Digestive System

A digestive system is a system that allows an organism to take in food, break it down, absorb its nutrients, and excrete what is not usable. All organisms that cannot internally make their own food (as plants do) must ingest or eat it, and therefore must have a digestive system in some form. Different types of animals have different digestive systems according to their main diet and the amount they eat.

All living things need food in order to continue to live, grow, and reproduce. Except for green plants and some algae that can make their own food using the Sun's energy, all other living things get their energy from eating other living things, such as plants or animals. However, these plants and animals used as food are made up of large molecules that cannot be used by an organism's cells unless they are changed into smaller molecules that can be absorbed. The entire process by which food is converted into a form the body can use is called digestion. This process of digestion is carried out by the organism's digestive system. Digestive systems range from the very simple, primitive systems of one-celled organisms to the complex, many-organ systems used by vertebrates (animals with backbones).

INTRACELLULAR DIGESTION

Very simple, single-celled organisms practice what is called intracellular digestion in which they engulf or surround a food particle with their outer membrane. During this type of digestion, these organisms literally bring the food particle inside the cell. Strong enzymes (proteins that control the rate of chemical changes) break the food particle down into its usable components, which are then absorbed into the cell's cytoplasm (the jelly-like fluid inside a cell). Waste products are packaged up and passed back out through the cell membrane.

EXTRACELLULAR DIGESTION

Other slightly more complex organisms like a sponge may have a mouth that leads to a large, open body cavity. Organisms that have only one opening through which passes both their food and their waste are said to have an incomplete digestive system. Flatworms and hydras have this type of digestive system, sometimes called a blind gut. Food enters its mouth and is partially digested by chemicals released into its gut. This is called extracellular digestion because it occurs inside the gut cavity and not inside a cell. Once the food has been broken down, the smaller bits can be absorbed by the cells that line the gut. Waste products are passed back out through the mouth. Because of this two-way traffic, the organism's cavity cannot be subdivided into specialized compartments.

More complex organisms have more complicated digestive systems. These are called complete digestive systems. Higher up the evolutionary ladder, the blind gut develops a separate opening for waste removal, called the anus. This is seen in earthworms, clams, crabs, spiders, and starfish, among others, who have the simplest form of a complete digestive system. Food enters the mouth, is broken down, and passes in one

direction through a straight digestive tube where it is absorbed. Waste passes out of the body through an anus at the organism's other end. Earthworm waste called casts are deposited on the soil's surface, adding needed nutrients.

WILLIAM BEAUMONT

American surgeon William Beaumont (1785–1853) conducted pioneering studies on how the human stomach works. His highly accurate and well-documented firsthand observations of the stomach proved that digestion is basically a chemical process.

If U.S. Army surgeon William Beaumont had not been in the right place at the right time, he would never have become a pioneer of science. Until the event occurred that changed his life, Beaumont had done nothing remarkable. Born in Lebanon, Connecticut, on a farm, he was able to study medicine at St. Albans in Vermont. By 1812, he became an assistant surgeon in the U.S. Army. Three years later, he left the army and tried private practice in Plattsburgh, New York. However, in 1820 he returned to the army and was eventually transferred to the frontier post of Fort Mackinac in Michigan. While there, a bizarre medical event occurred on June 6, 1822. That day, Beaumont was called upon to assist a patient who had been accidentally shot at close range with a shotgun. What Beaumont found was a young French-Canadian trapper named Alexis St. Martin who had been struck by the blast on his left side. The shotgun had taken a deep chunk out of his side and no one expected the young man to live through the night. Beaumont tended him with care and skill nonetheless, and to everyone's surprise he remained alive. Beaumont continued his care, changing his bandages every day for a year, and in time, St. Martin was fully recovered.

However, despite the fact that he had regained his full strength and seemed normal, St. Martin was not literally whole. His wound never fully closed, and he was left with an inch-wide opening through which Beaumont could insert his finger all the way into St. Martin's stomach. The trapper had been left with what is called a permanent traumatic fistula, meaning he had a hole in his side that led directly to his stomach. About a year later when St. Martin needed to take some medicine from Beaumont, the surgeon decided to try an experiment and administered the medicine directly into the stomach rather than orally as he would have done normally. Beaumont soon found that the medicine worked on St. Martin exactly as it would have if it had been administered the regular way. This led Beaumont to realize that he had a unique research opportunity at hand, and he soon began a series of experiments and observations on his subject. First Beaumont attached small chunks of food to a string and inserted them directly into the St. Martin's stomach. Beaumont would then withdraw the string and observe the results of digestion hour by hour. Later, by using a hand lens, he began actually looking into his patient's stomach to see how it behaved. He was also able to extract and analyze samples of gastric juice and stomach contents, establishing that digestion is indeed a chemical process. He could also observe the muscular movements of the stomach. Over the next few years, Beaumont conducted more than two hundred carefully detailed experiments and, in 1833, published his findings in Experiments and Observations on the Gastric Juice and the Physiology of Digestion. As the first, well-documented and accurate observation of the digestive processes of a living human being, Beaumont's book was a one-of-kind source on the process of digestion. It also suggested to some scientists the possibility of using artificial fistulas on their research animals as a way of learning more about their bodies.

The human side of this story should also be noted. By about 1835, St. Martin had had enough, not only of Beaumont's experiments, but also of the bullying Beaumont himself. Poor St. Martin refused to cooperate anymore and eventually just ran away from Beaumont for good and returned to his native Canada. In his own way, the wounded trapper who lived to the ripe old age of eighty-two, had made his own contributions toward understanding more about the human body.

HOW THE DIGESTIVE SYSTEM WORKS

As organisms became more complex and evolved into what are called the higher animals, their digestive systems developed an alimentary tract or alimentary canal with specialized structures and compartments. Among vertebrates, a basic digestive scheme came about that resulted in structures responsible for: receiving food, conducting and storing food, breaking food down and absorbing its useful nutrients, and absorbing water while eliminating wastes. The alimentary canal in vertebrates is able to move food from the mouth to the anus entirely on its own. It does this by contracting its circular muscles and pushing food further along. This happens in a rhythmic wave called peristalsis. The canal itself is constantly lubricated by mucus, making the food move easily when the circular muscles contract. Automatically, these important muscles are regularly contracting and relaxing, pushing food and waste along as if a fist were closing on a tube, squeezing its contents in a certain direction.

In all vertebrates, food encounters four major areas in the alimentary canal where it is received and ingested (swallowed), broken down, absorbed and eliminated. The mouth is responsible for receiving food, and it is usually designed with the animal's diet in mind. The mouth may have teeth which, helped by the tongue, mechanically break food down into smaller pieces. Some species, like snakes, smell with their tongue, while others, such as frogs, capture their food with it. Since mammals chew their food in their mouth, they also have salivary glands that secrete enzymes that moisten their food and begin the chemical breakdown process. The esophagus connects to the stomach and moves the food downward after it is swallowed. Unlike most vertebrates, however, birds have an enlarged esophagus called a crop that is used to store food before it is sent to the stomach. It also can produce a "milky" substance that a parent bird can regurgitate (spit back up) and feed its young.

In most vertebrates, the real grinding and digestion of food is carried out in the stomach. This is a muscular pouch that works to mix the food with a highly acidic combination of chemicals that break it down or dissolve it further. Powerful stomach muscles churn up the food while certain cells lining the stomach produce gastric juice (mainly hydrochloric acid) that turn the broken-down food into a milky substance known as chyme. The stomach itself is protected from the strong acid by a thick layer of mucus. As the stomach fills with chyme, it gradually releases small amounts into the duodenum, the top part of the small intestine. The stomach of some vertebrates, like birds, who swallow their food without chewing (like birds), are called gizzards and often contain pebbles that mechanically break up the food for them as their muscle walls contract. Some plant-eating mammals called ruminants, like cows and horses, have special stomach chambers that help break down their difficult-to-digest diet. They also must have certain microorganisms in their gut in order to break down their food since mammals do not produce the digestive enzymes needed to break down cellulose (tough plant walls).

Once the now-liquid food passes into the duodenum, it begins its final trip to becoming completely digested and prepared for absorption. At this point, the liver and the pancreas come into play as both have ducts leading into the duodenum. The pancreas secretes enzymes or pancreatic juice that is alkaline and counteracts the strong acid made by the stomach. It also breaks down food molecules in the duodenum. The liver produces a fluid called bile that is essential if the body is to digest fats. Bile emulsifies (breaks down) large fat globules so they can be absorbed. Without bile, which is stored in a sac called the gall bladder, most of the fat would pass through the digestive system undigested. From the duodenum, the digested food flows into the small intestine, called the ileum in humans. The small intestine has densely folded structures called villi that look like tiny tongues or fingers. These villi absorb the molecules of nutrients and then rapidly send them into the bloodstream. The parts of food that have not been digested or absorbed continue to pass down into the large intestine (called the colon in humans) in the chyme. The large intestine's job is to absorb nearly all its water, and what is left arrives at the rectum where it is stored as feces. When a sufficient quantity has accumulated, the feces are expelled through the anus. This is called elimination.

Although mammals all have the same basic digestive system, there are differences that reflect their diet. Carnivores, or meat-eaters who have special teeth and live by hunting, have fairly short digestive systems since meat is mostly protein and requires very little hard work to digest. Some carnivores swallow their prey whole and thus do not need to have salivary glands. Herbivores (plant eaters) have to consume huge amounts of vegetative matter that is also hard to break down. As a consequence, their digestive systems are much more complicated than that of a carnivore. For example, a cow has four stomachs and an extremely long and coiled intestine. Some animals, like rodents and rabbits, even reingest their waste pellets so that their food passes through their systems twice. Since their digestion is basically incomplete, they must take in their partially digested fiber and have it pass through their digestive system again to benefit from it fully.