The means by which digested nutrients reach the blood is known as absorption. Most absorption takes place through the villi in the mucosa of the small intestine (see Fig. 15-9). Within each villus is an arteriole and a venule bridged with capillaries. Simple sugars, small proteins (peptides), amino acids, some simple fatty acids, and most of the water in the digestive tract are absorbed into the blood through these capillaries. From here, they pass by way of the portal system to the liver, to be processed, stored, or released as needed.
Absorption of Fats
Most fats have an alternative method of reaching the blood. Instead of entering the blood capillaries, they are absorbed by the villi’s more permeable lymphatic capillaries called lacteals. The absorbed fat droplets give the lymph a milky appearance. The mixture of lymph and fat globules that drains from the small intestine after fat has been digested is called chyle. Chyle merges with the lymphatic circulation and eventually enters the blood when the lymph drains into veins near the heart. The absorbed fats then circulate to the liver for further processing.
Figure 15-9 A villus of the small intestine. Each villus has blood vessels and a lacteal (lymphatic capillary) for absorption of nutrients.
Absorption of Vitamins and Minerals
Minerals and vitamins ingested with food are also absorbed from the small intestine. The minerals and some of the vitamins dissolve in water and are absorbed directly into the blood. Other vitamins are incorporated in fats and are absorbed along with the fats. Vitamin K and some B vitamins are produced by bacterial action in the colon and are absorbed from the large intestine.
Control of Digestion
As food moves through the digestive tract, its rate of movement and the activity of each organ it passes through must be carefully regulated. If food moves too slowly or digestive secretions are inadequate, the body will not get enough nourishment. If food moves too rapidly or excess secretions are produced, digestion may be incomplete or the lining of the digestive tract may be damaged. There are two types of control over digestion: nervous and hormonal. Both illustrate the principles of feedback control. The nerves that control digestive activity are located in the submucosa and between the muscle layers of the organ walls. Instructions for action come from the autonomic (visceral) nervous system. In general, parasympathetic stimulation increases activity, and sympathetic stimulation decreases activity. Excess sympathetic stimulation, as in stress, can block the movement of food through the digestive tract and inhibit the secretion of mucus, which is crucial in protecting the lining of the digestive tract. The digestive organs themselves produce the hormones involved in the regulation of digestion. The following is a discussion of some of these controls. The sight, smell, thought, taste, or feel of food in the mouth stimulates, through the nervous system, the secretion of saliva and the release of gastric juice. Once in the stomach, food stimulates the release into the blood of the hormone gastrin, which promotes stomach secretions and motility (movement).
When chyme enters the duodenum, nerve impulses inhibit movement of the stomach, so that food will not move too rapidly into the small intestine. This action is a good example of negative feedback. At the same time, hormones released from the duodenum not only function in digestion, but also feed back to the stomach to reduce its activity. Gastric-inhibitory peptide (GIP) is one such hormone. It acts on the stomach to inhibit the release of gastric juice. Its more important action is to stimulate insulin release from the pancreas when glucose enters the duodenum. Another of these hormones, secretin stimulates the pancreas to release water and bicarbonate to dilute and neutralize chyme. Cholecystokinin (CCK), stimulates the release of enzymes from the pancreas and causes the gallbladder to release bile.
Hunger and Appetite
Hunger is the desire for food, which can be satisfied by the ingestion of a filling meal. Hunger is regulated by hypothalamic centers that respond to the levels of nutrients in the blood. When these levels are low, the hypothalamus stimulates a sensation of hunger. Strong, mildly painful contractions of the empty stomach may stimulate a feeling of hunger. Messages received by the hypothalamus reduce hunger as food is chewed and swallowed and begins to fill the stomach. The short-term regulation of food intake works to keep the amount
of food eaten within the limits of what the intestine can process. The longterm regulation of food intake maintains appropriate blood levels of certain nutrients. Appetite differs from hunger in that, although it is basically a desire for food, it often has no relationship to the need for food. Even after an adequate meal that has relieved hunger, a person may still have an appetite for additional food. A variety of factors, such as emotional state, cultural influences, habit, and memories of past food intake, can affect appetite.
A chronic loss of appetite, called anorexia, may be caused by a great variety of physical and mental disorders. Because the hypothalamus and the higher brain centers are involved in the regulation of hunger, it is possible that emotional and social factors contribute to the development of anorexia. Anorexia nervosa is a psychological disorder that predominantly afflicts young women. In a desire to be excessively thin, affected people literally starve themselves, sometimes to the point of death. A related disorder, bulimia, is also called the binge-purge syndrome. Affected individuals eat huge quantities of food at one time, and then induce vomiting or take large doses of laxatives to prevent absorption of the food. These disorders stress all body systems. In women, a lack of estrogen may cause menstrual periods to cease. Loss of bone mass may lead to osteoporosis. Degeneration of the myocardium can result in heart failure. Mental function is impaired. The reflux of acidic substances in bulimia causes erosion of the esophagus and destruction of tooth enamel.
Leptin: The Weight-Loss Hormone
Despite day-to-day variations in food intake and physical activity, a healthy individual maintains a constant body weight and energy reserves of fat over long periods. Clearly, long-term negative feedback mechanisms are at work, but until recently scientists did not understand them. With the discovery of the hormone leptin (from the Greek word leptos, meaning thin), researchers have been able to piece together one longterm mechanism for regulating weight. Leptin is produced by adipocytes, the cells in adipose tissue. Fat storage that occurs when food intake exceeds the body’s demands stimulates adipocytes to release more leptin into the bloodstream. Centers in the hypothalamus respond to the increased leptin by decreasing food intake and increasing energy expenditure, which result in weight loss. If this feedback mechanism is disrupted, obesity will result. For example, mice with a genetic mutation that prevents them from making leptin are obese. Injecting the mice with leptin causes them to lose weight. After discovering leptin and demonstrating that it could reverse obesity in genetically obese mice, researchers hoped that leptin could be used to treat obesity in humans. It is now known that unlike genetically obese mice, the vast majority of obese humans are able to make leptin. Human obesity appears to be caused by an inability of the hypothalamus to respond to leptin, rather our inability to make the hormone.