Human Digestive System

Intestinal villi are small, finger-like projections that protrude from the epithelial lining of the intestinal wall. Villi increase the internal surface area of the intestinal walls. Increased surface area allows for increased intestinal wall area that is available for absorption.

Bile is produced by liver and is stored in gall bladder.

Amylase is an enzyme that catalyzes the hydrolysis of starch into sugars. Amylase is present in the saliva of humans and some other mammals, where it begins the chemical process of digestion. Foods that contain large amounts of starch but little sugar, such as rice and potatoes, may acquire a slightly sweet taste as they are chewed because amylase degrades some of their starch into sugar.

It is an intestinal hormone that stimulates secretion of mucus from Brunner's gland.

Cholecystokinin is produced by duodenum in response to the presence of food. It causes contraction of gall bladder for releasing bile.

After being chewed and swallowed, the food enters the esophagus. It uses rhythmic, wave-like muscle movements (called peristalsis) to force food from the throat into the stomach. This muscle movement gives us the ability to eat or drink even when we are upside-down.

The striated duct is well developed in the submandibular gland. The epithelium of the striated duct participates in ion and water transport and secretes kallikrein. A segment of this duct is lined by cuboidal-to-columnar epithelial cells with basal infoldings containing numerous mitochondria.

Kupffer cell activation is responsible for early ethanol-induced liver injury, common in chronic alcoholics. Chronic alcoholism and liver injury deal with a two hit system. The second hit is characterised by an activation of the Toll-like receptor 4 (TLR4) and CD14, receptors on the Kupffer cell that internalise endotoxin (lipopolysaccharide or LPS).

Different phases of digestion take place including the cephalic phase, gastric phase and intestinal 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.

Water and lipids are absorbed by passive diffusion throughout the small intestine.

Peyer's patches are covered by specialszed epithelium (follicle-associated epithelium). Some of these epithelial cells have surfaces, which seem folded under the scanning electron microscope. These microfold, or M, cells sample and actively transport particulate antigens from the lumen into the ‘dome’ area, where priming of both T and B lymphocytes occurs.

Pancreatic amylase digests all the remaining starch in the duodenum. Amylase digests starch molecules from the ends of the chains in two-glucose units, forming the disaccharide maltose. Glycogen is also digested here.

Cathartic is a substance that accelerates defecation. This is in contrast to a laxative, which is a substance which eases defecation, usually by softening feces. It is possible for a substance to be both a laxative and a cathartic. However, agents such as psyllium seed husks increase the bulk of the feces.

Pepsin is a stomach enzyme that breaks down proteins into smaller peptides (peptones) during digestion.

It stores and concentrates bile, and then releases it into the duodenum to help absorb and digest fats.