Small Intestine

Small Intestine

Extending from the pyloric sphincter to the large intestine, the small intestine is a tubular organ with many loops and coils filling much of the abdominal cavity. It receives secretions from the liver and ­pancreas, completing digestion of the nutrients in chyme. It also absorbs the products of digestion and transports the remaining residues to the large intes-tine. The small intestine is the longest portion of the alimentary canal, yet its diameter is only about half that of the large intestine. It is approximately 6–7 m or about 20 feet in length but only about 2.5–4 cm or 1–1.6 inches in diameter. Because of loss of muscle tone, the small intestine is smaller in a cadaver, only 2–4 m or 7–13 feet.

The small intestine plays the major role in the diges-tion and absorption of nutrients, with 90% of nutrient absorption occurring there. Most of the remaining absorption of nutrients occurs in the large intestine. The small intestine consists of three portions: duodenum, jejunum, and ileum (FIGURE 24-10). The duodenum is mostly retroperitoneal, whereas the other portions are intraperitoneal. The lining of the small intestine has approximately 800 transverse folds known as plicae circulares. They do not disappear when the small intes-tine is filled, unlike the rugae of the stomach. The plicae circulares increase the absorptive surface area of the small intestine.

The duodenum is about 25 cm or 10 inches long and 5 cm or 2 inches in diameter and relatively immovable. It is located posterior to the parietal peritoneum, following a C-shaped path by passing anterior to the right kidney and upper three lumbar vertebrae. The duodenum is curved around the head of the pancreas. It functions as a mixing area, receiv-ing chyme from the stomach and digestive secre-tions from the pancreas and liver. The duodenum is the shortest portion of the small intestine yet has the most interesting functions. The bile duct delivers bile from the liver. The main pancreatic duct brings pancreatic juice from the pancreas. These ducts unite in the duodenal wall at the hepatopancreatic ampulla, which resembles a bulb and opens into the duodenum through the major duodenal papilla. The hepatopancreatic sphincter is a smooth muscle valve that controls entry of the substances from the liver and pancreas. The remainder of the small intestine is more mobile, lying free inside the peritoneal cavity.

The jejunum is the proximal two-fifths of the small intestine and is about 2.5 m or 8 feet in length and about 4 cm or about 1.5 inches in diameter. It links the duodenum with the ileum. Most chemical diges-tion and nutrient absorption occur in the jejunum. The jejunum, along with the next segment, appears like coiled sausage in the central, lower abdominal cavity.

The final segment of the small intestine is called the ileum, which is not distinctly separate from the jejunum but has a smaller diameter, with a thinner, less vascular, and less active wall. It is the longest portion of the small intestine, totaling about 3.6 m or 12 feet in length. The ileum ends at the ileocecal valve, which controls the flow of substances from the ileum into the large intestine.

The mesentery is a double-layered fold of peritoneal­ membrane that suspends the jejunum and ileum from the posterior abdominal wall (­FIGURE 24-11). The mesentery supports the blood vessels, lymphatic vessels, and nerves that supply the intestinal wall. A double fold of peritoneal mem-brane called the greater omentum drapes from the stomach over the transverse colon and folds of the small intestine. If the alimentary canal walls become infected, the greater omentum’s cells may adhere to the inflamed area, helping to prevent the infection from spreading to the peritoneal cavity.

The small intestine is served by parasympathetic nerve fibers from the vagus nerve and sympathetic nerve fibers from the thoracic splanchnic nerves. Both types are relayed via the superior mesenteric and celiac plexuses. Arterial blood is supplied mostly from the superior mesenteric artery. The similarly routed veins mostly drain into the superior mesenteric vein. Nutrient-rich blood from the small intestine drains through these veins to the hepatic portal vein, and is then carried to the liver.

Microscopic Structures of the Small Intestine

The length of the small intestine provides a large sur-face area for absorbing nutrients. This is increased by more than 600 times by the presence of its circular folds, villi, and microvilli. These specialized structures are more numerous near the distal small intestine, so it is here where most absorption occurs. The total intestinal surface area of the small intestine is approx-imately 200 square meters.

The small intestine’s circular folds (plicae circu-laris) of the mucosa and submucosa are deep and do not change. Each fold is about 1 cm or about 4/10 inch in height and functions to force chyme into a spiral-ing movement through the lumen of the small intes-tine. This slows down its movement, so full nutrient absorption has time to occur.

The inner wall of the small intestine has many tiny finger-like projections of the mucous membrane called intestinal villi (FIGURE 24-12). The villi are slightly more than 1 mm or about 4/100 inch in length and give the inner wall a soft texture similar to vel-vet fabric. They are most numerous in the duodenum and proximal jejunum, decreasing in size along the way from the duodenum to the ileum. The villi help to contract the intestinal contents and increase the sur-face area of the intestinal lining to aid in absorption of digestive products. In the duodenum, the villi appear like leaves.

Each villus has a layer of absorptive, simple columnar epithelium consisting of enterocytes, a core of connective tissue with capillaries, a lymphatic cap-illary called a lacteal, and nerve fibers. The cores have dense capillary beds. The lacteals are actually wide lymphatic capillaries. Nutrients are carried away by blood capillaries and lacteals. Extremely long and densely packed microvilli are found in the absorptive mucosal cells. Therefore, the mucosal surface appears “fuzzy” and is referred to as the brush border.

In the small intestine, the major mixing move-ment is segmentation, meaning small, ring-like con-tractions occur periodically to cut the chyme into segments and move it back and forth. This slows its movement through the small intestine. These weak peristaltic waves result in chyme taking between three and 10 hours to move through the small intestine.

Overdistention or irritation of the small intestine results in a peristaltic rush, which moves the small intestine’s contents into the large intestine quickly. Normal absorption of nutrients, water, and electro-lytes, therefore, does not occur in this situation. Peri-staltic rush results in diarrhea, and prolonged diarrhea results in water and electrolyte imbalances.

The small intestine connects to the large intes-tine via a valve called the ileocecal sphincter. This sphincter is usually constricted to keep the contents of each intestine separate. After eating, peristalsis in the ileum relaxes the sphincter to force some small intes-tine contents into the cecum of the large intestine.

Intestinal Glands

Intestinal glands are located between the bases of adjacent villi and extend downward into the mucous membrane. There are five types of cells in the mucosal epithelium:

■■ Enterocytes: These make up most of the ­epithelium, and are simple, columnar absorptive cells. The enterocytes are bound with tight junctions and have many microvilli. They are mostly responsible for absorption of nutrients and electrolytes within the villi. Inside the crypts, they are the main cells that secrete intestinal juice, which is watery and contains mucus. This juice carries absorbing nutrients from the chyme.

■■ Goblet cells: Which secrete mucus, exist through out the mucosa of the small intestine, in its villiand crypts. Specialized mucous-secreting glands in the proximal duodenum secrete thick, alkaline mucus when specifically stimulated to do so. At the bases of the villi, intestinal glands secrete high volumes of a watery fluid with a nearly ­neutral pH. This fluid lacks digestive enzymes. The enzymes in the luminal surfaces of the intestinal mucosa break down food just before absorption. These enzymes include pep-tidases, sucrose, maltase, lactase, and intestinal lipase. TABLE 24-4 summarizes these enzymes, their sources, and their actions. When stimu-lated by chyme, goblet cells and intestinal glands secrete their products. As the intestinal wall dis-tends, it activates nerve plexuses, stimulating parasympathetic reflexes that trigger release of small intestine secretions.

■■ Enteroendocrine cells: The source of enterogas-trones such as secretin and cholecystokinin. They are mostly presented throughout the crypts, but are found in lesser numbers in the villi.

■■ Paneth cells: Existing deep inside the crypts, these specialized secretory cells strengthen the defenses of the small intestine by releasing defensins, ­lysozyme, and other antimicrobial agents. Their secretions destroy various bacteria and aid in the determination of which bacteria form colonies within the intestinal lumen.

■■ Stem cells: Continuously dividing deep in the crypts, their daughter cells differentiate into all other types of cells. Most daughter cells, except for Paneth cells, differentiate while migrating up the villi. The Paneth cells migrate to the bottoms of the crypts. At the tips of the villi, epithelial cells undergo apoptosis. They are shed, and the villus epithelium is renewed every three to five days.

Intestinal Juice

Nearly 1–2 liters or about 1–2 quarts of intestinal juice are secreted by the intestinal glands every day. Hyper-tonic or acidic chyme is the major causative factor for this secretion because it causes the intestinal mucosa to be distended or irritated. In normal conditions, the intestinal juice is isotonic with the blood plasma and slightly alkaline, between 7.4 and 7.8 pH. Although mostly made of water, it also contains mucus secreted by the duodenal glands and mucosal goblet cells. Intestinal juice does not have high enzyme content because intestinal enzymes are mostly found at the brush border.

1. Explain the actions of the circular folds, villi, microvilli, and brush border.

2. What is the importance of the mesenteries?

3. Describe the three portions of the small intestine.

4. What are the five types of cells in the mucosal epithelium of the small intestine?