The terminal ileum

The terminal ileum is the distal end of the small intestine that intersects with the large intestine. It contains the ileocecal sphincter, a smooth muscle sphincter that controls the flow of chyme into the large intestine.

Anatomy of the terminal ileum

The terminal ileum is located on the right side of the abdominopelvic cavity in the umbilical and hypogastric regions. It is a tube about 1.25 to 1.5 inches (3 to 4 cm) long at the end of the ileum and terminates at the ileocecal sphincter. The ileocecal sphincter is a band of smooth muscle that controls the flow of chyme from the ileum into the cecum of the large intestine.

The jejunum

The jejunum

The jejunum is the middle segment of the small intestine found between the duodenum and the ileum. Most of the nutrients present in food are absorbed by the jejunum before being passed on to the ileum for further absorption.

Anatomy of the jejunum

The jejunum is a continuation of the small intestine following the duodenum. It begins at the duodenojejunal flexure, where the small intestine turns sharply toward the anterior direction. From the duodenojejunal flexure, the jejunum follows a convoluted path through the abdomen before continuing as the ileum. While the jejunum does not have an anatomical landmark to separate it from the ileum, it slowly changes its anatomical structure along its length as it transitions into the ileum.

The duodenum

The duodenum

The duodenum is the first and shortest segment of the small intestine. It receives partially digested food (known as chyme) from the stomach and plays a vital role in the chemical digestion of chyme in preparation for absorption in the small intestine. Many chemical secretions from the pancreas, liver and gallbladder mix with the chyme in the duodenum to facilitate chemical digestion.

Located inferior to the stomach, the duodenum is a 10-12 inch (25-30 cm) long C-shaped, hollow tube. The duodenum is a part of the gastrointestinal (GI) tract, attached to the pyloric sphincter of the stomach on its superior end and to the jejunum of the small intestine on its inferior end. The pancreas, liver and gallbladder all deliver their digestive secretions into the duodenum through an orifice known as the ampulla of Vater, which is located roughly in the middle of the duodenum on the left side.

The small intestine

The small intestine is a long, highly convoluted tube in the digestive system that absorbs about 90% of the nutrients from the food we eat. It is given the name “small intestine” because it is only 1 inch in diameter, making it less than half the diameter of the large intestine. The small intestine is, however, about twice the length of the large intestine and usually measures about 10 feet in length.

The small intestine winds throughout the abdominal cavity inferior to the stomach. Its many folds help it to pack all 10 feet of its length into such a small body cavity. A thin membrane known as the mesentery extends from the posterior body wall of the abdominal cavity to surround the small intestine and anchor it in place. Blood vessels, nerves, and lymphatic vessels pass through the mesentery to support the tissues of the small intestine and transport nutrients from food in the intestines to the rest of the body.

The spleen

The spleen is a brown, flat, oval-shaped lymphatic organ that filters and stores blood to protect the body from infections and blood loss.

Protected by our ribs, the spleen is located between the stomach and the diaphragm in the left hypochondriac region of the abdominal body cavity. The splenic artery branches off from the aorta and the celiac trunk to deliver oxygenated blood to the spleen, while the splenic vein carries deoxygenated blood away from the spleen to the hepatic portal vein. A tough connective tissue capsule surrounds the soft inner tissue of the spleen.

Physiology of the pancreas

The exocrine portion of the pancreas plays a major role in the digestion of food. The stomach slowly releases partially digested food into the duodenum as a thick, acidic liquid called chyme. The acini of the pancreas secrete pancreatic juice to complete the digestion of chyme in the duodenum. Pancreatic juice is a mixture of water, salts, bicarbonate, and many different digestive enzymes. The bicarbonate ions present in pancreatic juice neutralize the acid in chyme to protect the intestinal wall and to create the proper environment for the functioning of pancreatic enzymes. The pancreatic enzymes each specialize in digesting specific compounds found in chyme.

- Pancreatic amylase breaks large polysaccharides like starches and glycogen into smaller sugars such as maltose, maltotriose, and glucose. Maltase secreted by the small intestine then breaks maltose into the monosaccharide glucose, which the intestines can directly absorb.
- Trypsin, chymotrypsin, and carboxypeptidase are protein-digesting enzymes that break proteins down into their amino acid subunits. These amino acids can then be absorbed by the intestines.
- Pancreatic lipase is a lipid-digesting enzyme that breaks large triglyceride molecules into fatty acids and monoglycerides. Bile released by the gallbladder emulsifies fats to increase the surface area of triglycerides that pancreatic lipase can react with. The fatty acids and monoglycerides produced by pancreatic lipase can be absorbed by the intestines.
- Ribonuclease and deoxyribonuclease are nucleases, or enzymes that digest nucleic acids. Ribonuclease breaks down molecules of RNA into the sugar ribose and the nitrogenous bases adenine, cytosine, guanine and uracil. Deoxyribonuclease digests DNA molecules into the sugar deoxyribose and the nitrogenous bases adenine, cytosine, guanine, and thymine.

The pancreas

The pancreas is a glandular organ in the upper abdomen, but really it serves as two glands in one: a digestive exocrine gland and a hormone-producing endocrine gland. Functioning as an exocrine gland, the pancreas excretes enzymes to break down the proteins, lipids, carbohydrates, and nucleic acids in food. Functioning as an endocrine gland, the pancreas secretes the hormones insulin and glucagon to control blood sugar levels throughout the day. Both of these diverse functions are vital to the body’s survival.

Physiology of the gallbladder

The gallbladder acts as a storage vessel for bile produced by the liver. Bile is produced by hepatocytes cells in the liver and passes through the bile ducts to the cystic duct. From the cystic duct, bile is pushed into the gallbladder by peristalsis (muscle contractions that occur in orderly waves). Bile is then slowly concentrated by absorption of water through the walls of the gallbladder. The gallbladder stores this concentrated bile until it is needed to digest the next meal.

Foods rich in proteins or fats are more difficult for the body to digest when compared to carbohydrate-rich foods (see Macronutrients). The walls of the duodenum contain sensory receptors that monitor the chemical makeup of chyme (partially digested food) that passes through the pyloric sphincter into the duodenum. When these cells detect proteins or fats, they respond by producing the hormone cholecystokinin (CCK). CCK enters the bloodstream and travels to the gallbladder where it stimulates the smooth muscle tissue in the walls of the gallbladder.

The gallbladder

The gallbladder is a small storage organ located inferior and posterior to the liver. Though small in size, the gallbladder plays an important role in our digestion of food. The gallbladder holds bile produced in the liver until it is needed for digesting fatty foods in the duodenum of the small intestine. Bile in the gallbladder may crystallize and form gallstones, which can become painful and potentially life threatening.

Physiology of the stomach

In the mouth, we chew and moisten solid food until it becomes a small mass known as a bolus. When we swallow each bolus, it then passes through the esophagus to the stomach where it is stored along with other boluses and liquids from the same meal.

The size of the stomach varies from person to person, but on average it can comfortably contain 1-2 liters of food and liquid during a meal. When stretched to its maximum capacity by a large meal or overeating, the stomach may hold up to 3-4 liters. Distention of the stomach to its maximum size makes digestion difficult, as the stomach cannot easily contract to mix food properly and leads to feelings of discomfort.