29.3 Introduction to the Small and Large Intestines

Small and Large Intestines

The largest GI organ is the small intestine (Figure 29.6). The duodenum, jejunum, ileum, and ileocecal valve comprise the almost 20 feet of the small intestine. The primary functions of the small intestine are absorption and digestion; the small intestine is responsible for 90%–95% of nutrient absorption. The muscles of the small intestine further mix the chyme with digestive fluids for continued breakdown. The small intestine receives digestive juices from the liver, gallbladder, and pancreas. Bile acids from the liver and gallbladder enter the small intestine and break down fat for absorption in the small intestine. The pancreatic enzymes lipase, protease, and amylase travel through the pancreatic duct to the small intestine to facilitate digestion. Lipase, together with bile salts, breaks down fats and aids absorption of fat-soluble vitamins (A, D, E, K). Poor fat absorption may result in steatorrhea (fatty bowel movements). Protease breaks down protein for absorption. Amylase is released from the pancreas into the proximal small intestine to break down and absorb carbohydrates.

Figure 29.6 The three main regions of the small intestine are the duodenum, jejunum, and ileum. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Unique to the small intestine are tiny fingerlike projections called villi that line the walls of the small intestine. The villi protrude into the lumen to absorb water and nutrients into the circulatory system. The highly vascular villi line the entire small intestine to accomplish most of the digestion and absorption processes as peristalsis continues to move nutrients and digestive waste along the alimentary canal. The blood capillaries absorb carbohydrates and proteins. The lymph capillaries, also called lacteals, absorb fats. Disorders affecting the intestinal villi of the small intestine may result in malabsorption or nutritional disorders.

The large intestine, also known as the colon, receives waste products from the small intestine. Water is further absorbed from the waste products in the large intestine and forms stool. Peristaltic movements push stool toward the rectum. Under expected conditions, when waste products reach the rectum, most of the water has been reabsorbed into the body, leaving solid stool for evacuation from the body. The large intestine receives unabsorbed and undigested food material (waste products), absorbs water and electrolytes, and forms feces. The large intestine maintains over 500 species of bacteria that promote fermentation of indigestible material in the colon. The bacteria in the large intestine are responsible for absorbing vitamin K and B vitamins, including biotin (Thursby & Juge, 2017).

Irritable bowel syndrome (IBS) is a common disorder affecting the intestine. The symptoms of IBS range from mild to severe and vary from person to person. Symptoms may include bloating, gas, abdominal pain or discomfort, abdominal spasms, cramping, and constipation or diarrhea (or both). Because people experience varying types of symptoms, IBS is further categorized as IBS-C (constipation), IBS-D (diarrhea), IBS-M (mixed symptoms of diarrhea and constipation), and IBS-U (unsubtyped, with varying inconsistent symptoms). Although IBS is a chronic condition, most people with IBS can manage their disease with lifestyle changes such as consistently eating healthy, balanced meals; staying hydrated; and reducing stress.

Table 29.2 lists common GI conditions, associated general manifestations, and drugs commonly used to treat these conditions. Table 29.3 presents common actions of digestive enzymes.