as the embryo develops.

LUNGS.

Lungs are composed of many small chambers or air sacs surrounded by blood capillaries. Thus, they work with the circulatory system, which transports oxygen from inhaled air to all tissues of the body and also transports carbon dioxide from body cells to the lungs to be exhaled. After air enters the lungs, oxygen moves into the bloodstream through the walls of these capillaries. It then passes from the lung capillaries to the different muscles and organs of the body.

Although they are common to amphibians, reptiles, birds, and mammals, lungs differ enormously throughout the animal kingdom. Frogs, for instance, have balloon-like lungs that do not have a very large surface area. By contrast, if the entire surface of an adult male human's lungs were spread flat, it would cover about 750 sq. ft. (70 m2), approximately the size of a handball court. The reason is that humans have about 300 million gas-filled alveoli, tiny protrusions inside the lungs that greatly expand the surface area for gas exchange.

Birds have specialized lungs that use a mechanism called crosscurrent exchange, which allows air to flow in one direction only, making for more efficient oxygen exchange. They have some eight thin-walled air sacs attached to their lungs, and when they inhale, air passes through a tube called the bronchus and enters posterior air sacsthat is, sacs located toward the rear. At the same time, air in the lungs moves forward to anterior air sacs, or ones located near the bird's front. When the bird exhales, air from the rear air sacs moves to the outside environment, while air from the front moves into the lungs. This efficient system moves air forward through the lungs when the bird inhales and exhales and makes it possible for birds to fly at high altitudes, where the air has a low oxygen content.

Humans and other mammals have lungs in which air moves in and out through the same pathway. This is true even of dolphins and whales, though they differ from humans in that they do not take in nutrition through the same opening. In fact, terrestrial mammals, such as the human, horse, or dog, are some of the only creatures that possess two large respiratory openings: one purely for breathing and smelling and the other for the intake of nutrients as well as air (i.e., oxygen in and carbon dioxide out).

REAL-LIFE APPLICATIONS

Anaerobic Respiration

Activity that involves oxygen is called aerobic; hence the term aerobic exercise, which refers to running, calisthenics, biking, or any other form of activity that increases the heart rate and breathing. Activity that does not involve oxygen intake is called anaerobic. Weightlifting, for instance, will increase the heart rate and rate of breathing if it is done intensely, but that is not its purpose and it does not depend on the intake and outflow of breath. For that reason, it is called an anaerobic exercisethough, obviously, a person has to keep breathing while doing it.

In fact, a person cannot consciously stop breathing for a prolonged period, and for this reason, people cannot kill themselves simply by holding their breath. A buildup of carbon dioxide and hydrogen ions (electrically charged atoms) in the bloodstream stimulates the breathing centers to become active, no matter what we try to do. On the other hand, if a person were underwater, the lungs would draw in water instead of air, and though water contains air, the drowning person would suffocate.

ANAEROBIC BACTERIA.

Some creatures, however, do not need to breathe air but instead survive by anaerobic respiration. This is true primarily of some forms of bacteria, and indeed scientists believe that the first organisms to appear on Earth's surface were anaerobic. Those organisms arose when Earth's atmosphere contained very little oxygen, and as the composition of the atmosphere began to incorporate more oxygen over the course of many millions of years, new organisms evolved that were adapted to that condition.

The essay on paleontology discusses Earth's early history, including the existence of anaerobic life before the formation of oxygen in the atmosphere. The appearance of oxygen is a result of plant life, which produces it as a byproduct of the conversion of carbon dioxide that takes place in photosynthesis. Plants, therefore, are technically anaerobic life-forms, though that term usually refers to types of bacteria that neither inhale nor exhale oxygen. Anaerobic bacteria still exist on Earth and serve humans in many ways. Some play a part in the production of foods, as in the process of fermentation. Other anaerobic bacteria have a role in the treatment of sewage. Living in an environment that would kill most creaturesand not just because of the lack of oxygenthey consume waste materials, breaking them down chemically into simpler compounds.

HUMANS AND ANAEROBIC RESPIRATION.

Even in creatures, such as humans, that depend on aerobic respiration, anaerobic respiration can take place. Most cells are able to switch from aerobic to anaerobic respiration when necessary, but they generally are not able to continue producing energy by this process for very long. For example, a person who exercises vigorously may be burning up glucose faster than oxygen is being pumped to the cells, meaning that cellular respiration cannot take place quickly enough to supply all the energy the body needs. In that case, cells switch over to anaerobic respiration, which results in the production of lactic acid, or C3H6O3. One advantage of anaerobic respiration is that it can take place very quickly and in short bursts, as opposed to aerobic respiration, which is designed for slower and steadier use of muscles. The disadvantage is that anaerobic respiration produces lactic acid, which, when it builds up in muscles that are overworked, causes soreness and may even lead to cramps.

LACTIC ACID IN THE BODY.

Eventually, the buildup of lactic acid is carried away in the bloodstream, and the lactic acid is converted to carbon dioxide and water vapor, both of which are exhaled. But if lactic acid levels in the bloodstream rise faster than the body can neutralize them, a state known as lactic acidosis may ensue. Lactic acidosis rarely happens in healthy people and, more often than not, is a result of the body's inability to obtain sufficient oxygen, as occurs in heart attacks or carbon monoxide or cyanide poisoning or in the context of diseases such as diabetes.

The ability of the body to metabolize lactic acid is diminished significantly by alcohol, which impairs the liver's ability to carry out normal metabolic reactions. For this reason, alcoholics often have sore muscles from lactic acid buildup, even though they may not exercise. Lactic acid also can lead to a buildup of uric acid crystals in the joints, in turn causing gout, a very painful disease.

LACTIC ACID IN FOOD AND INDUSTRY.

Lactic acid is certainly not without its uses, and it is found throughout nature. When lactose, or milk sugar, is fermented by the action of certain bacteria, it causes milk to sour. The same process is used in the manufacture of yogurt, but the reaction is controlled carefully to ensure the production of a consumable product. Lactic acid also is applied by the dairy industry in making cheese. Molasses contains lactic acid, a product of the digestion of sugars by various species of bacteria, and lactic acid also is used in making pickles and sauerkraut, foods for which a sour taste is desired.

A compound made from lactic acid is used as a food preservative, but the applications of lactic acid extend far beyond food production. Lactic acid is important as a starting material for making drugs in the pharmaceutical industry. Additionally, it is involved in the manufacturing of lacquers and inks; is used as a humectant, or moisturizer, in some cosmetics; is applied as a mordant, or a chemical that helps fabrics accept dyes, to textiles; and is employed in tanning leather.

Respiratory Disorders

In almost any bodily system, there are bound to be disorders, or at least the chance that disorders may occur. This is particularly the case with something as complex as the respiratory system, because the more complex the system, the more things that can go wrong. Among the respiratory disorders that affect humans is a whole range of ailments from the common cold to emphysema, and from the flu to cystic fibrosis.

THE COMMON COLD.

Colds are among the most common conditions that affect the respiratory system, though what we call the common cold is actually an invasion by one of some 200 different types of virus. Thus, it is really not one ailment but 200, though these are virtually identical, but the large number of viral causative agents has made curing the cold an insurmountable task.

When you get a cold, viruses establish themselves on the mucus membrane that coats the respiratory passages that bring air to your lungs. If your immune system is unsuccessful in warding off this viral infection, the nasal passages become inflamed, swollen, and congested, making it difficult to breathe.

Coughing is a reflex action whereby the body attempts to expel infected mucus or phlegm. It is essential to removing infected secretions from the body, but of course it plays no role in actually bringing a cold to an end. Nor do antibiotics, which are effective against bacteria but not viruses (see Infection). Only when the body builds up its own defense to the coldassuming the sufferer has a normally functioning immune systemis the infection driven away.

INFLUENZA AND ALLERGIES.

Influenza, a group of viral infections that can include swine flu, Asian flu, Hong Kong flu, and Victoria flu, is often far more serious than the common cold. A disease of the lungs, it is highly contagious, and can bring about fever, chills, weakness, and aches. In addition, influenza can be fatal: a flu epidemic in the aftermath of World War I, spread to far corners of the globe by returning soldiers, killed an estimated 20 million people.

Respiratory ailments often take the form of allergies such as hay fever, symptoms of which include sneezing, runny nose, swollen nasal tissue, headaches, blocked sinuses, fever, and watery, irritated eyes. Hay fever is usually aggravated by the presence of pollen or ragweed in the air, as is common in the springtime. Other allergy-related respiratory conditions may be aggravated by dust in the air, and particularly by the feces of dust mites that live on dust particles.

BRONCHIAL AILMENTS.

Allergic reactions can be treated by antihistamines (see The Immune System for more about allergies), but simple treatments are not available for such complex respiratory disorders as asthma, chronic bronchitis, and emphysema. All three are characterized by an involuntary constriction in the walls of the bronchial tubes (the two divisions of the trachea or windpipe that lead to the right and left lungs), which causes the tubes to close in such a way that it becomes difficult to breathe.

Emphysema can be brought on by cigarette smoking, and indeed some heavy smokers die from that ailment rather than from lung cancer. On the other hand, a person can contract a bronchial illness without engaging in smoking or any other activity for which the sufferer could ultimately be blamed. Indeed, small children may have asthma. One treatment for such disorders is the use of a bronchodilator, a medicine used to relax the muscles of the bronchial tubes. This may be administered as a mist through an inhaler, or given orally like other medicine.

TUBERCULOSIS AND PNEUMONIA.

More severe is tuberculosis, an infectious disease of the lungs caused by bacteria. Tuberculosis attacks the lungs, leading to a chronic infection with such symptoms as fatigue, loss of weight, night fevers and chills, and persistent coughing that brings up blood. Without treatment, it is likely to be fatal. Indeed, it was a significant cause of death until the introduction of antibiotics in the 1940s, and it has remained a problem in underdeveloped nations. Additionally, thanks to mutation in the bacteria themselves, strains of the disease are emerging that are highly resistant to antibiotics.

Another life-threatening respiratory disease is pneumonia, an infection or inflammation of the lungs caused by bacteria, viruses, mycoplasma (microorganisms that show similarities to both viruses and bacteria), and fungi, as well as such inorganic agents as inhaled dust or gases. Symptoms include pleurisy (chest pain), high fever, chills, severe coughing that brings up small amounts of mucus, sweating, blood in the sputum (saliva and mucus expelled from the lungs), and labored breathing.

In 1936, pneumonia was the principal cause of death in the United States. Since then, it has been controlled by antibiotics, but as with tuberculosis, resistant strains of bacteria have developed, and therefore the number of cases has increased. Today, pneumonia and influenza combined are among the most significant causes of death in the United States (see Diseases).

LUNG CANCER AND CYSTIC FIBROSIS.

Respiratory ailments may also take the form of lung cancer, which may or may not be a result of smoking. Cigarette smoking and air pollution are considered to among the most significant causes of lung cancer, yet people have been known to die of the disease without being smokers or having been exposed to significant pollution.

One particularly serious variety of respiratory illness is cystic fibrosis, a genetic disorder that causes a thick mucus to build up in the respiratory system and in the pancreas, a digestive organ. (For more about genetic disorders, see Heredity; for more on role of the pancreas, see Digestion.) In the United States, the disease affects about one in every 3,900 babies born annually. No cure for cystic fibrosis exists, and the disease is invariably fatal, with only about 50% of sufferers surviving into their thirties.

Lung complications are the leading cause of death from cystic fibrosis, and most symptoms of the disease are related to the sticky mucus that clogs the lungs and pancreas. People with cystic fibrosis have trouble breathing, and are highly susceptible to bacterial infections of the lungs. Coughing, while it may be irritating and painful if you have a cold, is necessary for the expulsion of infected mucus, but mucus in the lungs of a cystic fibrosis is too thick to be moved. This makes it easy for bacteria to inhabit the lungs and cause infection.

WHERE TO LEARN MORE

Bryan, Jenny. Breathing: The Respiratory System. New York: Dillon Press, 1993.

Cellular Metabolism and Fermentation. Estrella Mountain Community College (Web site). <http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookGlyc.html>.

Kimball, Jim. "The Human Respiratory System." Kim ball's Biology Pages (Web site). <http://www.ultranet.com/~jkimball/BiologyPages/P/Pulmonary.html>.

Levesque, Mireille, Letitia Fralick, and Joni McDowell. "Respiration in Water: An Overview of Gills." University of New Brunswick (Web site). <http://www.unb.ca/courses/biol4775/SPAGES/SPAGE13.HTM>.

Llamas, Andreu. Respiration and Circulation. Milwaukee: Gareth Stevens, 1998.

Paustian, Timothy. Anaerobic Respiration. Department of Bacteriology, University of Wisconsin-Madison (Web site). <http://www.bact.wisc.edu/microtextbook/Metabolism/RespAnaer.html>.

Roca, Núria, and Marta Serrano. The Respiratory System, the Breath of Life. Illus. Antonio Tenllado. New York: Chelsea House Publishers, 1995.

Silverstein, Alvin, and Virginia B. Silverstein. The Respiratory System. New York: Twenty-First Century Books, 1994.

KEY TERMS

AEROBIC:

Oxygen-breathing.

ANAEROBIC:

Non-oxygen-breathing.

ATP:

Adenosine triphosphate, an energy carrier formed when a simpler compound, adenosine diphosphate (ADP), combines with a phosphate group.

CAPILLARY:

A very small blood vessel. Capillaries form networks throughout the body.

CARBOHYDRATES:

Naturally occurring compounds, consisting of carbon, hydrogen, and oxygen, whose primary function in the body is to supply energy. Included in the carbohydrate group aresugars, starches, cellulose, and various other substances. Most carbohydrates are produced by green plants in the process of undergoing photosynthesis.

CELLULAR RESPIRATION:

A process that, when it takes place in the presence of oxygen, involves the intake of organic substances, which are broken down into carbon dioxide and water, with the release of considerable energy.

CIRCULATORY SYSTEM:

The parts of the body that work together to move blood and lymph. They include the heart, blood vessels, blood, and the lymphatic glands, such as the lymph nodes.

COMPOUND:

A substance in which atoms of more than one element are bonded chemically to one another.

FERMENTATION:

A process, involvingenzymes, in which a compound rich in energy is broken down into simpler substances.

GLUCOSE:

A monosaccharide (sugar) that occurs widely in nature and which is the form in which animals usually receive carbohydrates. Also known as dextrose, grape sugar, and corn sugar.

HEMOGLOBIN:

An iron-containing protein in human red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide fromthem. Hemoglobin is known for its deep red color.

LYMPH:

The portion of the blood that includes white blood cells and plasma but not red blood cells.

LYMPH NODES:

Masses of tissue, at certain places in the body, that act as filters for blood.

METABOLISM:

The chemical process by which nutrients are broken down and converted into energy or are used in the construction of new tissue or other material in the body. All metabolic reactions are either catabolic or anabolic.

MONOSACCHARIDE:

The simplest type of carbohydrate. Monosaccharides, which cannot be broken down chemically into simpler carbohydrates, also are known as simple sugars.

NUTRIENT:

Materials that supply energy or the materials to form new tissue for organisms. They include proteins, carbohydrates, lipids (fats), vitamins, and minerals.

PHOSPHATE GROUP:

A group (that is, a combination of atoms from two or more elements that tend to bond with other elements or compounds in certain characteristic ways) involving a phosphate, or a chemical compound that contains oxygen bonded to phosphorus.

PHOTOSYNTHESIS:

The biological conversion of light energy (that is, electromagnetic energy) from the Sun to chemical energy in plants. In this process carbondioxide and water are converted to carbohydrates and oxygen.

RESPIRATION:

A term that can refer either to cellular respiration (see definition) or, more commonly, to the process by which an organism takes in oxygen and releases carbon dioxide. Sometimes a distinction is made between external respiration, or an exchange of gases with the external environment, and internal respiration, an exchange of gases between the body's cells and the blood.

SIMPLE SUGAR:

A monosaccharide, or simple carbohydrate.

TISSUE:

A group of cells, along with the substances that join them, that forms part of the structural materials in plants oranimals.

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Respiration

Respiration

The term respiration has two relatively distinct meanings in biology. First, respiration is the process by which an organism takes oxygen into its body and then releases carbon dioxide from its body. In this respect, respiration can be regarded as roughly equivalent to "breathing." In some cases, this meaning of the term is extended to mean the transfer of the oxygen from the lungs to the bloodstream and, eventually, into cells. On the other hand, it may refer to the release of carbon dioxide from cells into the bloodstream and, thence, to the lungs.

Words to Know

Aerobic respiration: Respiration that requires the presence of oxygen.

Anaerobic respiration: Respiration that does not require the presence of oxygen.

ATP (adenosine triphosphate): High-energy molecule that cells use to drive energy-requiring processes such as biosynthesis (the production of chemical compounds), growth, and movement.

Capillaries: Very thin blood vessels that join veins to arteries.

Diffusion: Random movement of molecules that leads to a net movement of molecules from a region of high concentration to a region of low concentration.

Fermentation: A chemical reaction by which carbohydrates, such as sugar, are converted into ethyl alcohol.

Gill: An organ used by some animals for breathing consisting of many specialized tissues with infoldings. It allows the animal to absorb oxygen dissolved in water and expel carbon dioxide to the water.

Glucose: also known as blood sugar, a simple sugar broken down in cells to produce energy.

Glycolysis: A series of chemical reactions that takes place in cells by which glucose is converted into pyruvate.

Hemoglobin: Blood protein that can bind with oxygen.

Lactic acid: Similar to lactate, a chemical compound formed in cells from pyruvate in the absence of oxygen.

Pyruvate: The simpler compound glucose is broken down into during the process of glycolysis.

Trachea: A tube used for breathing.

Second, respiration also refers to the chemical reactions that take place within cells by which food is "burned" and converted into carbon dioxide and water. In this respect, respiration is the reverse of photosynthesis, the chemical change that takes place in plants by which carbon dioxide and water are converted into complex organic compounds. To distinguish from the first meaning of respiration, this "burning" of foods is also referred to as aerobic respiration.

Respiration mechanisms

All animals have some mechanism for removing oxygen from the air and transmitting it into their bloodstreams. The same mechanism is used to expel carbon dioxide from the bloodstream into the surrounding environment. In many cases, a special organ is used, such as lungs, trachea, or gills. In the simplest of animals, oxygen and carbon dioxide are exchanged directly between the organism's bloodstream and the surrounding environment. Following are some of the mechanisms that animals have evolved to solve this problem.

Direct diffusion. In direct diffusion, oxygen passes from the environment through cells on the animal's surface and then into individual cells inside. Sponges, jellyfish, and terrestrial flatworms use this primitive method of respiration. These animals do not have special respiratory organs. Microbes, fungi, and plants all obtain the oxygen they use for cellular respiration by direct diffusion through their surfaces.

Diffusion into blood. In diffusion into the blood, oxygen passes through a moist layer of cells on the body surface. From there, it passes through capillary walls and into the blood stream. Once oxygen is in the blood, it moves throughout the body to different tissues and cells. This method also does not rely upon special respiratory organs and is thus quite primitive. However, it is somewhat more advanced than direct diffusion. Annelids (segmented worms) and amphibians use this method of respiration.

Tracheae. In tracheal respiration, air moves through openings in the body surface called spiracles. It then passes into special breathing tubes called tracheae (singular, trachea) that extend into the body. The tracheae divide into many small branches that are in contact with muscles and organs. In small insects, air moves into the tracheae simply by molecular motion. In large insects, body movements assist tracheal air movement. Insects and terrestrial arthropods (organisms with external skeletons) use this method of respiration.

Gills. Fish and other aquatic animals use gills for respiration. Gills are specialized tissues with many infoldings. Each gill is covered by a thin layer of cells and filled with blood capillaries. These capillaries take up oxygen dissolved in water and expel carbon dioxide dissolved in blood.

Lungs. Lungs are special organs in the body cavity composed of many small chambers filled with blood capillaries. After air enters the lungs, oxygen diffuses into the blood stream through the walls of these capillaries. It then moves from the lung capillaries to the different muscles and organs of the body. Humans and other mammals have lungs in which air moves in and out through the same pathway. In contrast, birds have more specialized lungs that use a mechanism called crosscurrent exchange. Crosscurrent exchange allows air to flow in one direction only, making for more efficient oxygen exchange.

Movement of gases through the body

In direct diffusion and tracheal systems, oxygen and carbon dioxide move back and forth directly between cells and the surrounding environment. In other systems, some mechanism is needed to carry these gases between cells and the outside environment. In animals with lungs or gills, oxygen is absorbed by the bloodstream, converted into an unstable (easily broken down) chemical compound, and then carried to cells. When the compound reaches a cell, it breaks down and releases the oxygen. The oxygen then passes into the cell.

In the reverse process, carbon dioxide is released from a cell into the bloodstream. There the carbon dioxide is used to form another unstable chemical compound, which is carried by the bloodstream back to the gills or lungs. At the end of this journey, the compound breaks down and releases the carbon dioxide to the surrounding environment.

Various animals use different substances to form these unstable compounds. In humans, for example, the substance is a compound known as hemoglobin. In the lungs, hemoglobin reacts with oxygen to form oxyhemoglobin. Oxyhemoglobin travels through the bloodstream to cells, where it breaks down to form hemoglobin and oxygen. The oxygen then passes into cells.

On the return trip, hemoglobin combines with carbon dioxide to form carbaminohemoglobin. In this (and other) forms, carbon dioxide is returned to the surrounding environment.

Animals other than humans use compounds other than hemoglobin for the transport of oxygen and carbon dioxide. Certain kinds of annelids (earthworms, various marine worms, and leeches), for example, contain a green blood protein called chlorocruorin that functions in the same way that hemoglobin does in humans.

Whatever substance is used, the compound it forms with oxygen and carbon dioxide must be unstable, it must break down easily. This property is essential if the oxygen and carbon dioxide are to be released easily at the end of their journeys into and out of cells, lungs, and gills.

Cellular respiration. Cellular respiration is a process by which the simple sugar glucose is oxidized (combined with oxygen) to form the energy-rich compound adenosine triphosphate (ATP). Glucose is produced in cells by the breakdown of more complex carbohydrates, including starch, cellulose, and complex sugars such as sucrose (cane or beet sugar) and fructose (fruit sugar). ATP is the compound used by cells to carry out most of their ordinary functions, such as production of new cell parts and chemicals, movement of compounds through cells and the body as a whole, and growth.

The overall chemical change that occurs in cellular respiration can be represented by a fairly simple chemical equation:

6C6H12O6 + 6 O2 6 CO2 + 6 H2O + 36 ATP

That equation says that six molecules of glucose (C6H12O6) react with six molecules of oxygen (O2) to form six molecules of carbon dioxide (CO2), six molecules of water (H2O) and 36 molecules of ATP.

Cellular respiration is, however, a great deal more complicated that this equation would suggest. In fact, nearly two dozen separate chemical reactions are involved in the overall conversion of glucose to carbon dioxide, water, and ATP. Those two dozens reactions can be grouped together into three major cycles: glycolysis, the citric acid (or Krebs) cycle, and the electron transport chain.

In glycolysis, glucose is broken down into a simpler compound known as pyruvate. Pyruvate, in turn, is converted in the citric acid cycle to a variety of energy-rich compounds, such as ATP and NADH (nicotinamide adenine dinucleotide). Finally, all of these energy-rich compounds are converted in the electron transport chain to ATP.

Anaerobic respiration. As the equation above indicates, cellular respiration usually requires the presence of oxygen and is, therefore, often known as aerobic (or "using oxygen") respiration. Another form of respiration is possible, one that does not make use of oxygen. That form of respiration is known as anaerobic (or "without oxygen") respiration.

Anaerobic respiration begins, as does aerobic respiration, with glycolysis. In the next step, however, pyruvate is not passed onto the citric acid cycle. Instead, it undergoes one of two other chemical reactions. In the first of these reactions, the pyruvate is converted to ethyl alcohol in a process known as fermentation. Fermentation is a well-known chemical reaction by which grapes, barley, rice, and other grains are used to make wine, beer, and other alcoholic beverages.

The second anaerobic reaction occurs when cells are unable to obtain oxygen by methods they normally use. For example, a person who exercises vigorously may not be able to inhale oxygen fast enough to meet the needs of his or her cells. (Glucose is used up faster than oxygen is supplied to the cells.) In that case, cells switch over to anaerobic respiration. They convert glucose to pyruvate and then to another chemical known as lactate or lactic acid (two forms of the same compound). As lactic acid begins to build up in cells, it causes an irritation similar to placing vinegar (acetic acid) in an open wound.

Most cells are able to switch from aerobic to anaerobic respiration when necessary. But they are generally not able to continue producing energy by this process for very long.

Scientists believe that the first organisms to appear on Earth's surface were anaerobic organisms. Those organisms arose when Earth's atmosphere contained very little oxygen. They had to produce the energy they needed, therefore, by mechanisms that did not require oxygen. As the composition of Earth's atmosphere changed to include more oxygen, organisms evolved to adapt to that condition.

[See also Bacteria; Blood; Diffusion; Fermentation; Metabolism; Oxygen family; Respiratory system; Yeast ]

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Respiration

Respiration

Organisms respire in order to obtain oxygen and get rid of carbon dioxide. Oxygen drives cellular metabolism, the process in which glucose from food is converted to usable energy resources in the form of adenosine triphosphate , (ATP). Carbon dioxide is a by-product of this reaction. Aquatic species obtain oxygen from water, while terrestrial species obtain it from air. These two media, water and air, are often associated with different respiratory strategies. This is partly because the amount of oxygen in air is far greater than that dissolved in water. In addition, air requires much less energy to pump than does water, which is considerably more dense.

Many organisms do not require special respiratory organs because they obtain an adequate supply of oxygen through diffusion across the body surface. This is known as cutaneous gas exchange. Cutaneous gas exchange is often employed by small animals, which have a high ratio of surface area to volume. Some larger animals, such as certain annelid worms, are also able to use cutaneous exchange because of their large surface areas and modest energy demands.

Larger organisms almost invariably possess special respiratory organs. Respiratory specializations can be grouped into three major categories: gills , lungs, and trachea . All three types of organs evolved to provide extensive surface areas for use in gas exchange.

Gills

Gills describe respiratory structures formed from external extensions of the body. They characterize diverse species, including some annelids, some arthropods such as horseshoe crabs and crustaceans, mollusks, certain echinoderms , and vertebrates such as fish and larval amphibians. Gills are typically found in aquatic environments, although some terrestrial species, such as terrestrial crustaceans, make use of them as well. Species with very active lifestyles tend to have more highly developed gills, with considerable surface areas for gas exchange. In the fishes, for example, a series of bony gill arches supports many primary gill filaments , each of which has numerous minute folds referred to as the secondary gill lamellae.

Oxygen is absorbed into the bloodstream as oxygenated water flows over the gills. Some aquatic species rely on natural currents to carry oxygenated water to them. Others expend energy to force water over the gills. Among fishes, ram ventilators swim rapidly with open mouths, forcing water to flow into the mouth, across the gills, and out through the spiracle or operculum . Active swimmers, such as sharks and tuna, often employ ram ventilation.

Other fish species use a complex series of mouth and opercular expansions and compressions to pump water over the gills. Many aquatic species, including fishes, are characterized by countercurrent exchange, a system in which oxygenated water and deoxygenated blood flow in opposite directions, maximizing the amount of oxygen absorption that occurs.

Lungs

Lungs characterize many terrestrial species. Unlike gills, lungs are series of internal branchings that function in respiration. In humans, for example, air flows initially into the trachea. The trachea then splits into two bronchi, which split several more times into smaller and smaller bronchioles. These end at small sacs called alveoli , which are closely surrounded by capillary blood vessels.

It is in the alveoli that gas exchange actually occurs, with oxygen diffusing across the alveoli into the bloodstream. The alveoli are lined with substances called surfactants, which help to prevent them from collapsing. Humans have approximately 150 million alveoli. As with gills, the surface area available for gas exchange can be quite large.

Lungs characterize a diverse array of terrestrial species, including gastropod mollusks such as snails and slugs, spiders (whose respiratory organs are called book lungs because they contain a series of lamellae that resemble pages), and most terrestrial vertebrates.

Gas exchange using lungs depends on ventilation, the process through which air is brought from the external environment into the lungs. Ventilation mechanisms vary from taxon to taxon. In amphibian species, air is actively gulped, or forced into the lung by positive pressure. Reptiles, birds, and mammals use negative pressure to ventilate the lungs. These species expand the volume of the thoracic cavity where the lungs lie, causing air to be drawn into the lungs.

Creating negative pressure in the thoracic cavity is accomplished in a variety of ways. Lizards and snakes use special muscles to expand their rib cages. Turtles extend forelimbs and hindlimbs out of their shells to create negative pressure. In crocodiles, the liver is pulled posteriorly, towards the rear of the animal, in order to expand the thoracic cavity. Mammals employ a combination of contracting their diaphragm, a muscular sheet that lies at the base of the thoracic cavity, and expanding the rib cage.

Birds have evolved unusually efficient respiratory systems, most likely because of the tremendous energy required for flight. The bird respiratory system includes large, well-developed lungs and a series of air sacs connected to the lungs and trachea. Some of the air sacs occupy the hollow spaces in larger bones such as the humerus and femur. The air sacs are involved in ventilating the lungs, and allow for a one-directional flow of air through the respiratory system. This is unique among the terrestrial vertebrates.

The respiratory exchange system of birds is described as crosscurrent exchange. Crosscurrent exchange is less efficient than the countercurrent exchange system of fishes. However, because the concentration of oxygen in air is much greater than in water, this method is extremely effective.

Cutaneous respiration may supplement gas exchange through lungs or gills in many species. It is critical in most amphibians, which are characterized by moist skins consisting of live cells in which oxygen can dissolve and then be taken up and used by the organism. Some amphibians have evolved special adaptations that make gas exchange across the skin more effective. In some aquatic salamanders , for example, the skin has become highly folded, an adaptation that increases the surface area available for gas absorption. There is even a large family of salamanders, the family Plethodontidae, in which the lungs have been lost entirely. This group relies almost entirely on cutaneous exchange. Similar developments are seen in other groups, including some species of slugs.

The Tracheal System

The tracheal system is a respiratory system that is unique to air-breathing arthropods such as millipedes, centipedes, and insects. The tracheal system consists of air-filled tubes that extend into the body from pores on the body surface known as spiracles. These tubules provide oxygen to tissues directly. Centipedes have one spiracle per segment, and millipedes have two per segment. The number of spiracles in insects varies, but there can be as many as ten pairs.

The spiracle is merely an opening to the environment in primitive insect species. However, more advanced groups can close their spiracles, and some are even outfitted with filtering devices. The ability to close the spiracle is advantageous because it allows for water conservation. Interestingly, the limits of the tracheal gas exchange system are believed to restrict insects to their generally small size. Insects, unlike animals such as vertebrates, do not use their circulatory systems to aid in the transport of oxygen.

Water is lost from all respiratory surfaces in air-breathing organisms. This is particularly serious for species that rely on cutaneous exchange, and explains why most species of amphibians are limited to fairly moist habitats.

Respiration is regulated either by the central nervous system or by more localized mechanisms. Vertebrates have a respiratory pacemaker in the medulla of the brain. In addition, respiratory rates are influenced by internal gas concentrations. Aquatic species usually regulate respiration based on internal oxygen levels, whereas terrestrial species tend to rely on internal carbon dioxide levels.

see also Digestion.

Jennifer Yeh

Bibliography

Curtis, Helena. Biology. New York: Worth Publishers, 1989.

Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W. W. Norton, 1996.

Hickman, Cleveland P., Larry S. Roberts, and Allan Larson. Animal Diversity. Dubuque, IA: William C. Brown, 1994.

Hildebrand, Milton, and Viola Hildebrand. Analysis of Vertebrate Structure. New York: John Wiley, 1994.

Withers, Philip C. Comparative Animal Physiology. Fort Worth, TX: Saunders College Publishing, 1992.

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respiration

respiration is the absorption of oxygen and the output of carbon dioxide that take part in the metabolic processes in the body; it includes the burning of foodstuffs in the tissues, the transport of the gases in the blood and their exchange in the lungs. All but the smallest animals, for example single cells like amoeba, require specialized transport and exchange organs to provide sufficient supply and removal of the large quantities of gases involved. A vigorously exercising athletic human might take up as much as six litres of oxygen per minute, and excrete a similar amount or more of carbon dioxide.

It is usual to divide respiration into external and internal components, linked by the bloodstream that transports the gases but has little metabolism of its own. In mammals, external respiration — the ventilation of the lungs — is achieved by breathing, the mechanical basis of respiration: the terms are sometimes used synonymously. In fishes there is equivalent ‘ventilation’ of the gills with water. At rest an adult human inhales about 6–8 litres of air per minute, of which about two thirds gets to the alveoli and takes part in gas exchange, the rest remaining in the air passages. Fresh air contains 21% oxygen and almost no carbon dioxide. In the lungs, the entry of oxygen into the blood and the release of carbon dioxide results in exhaled gas having about 5% less oxygen and 5% more carbon dioxide. The gases pass in and out of the blood by passive diffusion, due to their relative pressures in the blood and in the alveolar gas, just as a fizzy drink will lose its gas and go flat when exposed to air. At rest an average-sized adult will take up about 250 ml of oxygen each minute, and exhale about 200 ml of carbon dioxide. In vigorous exercise these values can increase over 20-fold in a trained athlete as a result of the increased metabolism in the muscles. The blood in the capillaries leaving the lungs, and therefore in the arteries carrying it round the body, have the same pressures of oxygen and carbon dioxide as those in the alveoli, at least in health. In some lung and heart disease, involving a problem of gas transfer, the arterial blood may have a lower oxygen and a higher carbon dioxide pressure than that in the alveoli, causing hypoxia and hypercapnia. In other types of lung disease these abnormalities in the blood result from inadequate breathing — failure to ventilate the alveoli with sufficient fresh air.

In the blood almost all the oxygen is carried combined with haemoglobin in the red cells, while carbon dioxide is taken up in both plasma and red cells. Once the blood has reached the tissues, internal respiration takes over. Oxygen will diffuse from the capillaries into metabolizing cells, again passively following its pressure gradient. Vigorously contracting muscle cells use up almost all the available oxygen inside them, and strongly pull a fresh supply from the blood by diffusion. The oxygen combines with food materials, mainly sugars and fats, to release heat and energy for contraction of muscle, with water and carbon dioxide as the main waste products. The water enters the general water pool of the body, while the carbon dioxide enters the blood and is carried to the lungs mainly as bicarbonate but also in combination with hemoglobin and plasma proteins. An appreciable amount of carbon dioxide, unlike oxygen, is also free in solution in the plasma. Such aerobic metabolism occurs similarly in the great majority of body cells.

This understanding of the chemical basis of respiration was only developed in the eighteenth century, with the chemical identification of oxygen and carbon dioxide, mainly by Joseph Priestley (1733–1804) and Antoine-Laurent Lavoisier (1743–94). For two thousand years before then there were many speculations about the meaning of breathing, the main idea being that its function was to cool the blood by physically mixing air with it in the arteries and veins. The cooling function seemed proved by the fact that exhaled gas was usually warmer than inhaled air, and the mixing with blood was deduced from the fact that when arteries and veins were cut the blood flowing out was often frothy. (We now know that if large veins are cut they will suck air into the circulation, and it will pass through the heart and lungs and make the haemorrhaging blood bubbly.) In the second century ad Galen wrote ‘respiration is useful to animals for the sake of the heart, which to some extent requires the substance of the air and besides needs very greatly to be cooled because of its burning heat.’ Even primitive men must have known that breathing was necessary for life; according to the Judaeo–Christian tradition, breath created it. ‘And the Lord God formed man of the dust of the ground and breathed into his nostrils the breath of life; and man became a living soul’ (Genesis 2:7). In the seventeenth century much experimental work established that one component of air was essential for life, and that another different fraction was exhaled. These were identified as oxygen and carbon dioxide a century later. There was much speculation about the ‘respiration’ of fetuses. Clearly they could not breathe air, since their lungs were full of liquid. Since the role of the blood circulation in gas exchange was not understood, the respiratory function of the placenta, the ‘external’ respiratory organ of the fetus, was not apparent.

Sophisticated and accurate methods of analysing respiratory gases were developed in the twentieth century, and the mechanisms of external respiration are now well defined. At about the same time development of biochemistry and, later, of molecular biology, led to an understanding of internal respiration. When oxygen combines with carbohydrates every molecule of oxygen creates one molecule of water and one in carbon dioxide. Thus the ratio of the exchange volumes of the two gases, the respiratory quotient, is 1; in practice this would only occur in someone living on a diet of, and metabolizing only carbohydrate — polished rice, for example. Fat and protein contain less oxygen than does carbohydrate, so more oxygen is needed for their consumption than the carbon dioxide that is produced. On an average diet in a developed country the ratio of carbon dioxide excreted to oxygen absorbed is about 0.8. However these values assume that there is enough oxygen for the metabolic needs of all the body — that we are at rest or in a state of entirely aerobic metabolism. ‘Aerobic exercise’ is exercise within this limit. If exercise is severe, not enough oxygen is available for the muscles, which pass an ‘anaerobic threshold’, and energy is provided additionally by the breakdown of carbohydrates without using oxygen and with the formation of lactic acid. This diffuses into the blood, causing a mild acidosis and acting on the bicarbonate to release carbon dioxide which is excreted in the lungs. (Lemon juice, put on self-raising flour, or bicarbonate of soda, will make it fizz as it releases carbon dioxide.) Thus in severe exercise we exhale more carbon dioxide than the oxygen we absorb. After the exercise we retain carbon dioxide in the body and most of the lactic acid is converted back into carbohydrate in the liver. Extra oxygen is needed for this, and the process is referred to as ‘repaying the oxygen debt’.

John Widdicombe


See also breathing; breathing in exercise; carbon dioxide; haemoglobin; lungs; metabolism; oxygen.

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Respiration

Respiration

Respiration refers to the mechanisms for obtaining oxygen from the air and delivering it to the tissues, while eliminating carbon dioxide from the body. It is related to cellular respiration, the biochemical processes that consume this oxygen and generate the carbon dioxide in the course of making adenosine triphosphate (ATP ). Respiration in the former sense involves four processes: (1) breathing, or ventilation of the lungs; (2) gas exchange between air and blood in the lungs; (3) gas transport in the blood; and (4) gas exchange between the blood and target tissues.

Respiratory Anatomy

The respiratory system consists of: (1) the nasal cavity, which warms, cleans, and humidifies inhaled air; (2) the pharynx , where the respiratory and digestive systems meet and then diverge again; (3) the larynx , or voice box, which contains the vocal cords; (4) the trachea, or windpipe, a tube about 12 centimeters (4.7 inches) long and 2.5 centimeters (just less than an inch) wide that passes behind the heart and branches like a Y at its lower end; (5) bronchi and bronchioles, air tubes that begin at the fork of the trachea and divide into smaller and smaller divisions within each lung; and (6) alveoli, millions of tiny air sacs in the lung.

Except in the walls of the bronchi and bronchioles, the lungs have no muscle; they do not pump air in and out of themselves like the heart pumping blood, but are passively ventilated as the chest expands and contracts. The muscles that drive pulmonary ventilation are the diaphragm, a sheet of muscle between the thoracic and abdominal cavities; the intercostal muscles between the ribs; and other muscles of the abdomen and thorax that aid the primary respiratory muscles.

Pulmonary Ventilation

Ventilation is a rhythmic process, like the heartbeat, but its pacemakers are in the brainstem rather than in the chest. The medulla oblongata of the brainstem contains an inspiratory center composed of neurons that send signals to the diaphragm and external intercostal muscles. When these muscles are stimulated, they contract and enlarge the thoracic cavity. This creates a partial vacuum in the lungs. With the atmospheric pressure outside the body now greater than the pressure in the lungs, air flows "downstream" into the lungs and inflates them.

Usually no muscular effort is needed to exhale. When these muscles stop contracting, the elasticity of the thoracic cage (ribs, cartilages, diaphragm, and ligaments) causes it to spring back by itself, squeezing air out of the lungs. When one needs to exhale more deeply, however, the expiratory center of the medulla sends signals to the internal intercostal muscles, which pull the ribs downward and produce an extra degree of chest compression. The abdominal muscles also aid by increasing pressure in the abdominal cavity, pushing up on the diaphragm. These muscles are important in public speaking, singing, shouting, playing wind instruments, and blowing out candles, for example.

In normal, relaxed breathing, most adults inhale a tidal volume averaging 500 milliliters (16.9 fluid ounces) of air in each respiratory cycle. With maximum effort, however, one can inhale a greater amount called the vital capacity, averaging about 4,700 milliliters (almost 159 fluid ounces) in adults.

Pulmonary Gas Exchange

About 70 percent of the air a person inhales fills the millions of alveoli in the lungs. Each alveolus is surrounded by a basketlike mesh of blood capillaries. The wall separating the inhaled air from the blood is only 0.5 micrometer thickonly one-fifteenth the diameter of a single red blood cellso it presents very little barrier to gas diffusion between the air and blood.

Oxygen has a concentration (partial pressure) of 104 mmHg in the alveolar air and 40 mmHg in the arriving capillary blood. Thus, it diffuses down its concentration gradient from the air, through the alveolar wall, into the blood. About 98.5 percent of this oxygen binds to the pigment hemoglobin in the red blood cells, and the other 1.5 percent dissolves in the blood plasma.

Carbon dioxide (CO2), has a partial pressure of 46 mmHg in the arriving blood and 40 mmHg in the alveolar air, so its concentration gradient dictates that it diffuses the other way, from blood to air, and is then exhaled. About 70 percent of this CO2 comes from the breakdown of carbonic acid in the blood; 23 percent from CO2 bound to hemoglobin, albumin, and other blood proteins ; and 7 percent from gas dissolved in the blood plasma.

Gas Transport

Blood leaving the lungs is therefore relatively high in O2 (oxygen in its diatomic form) and low in CO2. It travels via the pulmonary veins to the left side of the heart, which pumps it out into the systemic circulation. This division of the circulatory system delivers it to every organ of the body.

Systemic Gas Exchange

When the blood reaches the systemic blood capillaries, gases undergo processes that are essentially the reverse of what occurs in the pulmonary alveoli. The blood unloads O2, which diffuses into the tissue fluid and thus reaches the cells around the blood capillaries. At the same time, the CO2 generated by the metabolism of those cells diffuses into the blood to be carried away to the lungs for disposal.

Blood typically contains 95 mmHg O2 upon arrival at the systemic capillaries and 40 mmHg O2 upon leaving. Conversely, the blood has 40 mmHg of CO2 on arrival at the systemic capillaries and typically 46 mmHg CO2 when it leaves. The blood does not, however, unload the same amount of O2 to all tissues or pick up the same amount of CO2. The more active a tissue is, the warmer it is, the lower its O2 level is, and the lower its pH is (because it generates more CO2 and CO2 reduces the pH of body fluids). Heat, low O2, low pH, and other factors enhance O2 unloading and CO2 loading, so tissues that need the most oxygen and waste removal get more than less active tissues do. The biochemistry of hemoglobin is mainly responsible for this elegant adjustment of gas exchange to the individual needs of different tissues.

see also Blood; Blood Vessels; Brain; Gas Exchange; Heart and Circulation; Oxidative Phosphorylation

Kenneth S. Saladin

Bibliography

Ganong, William F. Review of Medical Physiology, 19th ed. Stamford, CT: Appleton and Lange, 1999.

Saladin, Kenneth S. Anatomy and PhysiologyThe Unity of Form and Function, 2nd ed. Dubuque, IA: McGraw-Hill, 2001.

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Respiration

Respiration

Respiration is the physiological process that produces high-energy molecules such as adenosine triphosphate (ATP). The high-energy compounds become the fuel for the various manufacturing and growth processes of the cell. Respiration involves the transfer of electrons in a chemically linked series of reactions. The final electron acceptor in the respiration process is oxygen.

Respiration occurs in all types of organisms, including bacteria , protists , fungi , plants, and animals. In eukaryotes , respiration is often separated into three separate components. The first is known as external respiration, and is the exchange of oxygen and carbon dioxide between the environment and the organism (i.e., breathing). The second component of respiration is internal respiration. This is the exchange of oxygen and carbon dioxide between the internal body fluids, such as blood, and individual cells. Thirdly, there is cellular respiration, which is the biochemical oxidation of glucose and consequent synthesis of ATP.

Cellular respiration in prokaryotes and eukaryotes is similar. Cellular respiration is an intracellular process in which glucose is oxidized and the energy is used to make the high-energy ATP compound. ATP in turn drives energy-requiring processes such as biosynthesis, transport, growth, and movement.

In prokaryotes and eukaryotes, cellular respiration occurs in three sequential series of reactions; glycolysis, the citric acid cycle, and the electron transport chain. In prokaryotes such as bacteria, respiration involves components that are located in the cytoplasm of the cell as well as being membrane-bound.

Glycolysis is the controlled breakdown of sugar (predominantly, glucose, a 6-carbon carbohydrate) into pyruvate, a 3-carbon carbohydrate. Organisms frequently store complex carbohydrates, such as glycogen or starch, and break these down into glucose that can then enter into glycolysis. The process involves the controlled breakdown of the 6-carbon glucose into two molecules of the 3-carbon pyruvate. At least 10 enzymes are involved in glucose degradation. The oxidation of glucose is controlled so that the energy in this molecule can be used to manufacture other high-energy compounds. Each round of glycolysis generates only a small amount of ATP, in a process known as substrate-level phosphorylation. For each glucose molecule that is broken down by glycolysis, there is a net gain of two molecules of ATP. Glycolysis produces reduced nicotinamide adenine dinucleotide (NADH), a high-energy molecule that can subsequently used to make ATP in the electron transfer chain. For each glucose molecule that is broken down by glycolysis, there is a net gain of two molecules of NADH. Finally, glycolysis produces compounds that can be used to manufacture compounds that are called fatty acids. Fatty acids are the major constituents of lipids, and are important energy storage molecules.

Each pyruvate molecule is oxidized to form carbon dioxide (a 1-carbon molecule) and acetyl CoA (a two carbon molecule). Cells can also make acetyl CoA from fats and amino acids. Indeed, this is how cells often derive energy, in the form of ATP, from molecules other than glucose or complex carbohydrates. Acetyl CoA enters into a series of nine sequential enzyme-catalyzed reactions, known as the citric acid cycle. These reactions are so named because the first reaction makes one molecule of citric acid (a 6-carbon molecule) from one molecule of acetyl CoA (a 2-carbon molecule) and one molecule of oxaloacetic acid (a 4-carbon molecule). A complete round of the citric acid cycle expels two molecules of carbon dioxide and regenerates one molecule of oxaloacetic acid.

The citric acid cycle produces two high-energy compounds, NADH and reduced flavin adenine dinucleotide (FADH 2), that are used to make ATP in the electron transfer chain. One glucose molecule produces 6 molecules of NADH and 2 molecules of FADH 2. The citric acid cycle also produces guanosine triphosphate (GTP; a high-energy molecule that can be easily used by cells to make ATP) by a process known as substrate-level phosphorylation. Finally, some of the intermediates of the citric acid cycle reactions are used to make other important compounds, in particular amino acids (the building blocks of proteins), and nucleotides (the building blocks of DNA ).

The electron transfer chain is the final series of biochemical reactions in respiration. The series of organic electron carriers are localized inside the mitochondrial membrane of eukaryotes and the single membrane of Gram-positive bacteria or the inner membrane of Gram-negative bacteria. Cytochromes are among the most important of these electron carriers. Like hemoglobin, cytochromes are colored proteins, which contain iron in a nitrogen-containing heme group. The final electron acceptor of the electron transfer chain is oxygen, which produces water as a final product of cellular respiration.

The main function of the electron transfer chain is the synthesis of 32 molecules of ATP from the controlled oxidation of the eight molecules of NADH and two molecules of FADH 2, made by the oxidation of one molecule of glucose in glycolysis and the citric acid cycle. The electron transfer chain slowly extracts the energy from NADH and FADH 2 by passing electrons from these high-energy molecules from one electron carrier to another, as if along a chain. As this occurs, protons (H+) are pumped across the membrane, creating a proton gradient that is subsequently used to make ATP by a process known as chemiosmosis.

Respiration is often referred to as aerobic respiration, because the electron transfer chain utilizes oxygen as the final electron acceptor. When oxygen is absent or in short supply, cells may rely upon glycolysis alone for their supply of ATP. Glycolysis presumably originated in primitive cells early in the Earth's history when very little oxygen was present in the atmosphere. The glycolysis process has been referred to as anaerobic respiration, although this term is little used today to avoid confusion.

See also Bacterial growth and division; Biochemistry

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respiration

respiration, process by which an organism exchanges gases with its environment. The term now refers to the overall process by which oxygen is abstracted from air and is transported to the cells for the oxidation of organic molecules while carbon dioxide (CO2) and water, the products of oxidation, are returned to the environment. In single-celled organisms, gas exchange occurs directly between cell and environment, i.e., at the cell membrane. In plants, gas exchange with the environment occurs in special organs, the stomates, found mostly in the leaves (see leaf; transpiration).

Organisms that utilize respiration to obtain energy are aerobic, or oxygen-dependent. Some organisms can live in the absence of oxygen and obtain energy from fuel molecules solely by fermentation or glycolysis; these anaerobic processes are much less efficient, since the fuel molecules are merely converted to end products such as lactic acid and ethanol, with relatively little energy-rich ATP produced during these conversions.

For individual respiratory organs, see separate articles.

Animal Respiration

In complex animals, where the cells of internal organs are distant from the external environment, respiratory systems facilitate the passage of gases to and from internal tissues. In such systems, when there is a difference in pressure of a particular gas on opposite sides of a membrane, the gas diffuses from the side of greater pressure to the side of lesser pressure, and each gas is transported independently of other gases. For example, in tissues where carbon dioxide concentration is high and oxygen concentration is low as a result of active metabolism, oxygen diffuses into the tissue and carbon dioxide diffuses out.

In lower animals, gas diffusion takes place through a moist surface membrane, as in flatworms; through the thin body wall, as in earthworms; through air ducts, or tracheae, as in insects; or through specialized tracheal gills, as in aquatic insect larvae. In the gills of fish the blood vessels are exposed directly to the external (aquatic) environment. Oxygen–carbon dioxide exchange occurs between the surrounding water and the blood within the vessels; the blood carries gases to and from tissues.

In other vertebrates, including humans, gas exchange takes place in the lungs. Breathing is the mechanical procedure in which air reaches the lungs. During inhalation muscular action lowers the diaphragm and raises the ribs; atmospheric pressure forces air into the enlarged chest cavity. In exhalation the muscles relax and the air is expelled. This combined rhythmic action takes place about 12–16 times per minute when the body is at rest. The rate of breathing is controlled mainly by a respiratory center in the brain stem that responds to changes in the level of hydrogen ion and carbon dioxide in the blood, as well as to other factors such as stress, temperature changes, and motor activities. Some residual air always remains in the lungs, but with each breath an additional quantity of fresh air, called tidal air, is inhaled. Artificial respiration is used for respiratory failure.

In higher vertebrates, oxygen-poor, carbon dioxide–rich blood from the right side of the heart is pumped into the lungs and flows through the net of capillaries surrounding the alveoli, the cup-shaped air sacs of the lungs; oxygen diffuses across the capillary membranes into the blood, and carbon dioxide diffuses in the opposite direction. The oxygen combines with the protein hemoglobin in red blood cells as the blood returns to the left side of the heart, is pumped throughout the body, and is released into tissue cells (see circulatory system). Carbon dioxide passes in the opposite direction, from the cells of the tissues to the red blood cells. In the blood, carbon dioxide exists in three forms: as bicarbonate ion, in which form it serves as a buffer, keeping blood acidity fairly constant; combined with hemoglobin; and as the dissolved free gas. Of these, only free carbon dioxide gas is available for diffusion from the blood into the lungs.

Biochemical Respiration

In biochemistry, respiration refers to the series of biochemical oxidations in which organic molecules are converted to carbon dioxide and water while the chemical energy thus obtained is trapped in a form useful to the cell. Biochemical respiration occurs in both plant and animal cells. Carbohydrates, amino acids, and fatty acids—the organic fuel molecules of the cell—can be converted to acetyl CoA, a derivative of acetic acid and coenzyme A.

Acetyl CoA then enters a series of reactions in the mitochondria, organelles in the cell's cytoplasm. The series of reactions, known as the Krebs cycle, converts the acetic acid portion of acetyl CoA to carbon dioxide, protons, and hydride ions, the latter usually as part of the coenzyme NADH. This molecule is oxidized back to NAD when it donates the hydride ion to the series of enzymes known as the electron transport chain. In a process called oxidative phosphorylation, each electron transport enzyme is in turn reduced (receives the hydride ion), then oxidized (donates a hydride ion to the next enzyme in the series), and the chemical energy liberated in this series of reactions is coupled to the synthesis of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and phosphoric acid.

ATP, the cell's form of energy storage and supply, furnishes the chemical energy needed for muscle contraction, protein synthesis, active transport of substances across membranes, and electrical impulses. At the end of the electron transport chain, a hydride ion is donated to an atom of oxygen; this pair, together with a proton from the surrounding solution, forms a molecule of water. Thus, in the overall process of cellular respiration, the fuel molecules are converted to carbon dioxide and water while the chemical energy gained is trapped in a useful form as ATP.

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respiration

respiration The metabolic process in animals and plants in which organic substances are broken down to simpler products with the release of energy, which is incorporated into special energy-carrying molecules (see ATP) and subsequently used for other metabolic processes. In most plants and animals respiration requires oxygen, and carbon dioxide is an end product. The exchange of oxygen and carbon dioxide between the body tissues and the environment is called external respiration (see ventilation). In many animals the exchange of gases takes place at respiratory organs (e.g. lungs in air-breathing vertebrates) and is assisted by respiratory movements (e.g. breathing). In plants oxygen enters through pores on the plant surface and diffuses through the tissues via intercellular spaces or dissolved in tissue fluids.

Respiration at the cellular level is known as internal (cellular or tissue) respiration and can be divided into two stages. In the first, glycolysis, glucose is broken down to pyruvate. This does not require oxygen and is a form of anaerobic respiration. In the second stage, the Krebs cycle, pyruvate is broken down by a cyclic series of reactions to carbon dioxide and water. This is the main energy-yielding stage and requires oxygen. The processes of glycolysis and the Krebs cycle are common to all plants and animals that respire aerobically (see aerobic respiration).

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respiration

respiration Series of chemical reactions by which complex molecules (food molecules) are broken down to release energy in living organisms. Enzymes control these reactions, which are an essential part of metabolism. There are two main types of respiration: aerobic and anaerobic. In aerobic respiration, oxygen provided by breathing combines with the breakdown products and is necessary for the reactions to take place. Breathing is therefore different from respiration, though the terms are commonly interchanged. Anaerobic respiration takes place in the absence of oxygen, releasing energy through glucose and other foods. In most living organisms, the energy released by respiration is used to convert the compound adenosine diphosphate (ADP) to adenosine triphosphate (ATP), which transports energy around the cell. At the site where the energy is needed, ATP converts back to ADP with the aid of a special enzyme, a process that releases energy. The first stages of respiration take place in a cell's cytoplasm and the later stages, known as the Krebs cycle, in the cell's mitochondria which contain enzymes. See also transpiration

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respiration

respiration
1. Oxidative reactions in cellular metabolism that involve the sequential degradation of food substances and the generation of a high-energy compound, ATP (adenosine triphosphate) in aerobic respiration with the use of molecular oxygen as a final hydrogen acceptor; ATP, carbon dioxide, and water are the products thus formed.

2. The physico-chemical processes involved in the transportation of oxygen to and carbon dioxide from the tissues.

3. (external respiration) The act of breathing.

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respiration

respiration
1. Oxidative reactions in cellular metabolism that involve the sequential degradation of food substances and the generation of a high-energy compound (ATP) in aerobic respiration with the use of molecular oxygen as a final hydrogen acceptor; ATP, carbon dioxide, and water are the products thus formed
.
2. The physico-chemical processes involved in the transportation of oxygen to and carbon dioxide from the tissues
.
3. (external respiration) The act of breathing.

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respiration

respiration (res-per-ay-shŏn) n. the process of gaseous exchange between an organism and its environment. external r. breathing: the stage of respiration in which oxygen is taken up by the capillaries of the lung alveoli and carbon dioxide is released from the blood. internal r. the stage of respiration in which oxygen is released to the tissues and carbon dioxide absorbed by the blood. See also lung.
respiratory (rĕs-pir-ă-ter-i) adj.

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respiration

res·pi·ra·tion / ˌrespəˈrāshən/ • n. the action of breathing: opiates affect respiration. ∎  chiefly Med. a single breath. ∎  Biol. a process in living organisms involving the production of energy, typically with the intake of oxygen and the release of carbon dioxide from the oxidation of complex organic substances.

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respiration

respiration
1. Oxidative reactions in cellular metabolism involving the sequential degradation of food substances and the use of molecular oxygen as a final hydrogen acceptor; ATP, carbon dioxide, and water are the products thus formed
.
2. The physico-chemical processes involved in the transportation of oxygen to and carbon dioxide from the tissues.

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