Digestive System

Segmentation involves stationary, rhythmic contractions of the small intestine that mix chyme with digestive juices and bring it into contact with the mucosa for absorption. Peristalsis is the movement of contents along the tract.

Brush border enzymes, located on the microvilli of the small intestine, perform the final stage of digestion by breaking down disaccharides and peptides into absorbable monomers.

The small intestine consists of the duodenum, jejunum, and ileum, in that order. The ileum is the final segment before the large intestine.

Bile emulsifies fats, which increases their surface area for lipase enzymes to work, but it does not chemically digest fats itself. Chemical digestion is performed by enzymes like pancreatic lipase.

Cholecystokinin (CCK) is released in response to fat in the duodenum and triggers gallbladder contraction to release bile into the small intestine.

Secretin is released by the duodenum in response to acidic chyme and stimulates the pancreas to secrete bicarbonate-rich fluid to neutralize the acid.

Histamine acts as a paracrine signal in the stomach to stimulate parietal cells to increase the secretion of hydrochloric acid (HCl).

The enterogastric reflex occurs during the intestinal phase of gastric regulation, where the duodenum sends signals to inhibit gastric activity as chyme enters the intestine.

A zymogen is an inactive enzyme precursor. Procarboxypeptidase is the inactive form of carboxypeptidase, which is activated in the small intestine.

Salivary amylase functions optimally at a neutral pH (around 7), which is found in the mouth. Pepsin requires a very acidic environment, and lipases have varying optimal ranges.

Parietal cells secrete HCl, which provides the acidic environment necessary to convert ferric iron (Fe3+) to ferrous iron (Fe2+), the form that can be absorbed.

Parietal cells are the specific gastric cells responsible for the secretion of hydrochloric acid (HCl) into the stomach lumen.

Chief cells are located in the gastric glands and are responsible for secreting pepsinogen, the inactive precursor to the protease pepsin.