Digest

Digestion is the process by which food and drink are broken down into their smallest parts so the body can use them to build and nourish cells and to provide energy.

Overview

Digestion involves the mixing of food, its movement through the digestive tract, and the chemical breakdown of the large molecules of food into smaller molecules. Digestion begins in the mouth, when we chew and swallow, and is completed in the small intestine.

Process of Digestion

The process of digestion has three stages: the cephalic phase, the gastric phase, and the intestinal phase.

Cephalic Phase

The cephalic phase occurs at the sight, thought and smell of food, which stimulate the cerebral cortex. Taste and smell stimuli are sent to the hypothalamus and medulla oblongata. After this it is routed through the vagus nerve and release of acetylcholine. Gastric secretion at this phase rises to 40% of maximum rate. Acidity in the stomach is not buffered by food at this point and thus acts to inhibit parietal (secretes acid) and G cell (secretes gastrin) activity via D cell secretion of somatostatin.

Gastric Phase

The gastric phase takes 3 to 4 hours. It is stimulated by distension of the stomach, presence of food in stomach and decrease in pH. Distention activates long and myenteric reflexes. This activates the release of acetylcholine, which stimulates the release of more gastric juices. As protein enters the stomach, it binds to hydrogen ions, which raises the pH of the stomach. Inhibition of gastrin and gastric acid secretion is lifted. This triggers G cells to release gastrin, which in turn stimulates parietal cells to secrete gastric acid. Gastric acid is about 0.5% hydrochloric acid (HCl), which lowers the pH of the stomach to about 2. Gastric acid also denatures proteins.

Intestinal Phase

The intestinal phase has two parts, the excitatory and the inhibitory. Partially digested food fills the duodenum. This triggers intestinal gastrin to be released. Enterogastric reflex inhibits vagal nuclei, activating sympathetic fibers causing the pyloric sphincter to tighten to prevent more food from entering, and inhibits local reflexes.

Role of Digestive Organs

Digestion involves several organs, each playing a specific role in the process. These include the mouth, esophagus, stomach, small intestine, large intestine, and rectum.

Disorders of Digestion

There are many disorders of digestion that require clinical care by a physician or other healthcare professional. These include gastroesophageal reflux disease (GERD), gallstones, celiac disease, Crohn's disease, ulcerative colitis, irritable bowel syndrome, hemorrhoids, diverticulosis, and anal fissure.

See Also

• Human digestive system
• Digestive enzyme
• Gastroenterology