Each organ of the mammalian digestive system has specialised food–processing functions.
The general principles of food processing are similar for a diversity of animals, so we can use the digestive system of mammals as a representative example. The mammalian digestive system consists of the alimentary canal and various accessory glands that secrete digestive juices into the canal through ducts.
After food is chewed and swallowed, it takes only 5–10 seconds for it to pass down the oesophagus and into the stomach, where it spends 2–6 hours being partially digested. Final digestion and nutrient absorption occur in the small intestine over a period of 5–6 hours. In 12–24 hours, any undigested material passes through the large intestine, and faeces are expelled through the anus.
The general principles of food processing are similar for a diversity of animals, so we can use the digestive system of mammals as a representative example. The mammalian digestive system consists of the alimentary canal and various accessory glands that secrete digestive juices into the canal through ducts.
Peristalsis, rhythmic waves of contraction by smooth muscles in the wall of the canal, pushes the food along the tract. At some of the junctions between specialised segments of the digestive tube, the muscular layer is modified into ringlike valves called sphincters, which close off the tube like drawstrings, regulating the passage of material between chambers of the canal. The accessory glands of the mammalian digestive system are three pairs of salivary glands, the pancreas, the liver and the gallbladder, which stores a digestive juice.
Using the human digestive system as a model, let′s now follow a meal through the alimentary canal, examining in more detail what happens to the food in each of the processing stations along the way.
The Oral Cavity, Pharynx, and Oesophagus
Both physical and chemical digestion of food begin in the mouth. During chewing, teeth of various shapes cut, smash, and grind food, making it easier to swallow and increasing its surface area. The presence of food in the oral cavity triggers a nervous reflex that causes the salivary glands to deliver saliva through ducts to the oral cavity. Even before food is actually in the mouth, salivation may occur in anticipation because of learned associations between eating and the time of day, cooking odours, or other stimuli. Humans secrete more than a liter of saliva each day.
Saliva contains a slippery glycoprotein (carbohydrate–protein complex) called mucin, which protects the lining of the mouth from abrasion and lubricates food for easier swallowing. Saliva also contains buffers that help prevent tooth decay by neutralising acid in the mouth. Antibacterial agents in saliva kill many of the bacteria that enter the mouth with food.
Chemical digestion of carbohydrates, a main source of chemical energy, begins in the oral cavity. Saliva contains salivary amylase, an enzyme that hydrolyses starch (a glucose polymer from plants) and glycogen (a glucose polymer from animals). The main products of this enzyme′s action are smaller polysaccharides and the disaccharide maltose.
The tongue tastes food, manipulates it during chewing, and helps shape the food into a ball called a bolus. During swallowing, the tongue pushes a bolus to the back of the oral cavity and into the pharynx.
The region we call our throat is the pharynx, a junction that opens to both the oesophagus and the windpipe (trachea). When we swallow, the top of the windpipe moves up so that its opening, the glottis, is blocked by a cartilaginous flap, the epiglottis. You can see this motion in the bobbing of the “Adam′s apple” during swallowing. This tightly controlled mechanism normally ensures that a bolus is guided into the entrance of the oesophagus.
Food or liquids may go “down the wrong pipe” because the swallowing reflex didn′t close the opening of the windpipe in time. The resulting blockage of airflow (choking) stimulates vigorous coughing, which usually expels the material. If it is not expelled quickly, the lack of airflow to the lungs can be fatal.
The oesophagus conducts food from the pharynx down to the stomach by peristalsis. The muscles at the very top of the oesophagus are striated (voluntary). Thus, the act of swallowing begins voluntarily, but then the involuntary waves of contraction by smooth muscles in the rest of the esophagus take over.
The Stomach
The stomach stores food and performs preliminary steps of digestion. This large organ is located in the upper abdominal cavity, just below the diaphragm. With accordionlike folds and a very elastic wall, the stomach can stretch to accommodate about 2 L of food and fluid. It is because the stomach can store an entire meal that we do not need to eat constantly. Besides storing food, the stomach performs important digestive functions: It secretes a digestive fluid called gastric juice and mixes this secretion with the food by the churning action of the smooth muscles in the stomach wall.
Gastric juice is secreted by the epithelium lining numerous deep pits in the stomach wall. With a high concentration of hydrochloric acid, gastric juice has a pH of about 2—acidic enough to dissolve iron nails. One function of the acid is to disrupt the extracellular matrix that binds cells together in meat and plant material. The acid also kills most bacteria that are swallowed with food. Also present in gastric juice is pepsin, an enzyme that begins the hydrolysis of proteins. Pepsin breaks peptide bonds adjacent to specific amino acids, cleaving proteins into smaller polypeptides, which are later digested completely to amino acids in the small intestine. Pepsin is one of the few enzymes that works best in a strongly acidic environment. The low pH of gastric juice denatures (unfolds) the proteins in food, increasing exposure of their peptide bonds to pepsin.
What prevents pepsin from destroying the cells of the stomach wall? First, pepsin is secreted in an inactive form called pepsinogen by specialised cells called chief cells located in gastric pits.
Other cells, called parietal cells, also in the pits, secrete hydrochloric acid. The acid converts pepsinogen to active pepsin by removing a small portion of the molecule and exposing its active site. Because different cells secrete the acid and pepsinogen, the two ingredients do not mix—and pepsinogen is not activated—until they enter the lumen (cavity) of the stomach. Activation of pepsinogen is an example of positive feedback: Once some pepsinogen is activated by acid, activation occurs at an increasingly rapid rate because pepsin itself can activate additional molecules of pepsinogen. Many other digestive enzymes are also secreted in inactive forms that become active within the lumen of the digestive tract.
The stomach′s second defense against self–digestion is a coating of mucus, secreted by the epithelial cells of the stomach lining. Still, the epithelium is constantly eroded, and mitosis generates enough cells to completely replace the stomach lining every three days. Gastric ulcers, lesions in this lining, are caused mainly by the acid–tolerant bacterium
Helicobacter pylori
Though treatable with antibiotics, gastric ulcers may worsen if pepsin and acid destroy the lining faster than it can regenerate.
About every 20 seconds, the stomach contents are mixed by the churning action of smooth muscles. You may feel hunger pangs when your empty stomach churns. (Sensations of hunger are also associated with brain centres that monitor the blood′s nutritional status and levels of the appetite–controlling hormones discussed earlier in this chapter.) As a result of mixing and enzyme action, what begins in the stomach as a recently swallowed meal becomes a nutrient–rich broth known as acid chyme.
Most of the time, the stomach is closed off at both ends. The opening from the oesophagus to the stomach, the cardiac orifice, normally dilates only when a bolus arrives. The occasional backflow of acid chyme from the stomach into the lower end of the oesophagus causes heartburn. (If backflow is a persistent problem, an ulcer may develop in the oesophagus.) At the opening from the stomach to the small intestine is the pyloric sphincter, which helps regulate the passage of chyme into the intestine, one squirt at a time. It takes about 2 to 6 hours after a meal for the stomach to empty in this way.
The Small Intestine
With a length of more than 6 m in humans, the small intestine is the longest section of the alimentary canal (its name refers to its small diameter, compared with that of the large intestine). Most of the enzymatic hydrolysis of food macromolecules and most of the absorption of nutrients into the blood occur in the small intestine.
Most digestion is completed early in this journey, while the chyme is still in the duodenum. The remaining regions of the small intestine, called the jejunum and ileum, function mainly in the absorption of nutrients and water
Absorption of Nutrients
To enter the body, nutrients in the lumen must cross the lining of the digestive tract. A few nutrients are absorbed in the stomach and large intestine, but most absorption occurs in the small intestine. This organ has a huge surface area—300 m2, roughly the size of a tennis court. Large circular folds in the lining bear fingerlike projections called villi, and each epithelial cell of a villus has many microscopic appendages called microvilli that are exposed to the intestinal lumen
(The microvilli′s shape is the basis of the term brush border for the intestinal epithelium.) This enormous microvillar surface is an adaptation that greatly increases the rate of nutrient absorption.
Penetrating the core of each villus is a net of microscopic blood vessels (capillaries) and a small vessel of the lymphatic system called a lacteal. (In addition to their circulatory system, vertebrates have an associated network of vessels—the lymphatic system—that carries a clear fluid called lymph. Nutrients are absorbed across the intestinal epithelium and then across the unicellular epithelium of the capillaries or lacteals. Only these two single layers of epithelial cells separate nutrients in the lumen of the intestine from the bloodstream.
In some cases, transport of nutrients across the epithelial cells is passive. The simple sugar fructose, for example, apparently moves by diffusion down its concentration gradient from the lumen of the intestine into the epithelial cells and then into capillaries. Other nutrients, including amino acids, small peptides, vitamins, and glucose and several other simple sugars, are pumped against concentration gradients by the epithelial membranes. This active transport allows the intestine to absorb a much higher proportion of the nutrients in the intestine than would be possible with passive diffusion.
Amino acids and sugars pass through the epithelium, enter capillaries, and are carried away from the intestine by the bloodstream. After glycerol and fatty acids are absorbed by epithelial cells, they are recombined into fats within those cells. The fats are then mixed with cholesterol and coated with proteins, forming small globules called chylomicrons, most of which are transported by exocytosis out of the epithelial cells and into lacteals
The lacteals converge into the larger vessels of the lymphatic system. Lymph, containing chylomicrons, eventually drains from the lymphatic system into large veins that return blood to the heart.
In contrast to the lacteals, the capillaries and veins that drain nutrients away from the villi all converge into the hepatic portal vein, a blood vessel that leads directly to the liver. This ensures that the liver—which has the metabolic versatility to interconvert various organic molecules—has first access to amino acids and sugars absorbed after a meal is digested. Therefore, blood that leaves the liver may have a very different balance of these nutrients than the blood that entered via the hepatic portal vein. For example, the liver helps regulate the level of glucose molecules in the blood, and blood exiting the liver usually has a glucose concentration very close to 0.1%, regardless of the carbohydrate content of a meal (see Figure 41.3). From the liver, blood travels to the heart, which pumps the blood and the nutrients it contains to all parts of the body.
The large intestine or colon, is connected to the small intestine at a T–shaped junction, where a sphincter (a muscular valve) controls the movement of material. One arm of the T is a pouch called the caecum. Compared to many other mammals, humans have a relatively small caecum. The human cecum has a fingerlike extension, the appendix, which is dispensable. (Lymphoid tissue in the appendix makes a minor contribution to body defense.) The main branch of the human colon is about 1.5 m long.
A major function of the colon is to recover water that has entered the alimentary canal as the solvent of the various digestive juices. About 7 L of fluid are secreted into the lumen of the digestive tract each day, which is much more liquid than most people drink. Most of this water is reabsorbed when nutrients are absorbed in the small intestine. The colon reclaims much of the remaining water that was not absorbed in the small intestine. Together, the small intestine and colon reabsorb about 90% of the water that enters the alimentary canal.
The wastes of the digestive tract, the faeces, become more solid as they are moved along the colon by peristalsis. The movement is sluggish, and it generally takes about 12 to 24 hours for material to travel the length of the organ. If the lining of the colon is irritated—by a viral or bacterial infection, for instance—less water than normal may be reabsorbed, resulting in diarrhea. The opposite problem, constipation, occurs when peristalsis moves the feces along the colon too slowly. An excess of water is reabsorbed, and the faeces become compacted.
Living in the large intestine is a rich flora of mostly harmless bacteria. One of the common inhabitants of the human colon is
Escherichia coli, a favourite research organism of molecular biologists. The presence of
Escherica coli in lakes and streams is an indication of contamination by untreated sewage. Intestinal bacteria live on unabsorbed organic material. As by–products of their metabolism, many colon bacteria generate gases, including methane and hydrogen sulfide. Some of the bacteria produce vitamins, including biotin, folic acid, vitamin K, and several B vitamins. These vitamins, absorbed into the blood, supplement our dietary intake of vitamins.
Faeces contain masses of bacteria, as well as cellulose and other undigested materials. Although cellulose fibers have no caloric value to humans, their presence in the diet helps move food along the digestive tract.
The terminal portion of the colon is called the rectum, where faeces are stored until they can be eliminated. Between the rectum and the anus are two sphincters, one involuntary and the other voluntary. One or more times each day, strong contractions of the colon create an urge to defecate.
