Respiratory System

The respiratory system is composed of those organs and processes that allow an animal to take oxygen into its body and expel carbon dioxide. All living things require oxygen to survive, and although respiratory systems may vary in size and the way they function, they share many basic features and operate on the same principles.

All animals need oxygen to survive, because oxygen is the fuel they use to convert their food into energy. As with any fuel that is consumed, there is always a byproduct given off, and in the case of animals, this waste product is carbon dioxide. Since all animals need to constantly take in oxygen and eliminate carbon dioxide, they need some system to perform this gas exchange, which is called respiration. Although a variety of systems and methods accomplish this, all operate on the same basic principle called diffusion.

DIFFUSION VITAL TO RESPIRATION

Diffusion can be described as the movement or spreading out of a substance from an area in which it is highly concentrated to an area of its lowest concentration. Diffusion takes place at the level of molecules, and since molecules are constantly in motion, they have a natural tendency to mix randomly with one another. Although it is not known exactly why molecules behave this way, they have a natural tendency to move from where they are all together to where there are the fewest of them, in an effort to spread themselves out more evenly. Although any given single molecule may not always behave this way, the net or overall movement of a group of molecules will always do so. This net movement is called diffusion. An everyday example of diffusion is the way in which tobacco smoke or strong perfume will spread throughout the still air of a closed room.

The respiratory system of an animal always carries out diffusion across a respiratory surface. There is always some sort of membrane across which the gases (oxygen and carbon dioxide) pass in and out of a body. When there is more oxygen in the environment outside a membrane than there is inside, oxygen will automatically drift through the membrane to equalize the pressure on both sides. Although this may sound like osmosis (which could be described as diffusion across a membrane), osmosis only involves the passage of a substance dissolved in a liquid solution. In respiration, however, gases are exchanged.

INTEGUMENTARY EXCHANGE

In the simplest animal respiratory system, gases are exchanged directly through the body tissues themselves. This is called integumentary exchange, since an animal's integument is its protective outer covering or skin. The one-celled organisms that practice this form of respiration are too simple to have any skin, and instead have a membrane that allows oxygen and carbon dioxide to diffuse right through it and reach every part of its "body." More complicated animals, like earthworms, also perform this type of "skin breathing," but they also use a simple type of circulatory system to move the gases from the inside of their body to their skin. Studying an earthworm also reveals a major characteristic of all respiratory systems—they must always be wet. The slimy, mucus-covered skin of an earthworm demonstrates that diffusion will only take place if the respiratory surface or membrane is always kept moist. If an earthworm dries out, it will suffocate.

TRACHEAL SYSTEM

Skin breathing may be fine for very small organisms, but when the outer area of an animal cannot provide a large enough surface area or is too thick or hardened to allow a good gas exchange, then the animal needs some type of specialized respiratory organs. One simple form of such specialized organs is the tracheal system found in insects. In this unique system, oxygen and carbon dioxide are exchanged through diffusion by means of a network of small, stiff tubes called tracheae that extend into the insect's body and are small enough to supply individual cells. Insects have no need of lungs or any connection to a circulatory system.

GILLS

The gills that fish use to breathe are the next most sophisticated system, and they are designed specifically to work in water. A gill is a collection of thin flaps (called lamellae) that are able to obtain oxygen dissolved in water. As water moves over the lamellae's thin membrane, dissolved oxygen diffuses into the fish's blood, while carbon dioxide diffuses out through the gills and into the water. Since water contains only five percent of the oxygen that air does, gills must be especially efficient.

They only will work, however, if water keeps moving over their surface, and a fish needs to keep its mouth open as its swims to keep the water moving over the gills. When a fish is not moving, it closes its gills, takes a mouthful of water, and forces it out its gills. Fish cannot breathe out of water because the filaments that contain their lamellae collapse when they are not supported by water. The gills of most fish are located behind their head.

THE HUMAN RESPIRATORY SYSTEM

Lungs are the respiratory organs used by mammals, birds, and reptiles, as well as some amphibians. Lungs exchange gases, since it is the job of the circulatory system to collect and distribute gases throughout the organism. The human lungs are located in the chest cavity, although the human respiratory system actually begins at the nose and mouth where air is inhaled and exhaled. The mouth and nose are connected to a common tube at the back of the throat called a pharynx. Air then passes through the larynx, also called the voice box, into a branching system that resembles an upside-down tree. The larynx flows into the trachea (the tree trunk), which divides into two large limbs called the right and left bronchi. Each of these branch off into multiple smaller bronchi, which continue dwindling down into smaller and smaller tubes that finally end in terminal bronchioles. At the end of these bronchioles, like the leaves of tree, are clusters of moist air sacs called alveoli where the actual exchange of gases takes place. Tiny, thin-walled blood vessels called capillaries create vast networks surrounding the alveoli. It is across these thin walls that oxygen is passed into the blood and carbon dioxide out of the blood by diffusion. Humans have a total of 300,000,000 alveoli in both lungs. Air is actually forced into the lungs by a large muscle beneath the lungs called a diaphragm. When its muscles contract and pull down, the ribs above are lifted upward and outward, and air rushes into the elastic lungs. When air is automatically sucked into the lungs as they expand (inspiration), it is called negative pressure breathing. Expiration occurs when the diaphragm muscles relax, allowing the lungs to go back to their retracted state. Air containing carbon dioxide is then forced out of the lungs. Breathing is controlled by a brain command that sends a signal every few seconds. The average adult human takes between twelve and fifteen breaths a minute.

Because the human respiratory system brings in substances from the outside environment, there are safeguards to fight infectious agents that may enter with the oxygen. The normal human lung is sterile, meaning that there are no bacteria or viruses present. The first line of defense includes hair in the nostrils that filters large particles. The epiglottis between the pharynx and larynx acts as a trap door and prevents food and other swallowed substances from entering the larynx and then the trachea. If it does, we cough involuntarily and say that "something went down the wrong pipe." The acts of sneezing and coughing are usually both started by irritants in the respiratory system, and work to forcibly expel them. Finally, mucus exists throughout the system and serves to trap dust and infectious organisms. Cells called macrophages line the respiratory tract and engulf and kill anything they consider an invader.

Plants can be said to "breathe" since they exchange gases during photosynthesis (which is actually the reverse of respiration since it takes in carbon dioxide and gives off oxygen in the process of making food). However, they do not have any respiratory system approximating that of an animal, since plants exchange gases through simple openings or pores in their leaves called stomata.

[See alsoBlood; Respiration ]