For: anaerobic digestion For: precipitation (chemistry)#Digestion

Digestion is the breaking down of chemicals in the body, into a form that can be absorbed. It is also the process by which the body breaks down chemicals into smaller components that can be absorbed by the blood stream. In mammals, preparation for digestion begins with the cephalic phase in which saliva is produced in the mouth and digestive enzymes are produced in the stomach. Mechanical and chemical digestion begin in the mouth where food is chewed, and mixed with saliva to break down starches. The stomach continues to break food down mechanically and chemically through the churning of the stomach and mixing with enzymes. Absorption occurs in the stomach and gastrointestinal tract, and the process finishes with excretion.

Overview

In most vertebrates, digestion is a multi-stage process in the digestive system, following ingestion of the raw materials, most often other organisms. The process of ingestion usually inolves some type of mechanical and chemical processing. Digestion is separated into four separate steps:

1. Ingestion: placing food into the mouth
2. Mechanical and chemical breakdown: mastication and the mixing of the resulting food bolus with water, acids, bile and enzymes in the stomach and intestine to break down complex molecules into simple structures
3. Absorption: movement of nutrients from the digestive system to the circulatory and lymphatic capillaries through osmosis, active transport, and diffusion
4. Egestion: Removal of undigested materials from the digestive tract through defecation

Underlying the process is muscle movement throughout the system through swallowing and peristalsis.

Phases of gastric secretion

• Cephalic phase - This phase occurs before food enters the stomach and involves preparation of the body for eating and digestion. Sight and thought stimulate the cerebral cortex. Taste and smell stimulus is 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 - This phase takes 3 to 4 hours. It is stimulated by distention of the stomach, presence of food in stomach and increase in pH. Distention activates long and myentric 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 to around pH 6. Inhibition of gastrin and HCl secretion is lifted. This triggers G cells to release gastrin, which in turn stimulates parietal cells to secrete HCl. HCl release is also triggered by acetylcholine and histamine.
• Intestinal phase - This phase has 2 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.