Digestion

Digestion is the process by which the body of a living organism breaks food down into very small parts that cells can use. Food consists of large molecules — carbohydrates (sugars and starches), proteins, and fats — that cannot pass directly through the lining of the digestive tract. Digestion converts these into smaller parts such as simple sugars, amino acids, and fatty acids.

In humans, digestion occurs along a long pathway beginning in the mouth and ending at the rectum. Along this pathway are organs that assist in breaking down food, including the stomach, the gall bladder, and the pancreas, which produce secretions that aid in digestion.

Types: mechanical and chemical

Digestion occurs in two forms. Mechanical digestion involves physical breakdown: teeth chew food, stomach muscles churn it, and the wavelike movement of the digestive tract known as peristalsis propels food forward. Chemical digestion is carried out by enzymes — proteins that speed up the breakdown process. For example, amylase in saliva begins the breakdown of starches, pepsin in the stomach acts on proteins, and lipase from the pancreas breaks down fats.

Stages of the pathway

Digestion begins in the mouth: teeth break food apart, and saliva starts to break down starches. Food then passes down the esophagus into the stomach, where stomach acid and pepsin break down proteins. It then enters the small intestine, which is the primary site of nutrient absorption. There it meets secretions from the pancreas and bile from the gall bladder, which breaks down fats. Absorption occurs through tiny projections called villi, which greatly increase the surface area for nutrient uptake.

The large intestine and bacteria

Material that remains after the small intestine passes into the large intestine. Its main functions are to absorb water and electrolytes and to form stool. The large intestine contains a large number of bacteria (microbiota), some of which ferment fiber that the body cannot break down, producing short-chain fatty acids and certain vitamins.

Regulation

Digestion is controlled by nerves and hormones. There is a nerve network specific to the digestive tract (the enteric nervous system) that regulates movement in a coordinated manner. Hormones such as gastrin, secretin, and cholecystokinin increase or decrease the timing and quantity of digestive secretions depending on the type of food in the pathway.