10.8: Small Intestine

The small intestine is a tube approximately 6 meters (20 feet) long and 2.5 cm (1 inch) in diameter that extends from the pyloric sphincter to the beginning of the large intestine (Figure 10.1). It fits into the expansive region below the stomach by being coiled and bent.

Secretion

Like the stomach, the small intestine has a fairly thick inner lining that secretes digestive juices. Materials in the intestinal secretions include water and a variety of digestive enzymes. The water dissolves small molecules and, with the enzymes, breaks down large nutrient molecules. The breakdown of nutrients is aided by secretions from the liver and pancreas.

Absorption

The small intestine is also the section of the GI tract where most nutrients are absorbed. As in the stomach, churning aids absorption by bringing materials into contact with the inner absorptive surface. Absorption by the small intestine is especially efficient because its lining has a series of inward foldings and many microscopic fingerlike projections (villi).

The inward foldings and villi permit rapid absorption by increasing the surface area that is in contact with dissolved nutrients. Absorption by the villi is aided by the presence of many capillaries and lymph vessels, which allow nutrients to enter the circulatory system quickly. Most absorbed nutrients enter the blood vessels, though fat enters the lymph vessels.

The absorption of three nutrients by the small intestine requires special assistance. First, iron can be absorbed effectively only if the stomach has treated it with adequate amounts of HCl. Iron functions in the production of red blood cells. Second, vitamin B12 can be absorbed in adequate amounts only if the stomach provides the small intestine with enough intrinsic factor. Vitamin B12 is also important for RBC production. Finally, calcium absorption requires the presence of activated vitamin D, which also allows the small intestine to increase the efficiency of calcium absorption when calcium in the diet or the body falls below desirable levels. Having adequate calcium is necessary for several functions, including maintenance of strong bones, muscle contraction, and nervous system activities.

Movements

As digestion and absorption continue, the contents of the small intestine are moved forward periodically by peristalsis. The basic actions and control mechanisms for peristalsis are similar to those in the stomach. By the time the contents have reached the end of the small intestine, almost all useful nutrients have been fully digested and absorbed. The remaining indigestible substances, wastes in bile from the liver, bacteria, and much water are pushed into the large intestine.

Age Changes

Aging seems to have little effect on the structure and functions of the small intestine. The age changes that have been observed, such as alterations in villi, apparently do not have important effects on intestinal functioning.

Lactase Secretion

One exception is a gradual decrease in the secretion of lactase, which splits lactose into two simple sugar molecules. Lactose is found in milk and many foods made from milk.

The decline in lactase varies from person to person with respect to time of onset and severity. Because of genetic factors, several groups (e.g., blacks, Asiatics, people of Mediterranean descent) have a significant decrease in lactase secretion during childhood or adolescence while most white people retain adequate lactase secretion well into adulthood. However, lactase secretion eventually becomes quite low in many older individuals. When it becomes too low, much of the lactose consumed in milk and milk products is not broken down. Certain types of bacteria in the intestine then use the undigested lactose for their own nutrition, resulting in much intestinal gas production. The gas can cause considerable discomfort or temporary disability. Such individuals are said to have lactose intolerance.

Many people with lactose intolerance avoid its consequences by abstaining from milk and foods containing milk. However, since dairy products are a major source of calcium, this can lead to calcium deficiency, which is a main risk factor for osteoporosis.

There are ways to avoid the adverse effects of lactose intolerance while still consuming milk and milk products. One way is to consume milk or milk products that have had lactose converted to other substances by bacterial action or lactase additives. Examples include certain types of yogurt and hard cheeses. Another way is to take lactase supplements. People who cannot use these methods should consume nondairy foods containing high levels of calcium, such as green leafy vegetables, canned fish, and calcium-supplemented orange juice.

Absorption

A second exception is a decline in the ability of the small intestine to absorb vitamins A, D, K and zinc. These decreases become important only for individuals whose diets contain low levels of these nutrients. The consequences of these deficiencies include skin and vision problems; weak bones; slow blood clotting; and decreased healing, immune function, and taste sensation, respectively.

Although the small intestine retains most of its absorptive power, its ability to absorb certain nutrients is adversely affected by other changes that often accompany aging. These changes include reduced production of HCl and intrinsic factor by the stomach and declining levels of active vitamin D.

Low HCl production reduces the absorption of iron and calcium and alters the numbers and types of bacteria that grow in the small intestine. As the bacteria change, the ability of the small intestine to absorb many nutrients declines. Individuals with marginal diets or severe HCl deficiencies are likely to develop iron or calcium deficiencies as well as other types of malnutrition. Individuals with very low intrinsic factor production, such as those with atrophic gastritis, and people with minimal vitamin B12 intake, are likely to have vitamin B12 deficiency and the resulting anemia.

Recall that vitamin D is produced in a series of steps and is finally activated by the kidneys. The amount of active vitamin D in the body usually decreases with age because of several factors. These factors include less absorption of dietary vitamin D by the small intestine; less exposure of the skin to sunlight; less vitamin D production by skin cells; less activation of vitamin D by the kidneys; and inadequate intake of vitamin D in the diet.

The dwindling levels of active vitamin D and a gradual decline in the ability of the small intestine to respond to vitamin D cause calcium absorption by the small intestine to decline. Older people with very low levels of vitamin D and those with a poor dietary intake of calcium are at high risk of developing calcium deficiency and osteoporosis.

Abnormal Changes

Peptic Ulcer

As was mentioned earlier, one abnormality in the small intestine is a duodenal peptic ulcer (Figure 10.3). Factors contributing to duodenal peptic ulcers include bacteria (H.pylori), emotional stress, excess caffeine consumption, and excess stomach acid production. The incidence of this condition does not change with age.

Figure 10.3 Peptic ulcers in the stomach and duodenum. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Unlike gastric peptic ulcers, the pain associated with duodenal peptic ulcers usually subsides after eating and intensifies when the stomach is empty. This pattern probably results because movement of acidic stomach contents into the small intestine is slowed when the stomach contains food. Other than the pain, the effects and complications of duodenal peptic ulcers are similar to those of gastric peptic ulcers.

Duodenal peptic ulcers can be prevented by avoiding the contributing factors. Treatment strategies are similar to those for gastric peptic ulcers, though the specific medications used and other details of the treatment may differ.

The functioning of the small intestine also is adversely affected by many abnormal changes and conditions that are more common among the elderly. Examples include poor diet; infections; poor circulation; diseases of the skin, stomach, liver, gallbladder, pancreas, or small intestine; hormone imbalances; medications; and surgery. When any of these factors lead to detrimental changes in the small intestine, serious malnutrition can result.