Digestive tract (comparative anatomy)

Digestion is the process by which food is broken down into molecules that are small enough to be absorbed into the body. Digestion takes place in the digestive tract of animals. The digestive tract is a continuous tube that acts on ingested food in a sequential manner. Each part of the digestive tract is adapted to reduce the size of food particles, either mechanically or enzymatically, until they are small enough to be absorbed into the body. Consideration of the mechanisms of food intake in lower animals will illustrate the evolution of complexity as an adaptation to the changing environments of these animals.

Digestive Tracts of Simple Animals

Sponges are primitive water-dwelling animals that are attached to a fixed point in the water. They bring food into their bodies from currents of water containing particulate food passing through openings in their outer wall. These currents are created by movements of flagella on cells called choanocytes. Food enters the choanocytes by phagocytosis. Phagocytosis is the process in which a cell surrounds a particle with extensions of its cell membrane until the particle is completely surrounded and thus becomes enclosed within a small sac, or vesicle, within the cell. Intracellular enzymes then digest the food particles dissolved in the fluid into their component molecules, which then become available to the metabolic systems in the cytoplasm of the cell. Some cells, called amoebocytes, carry the food particles to other cells in the sponge by crawling through the spaces between the cells. Their travel is by amoeboid motion, in which the cell sends out an extension, called a pseudopod, and then follows it.

This method of feeding and digestion is adequate for a sponge because most of the sponge cells are in close contact with the water currents in which it lives. Thus, these cells can have direct access to food carried in the water currents. The cells that are not in close contact with water currents can be adequately supplied by the amoebocytes. The digestion, or breakdown, must be carried on inside the cells by cytoplasmic enzymes because if these enzymes were released to the extracellular surface, they would be washed away.

The coelenterates, such as jellyfishes or hydras, are more advanced than sponges and require a more elaborate digestive mechanism. These water- dwelling animals are either attached to a surface or float in the water currents. Thus, like sponges, they are dependent on food carried in the water. These animals, however, can eat live prey as well as particulate food. They are equipped with tentacles that can reach out and trap smaller animals and paralyze them with poisoned darts called nematocysts. The tentacles then bring the food into a distinct body cavity, the gastrovascular cavity, through its one opening. The digestive cavity is, at least partially, not in direct contact with the water currents around the animal. Digestion can take place through extracellular enzymes secreted by the cells lining this cavity. The resulting molecules are then absorbed through the cell membranes. Amoebocytes also function in these animals. These animals have limited motion through musclelike cells, and this motion moves fluid within the gastrovascular cavity, thus carrying fluid to all parts of the animal.

Flatworms are more advanced than sponges and coelenterates and live in a moist, but not watery, environment. They have a distinct nervous and muscular system and can move to search for food. Their digestive tract, as that of sponges, has only a single opening. Food is pushed into this opening by the muscle action of the first part of the digestive tract, which can be protruded to the outside of the animal. Digestion is extracellular and carried to the rest of the animal through muscle contractions of the digestive tract. The digestive tract is highly branched and extends to all parts of the animal.

Digestive Tracts of More Complex Animals

Animals more highly evolved than flatworms, including roundworms, insects, fish, mammals, and birds, have a functionally similar digestive tract. These animals all have similar requirements, which have necessitated further, more efficient digestion. These animals are more active and thus must ingest more food. Their digestive tracts have two openings, allowing a continuous digestion: Food enters at one end and is excreted at the other. In contrast, an animal with only one digestive opening cannot excrete and ingest at the same time. The greater size of these more-evolved animals also requires that digested food be absorbed into the circulatory system so that distribution to the rest of the body cells is quick. The absorbing portion of the gut is therefore surrounded by blood vessels.

Further adaptations have required sophisticated specializations of the digestive tract. These include initial chewing devices that can mechanically reduce the size of food so that it can be swallowed. Parts of the digestive system have evolved to store food until it can be efficiently digested. This adaptation allows animals to eat sporadically, when food is available, and allows time for other activities, such as hunting or hiding. Other portions of the digestive tract have become specialized to secrete powerful enzymes that sequentially break down the molecules in food to smaller and smaller molecules. Last, the terminal portions of the digestive tract retain food and extract any remaining nutritional value and eliminate the rest at a convenient time. Many of these adaptations required the formation of a space, called a coelom, between the digestive tract and the rest of the body. This space allows the gut to coil and thus become much longer than the animal, with a resulting increase in the surface area available for digestion and absorption.

The mouth, or buccal cavity, is designed for the entry of food into the digestive tract. The lips and tongue are highly sensitive to the texture and taste of food. They are capable of very precise movements because their musculature is supplied with an extensive nerve supply. The tongue can move laterally, up and down, and in and out, because it has both longitudinal and circular muscles. Movements of the jaws during chewing (mastication) cause the teeth to crush and tear food in the mouth. The teeth have an outer covering of very tough enamel, which protects them against abrasion. Some animals have teeth that grow throughout their life and replace the worn-out ends. Salivary glands in the sides or base of the jaw secrete saliva through ducts that empty into the mouth at the sides of or under the tongue. Saliva has the primary function of lubricating and wetting chewed food. Saliva contains an enzyme, called salivary amylase, which begins the digestion of starch, although the digestion is greatly slowed after the food enters the acidic stomach lumen.

After the food has been reduced to small particles and mixed with saliva, it is swallowed (deglutition). Swallowing is partly a reflex action, controlled by a center in the base of the brain. The tongue rises to the roof of the mouth, pushing the rounded mass of chewed food, called a bolus, into the opening of the esophagus. Further propulsion is created by contraction of the area between the mouth and the esophagus, called the pharynx. The esophagus is a muscular tube leading to the stomach. Contractions of muscles which encircle the esophagus cause a moving ring of contraction, called peristalsis, which propels the bolus into the stomach.

Mucous Layers

The wall of the gastrointestinal tract is similar throughout its length. The layers, from lumen outward, are the mucosa, submucosa, submucosal nerve plexus, circular muscle, myenteric nerve plexus, longitudinal muscle, and the thin connective tissue covering called the serosa. The stomach and intestines are suspended from the back wall of the abdominal cavity by a sheet of connective tissue called the mesentery. Nerves, blood vessels, and lymphatic vessels reach the gut in the mesentery.

The cavity of the gut, or lumen, is lined by a single sheet of cells called the mucosa. The mucosa contains a wide variety of cell types. Most of the mucosa is composed of a cell type which is called columnar epithelium because the cells are longer than their diameter. Mucous, or goblet, cells secrete mucus, which is the viscous slippery material that protects the cells of the gut against mechanical abrasion and chemical attack. Other cells secrete enzymes into the lumen. Hydrochloric acid is secreted by parietal cells in the stomach. Other cells in the small intestine secrete basic bicarbonate ions. These cells provide the degree of acidity or basicity appropriate to the different regions of the gut. Other cells are adapted to absorb nutrients from or to secrete fluid into the lumen of the gut.

There are also many endocrine cells in the mucosa. These cells secrete hormones into the blood when they are stimulated by nerves or by the contents of the gut. These hormones control the degree of motility or secretion of the gut and the metabolic and physiological responses of the body following feeding. Indeed, the digestive system is the largest endocrine gland in the body. These same hormones are found in the brain, where they act as neurotransmitters, and in other endocrine glands. They have numerous functions revolving around the digestive tract. Some of these hormones can increase or decrease hunger. Others prepare the body for the nutrients that will be absorbed from the digestive tract so that the nutrients can be efficiently utilized. Certain hormones can be released by different types of nutrients in the lumen of the digestive tract. Other hormones can be released through the action of nerves when food is eaten.

The layer next to the mucosa, called the submucosa, is composed of fibrous connective tissue. It provides a mechanical support for the mucosa and contains the nerve and blood supply leading to and from the mucosa. The lymphatic vessels draining the mucosa also travel through the submucosa.

Nerve and Muscle Layers

The next, more external layer, is a sheet of nerves, called the submucous (Meissner) plexus. These nerves send fibers inward to the mucosa and outward to the other layers. They respond to the luminal contents and to other nerves and hormones. There are as many nerves in the gut as there are in the spinal cord. They are an intrinsic nervous system of the gut—that is, they begin and end in the gut. They are considered a separate category along with the autonomic (involuntary) and somatic (voluntary) nervous systems.

The next layer of the gut wall is a layer of visceral smooth muscle oriented circularly around the circumference of the gut. Contraction of these muscles causes a ring of contraction that may or may not move down the intestine. Next, there is another layer of nerves called the myenteric (Auerbach) plexus. Both nerve plexuses are responsible for controlling and integrating the functions of the intestine. Motility of the muscles of the gut, absorption of salt, water, and nutrients, and blood flow are all regulated by these nerves. The outermost layer of the gut is composed of visceral smooth muscle oriented longitudinally along the gut. Contractions of these muscles shorten the length of the gut.

There are also rings of smooth muscle, called sphincters, which control the movement from one part of the gut to the adjacent part. These sphincters are found between the esophagus and the stomach, the stomach and small intestine, the small and large intestine, and the large intestine and the outside.

Food that enters the stomach is partially digested by the enzyme pepsin, which is secreted by the chief cells of the gastric mucosa. Pepsin begins the digestion of protein. The hydrochloric acid secreted by the parietal cells has the functions of activating the pepsin and killing bacteria. The most necessary function of the stomach is storage of food (now reduced to a semiliquid state called acid chyme, or chyme) and slowly propelling it into the small intestine. Additionally, the stomach secretes a substance called intrinsic factor, required for absorption of vitamin B12, which promotes red blood cell formation.

Ruminants, such as cattle and sheep, have the end of the esophagus and the beginning of the stomach modified into large chambers, called the rumen and reticulum, in which food is stored. These portions of the stomach are alkaline because of the enormous volume of basic saliva secreted by the animal. Bacterial digestion of the chyme occurs in these chambers. In addition, the contents can be regurgitated into the mouth, and this cud is then chewed further. After the cud is chewed and reswallowed, it bypasses the previous chambers and enters a third chamber, called the omasum, where it is churned by muscular contractions. Finally, it enters the abomasum, which is similar to the stomach of other animals.

Birds have specialized adaptations of the stomach, called the crop and gizzard. The crop is a large structure at the beginning of the stomach that stores food until it enters the stomach. The gizzard is a muscular portion of the stomach that grinds the food. This grinding by the gizzard is necessary because birds have no teeth. Frequently, birds will ingest small stones, which are stored in the gizzard and help grind the food.

The Small Intestine

The stomach empties into the small intestine. The first portion of the small intestine is called the duodenum, the middle portion the jejunum, and the terminal portion the ileum. There are two large organs that are connected to the duodenum through ducts that empty into its lumen. These organs are the liver and the pancreas. The liver secretes bile salts, which are necessary to emulsify fats into small particles for absorption. Bile salts are stored in the gallbladder between meals. The gallbladder is connected, by a branch, to the duct leading from the liver to the duodenum. The pancreas secretes basic bicarbonate, which helps neutralize stomach acids that enter the duodenum. The pancreas also secretes many different digestive enzymes, which break down proteins, fats, carbohydrates, nucleic acids, and other large molecules. Thus, as soon as chyme enters the duodenum, it is immediately mixed with digestive enzymes and bile salts that enters the lumen from the pancreatic and bile ducts.

The chyme is mixed and propelled along the small intestine by longitudinal and circular muscle contractions. These contractions continually mix the chyme with the pancreatic enzymes and bile salts and present the digested molecules to the mucosal surface, where further digestion takes place. Most of the mucosal cells, called enterocytes, produce enzymes and absorb nutrients. Enterocytes are continuously formed in mucosal pits, called crypts. They migrate up tiny fingerlike projections, called villi, which protrude into the lumen of the gut. It takes about three days for the enterocyte to travel from the base of the crypt to the tip of the villi, and then it is sloughed into the lumen. The villi are thought to increase the surface area of the gut on which digestion and absorption take place. The enterocytes produce enzymes that are attached to the mucosal surface of the cells. These enzymes are responsible for the final stages of digestion, producing the smallest molecules, which are now in a form that can be absorbed by the intestine. Because the digestion takes place on the cell's surface, it is called contact digestion.

After molecules are in their completely digested form, they are absorbed by enterocytes, which transport them from the lumen of the gut to the circulatory or the lymphatic system. Most organic nutrients, such as amino acids, fats, and glucose, are absorbed in the first half of the small intestine, the duodenum and jejunum. Salt, water, and bile salts are absorbed primarily in the ileum. Absorption is virtually complete as long as the digestive system is functioning normally. Usually, the main problems that arise during gastrointestinal disorders are associated with malabsorption of fats. Fats require bile salts to be emulsified. Emulsification is necessary for enzymes to break down fats and also to reduce the final size of the fat microdroplet that results. If any step in this process is not functioning well, then the fats come out of suspension in the intestine and are excreted.

The final contents of the small intestine consist mostly of salts, water, indigestible fiber, and the debris from sloughed enterocytes. The small intestine empties into the large intestine, where some bacterial digestion occurs, which produces mostly small fatty acid molecules. The debris from these bacteria adds to the bulk of the undigested material. Muscle contraction propels these feces through the large intestine until it is eliminated by defecation. Sphincters control the final evacuation.

Studying the Digestive Tract

The structural features of the digestive tract can be determined by classical techniques of anatomical dissection and histological examination of the cellular characteristics of the different sections of the digestive tract. The secretions and the digestive steps can be determined by sampling the luminal contents. The sampling can be done by passing a tube through the digestive tract until the end reaches the desired portion and then withdrawing a sample for biochemical analyses.

Motility can be measured by attaching a balloon to a tube passed into the digestive tract and measuring the changes in pressure from muscle contractions. Absorption can be measured by perfusing a solution of known composition from one opening in a double tube and collecting the solution remaining after it has passed through the gut lumen from a second opening.

Motility of the intestine or the presence of obstructions that prevent the passage of food along the gastrointestinal tract can be observed by X-ray techniques. A liquid substance, such as a barium suspension, which is opaque to X-rays, is swallowed. A series of X-rays is taken, or continuous monitoring by an X-ray camera is used. Obstructions can be visualized from the buildup of barium above the blockade. The speed of movement can be estimated to determine if the overall motility of the gastrointestinal tract is abnormal. X-rays can also be used to directly determine the presence of abnormal structures such as gallstones, which form in the bile ducts, or tumors. The bile duct and gallbladder system can be visualized with X-rays by administering a radiopaque dye that is secreted by the liver into the duct system.

The overall integrity of the gastrointestinal tract can be determined by ingesting inert substances of different molecular sizes and determining if they appear in the blood. Normally, only relatively small molecules can penetrate the very tight mucosal lining of the gut, unless they are nutrients of the body. The penetration of larger molecules across the mucosa indicates leaks resulting from damage to the gastrointestinal lining.

Principal Terms

Absorption: The movement of nutrients out of the lumen of the gut into the body

Bile Salts: Organic compounds derived from cholesterol that are secreted by the liver into the gut lumen and that emulsify fats

Digestion: The process by which larger organic nutrients are broken down into smaller molecules in the lumen of the gut

Duodenum: The first part of the small intestine, where it joins the stomach

Enterocytes: The cells that line the lumen of the small intestine

Lumen: The central opening through the digestive tract, which is continuous from the mouth to the anus

Lymphatic Vessels: Very thin tubes that carry water, proteins, and fats from the gut to the bloodstream

Mucosa: The lining of the inner wall of the gut facing the lumen

Pancreas: An organ derived from the gut that secretes digestive enzymes; it is connected to the gut by a duct through which its secretions enter the gut

Plexus: A group of nerve cells and their connections to one another

Sphincter: A ring of muscle that can close off a portion of the gut

Bibliography

Arms, Karen, and Pamela S. Camp. Biology. 4th ed. Saunders College Publishing, 1995.

Ganong, William F. Review of Medical Physiology. 25th ed. Appleton and Lange, 2015.

Guyton, Arthur C., and Michael E Hall. Guyton and Hall Textbook of Medical Physiology. 14th ed. Elsevier, 2021.

Johnson, Leonard R., and David H. Alpers, et al., eds. Physiology of the Gastrointestinal Tract. 3d ed. 2 vols. Raven Press, 1994.

Johnson, Leonard R., and Thomas Gerwin, eds. Gastrointestinal Physiology. 9th ed. C. V. Mosby, 2019.

Tortora, Gerard J., and Bryan H. Derrickson. Principles of Anatomy and Physiology. 3rd ed. John Wiley and Sons Australia, 2022.