Hormonal Controls

Hormones are secreted into the blood; this is represented in the map by drawing arrows outside the symbolic GI tract. Hormones play an important role regulating gastric motility. They also control the secretion of gastric and pancreatic juice plus other secretory activities.

Regulation of Gastric Motility

The hormone gastrin is produced by cells lining gastric glands in the lower region of the stomach (labeled hexagon). Secretion is stimulated (solid arrow) by the presence of peptides and amino acids in the stomach. Caffeine is also a stimulant (solid arrow). Gastrin is carried by the blood stream to the smooth muscle of the stomach resulting in increased (solid arrow ) motility. Gastrin secretion is inhibited (dashed arrow) by increasing stomach acidity (H+).

The intestinal hormone cholecystokinin is secreted into the blood when peptides, fatty acids & monoglycerides are present. It inhibits gastric motility indirectly by inhibiting (dashed arrow) the cells that produce gastrin thus reducing motility.

Regulation of Exocrine Secretions

Gastric Juice

Chief cells--also called zymogenic cells--secrete pepsinogen into the stomach. Secretions are represented by solid-headed dashed arrows. (The secretions themselves will be discussed in a later tutorial.) Parietal cells secrete hydrochloric acid here represented by the active component, hydrogen ions, H+.

Peptides & amino acids and caffeine stimulate (solid arrows) the release of gastrin into the blood; excess acidity, H+ inhibits it (dashed arrow). This hormone directly stimulates the chief cells (solid arrow) while its affect on the parietal cells is indirect. Cells that release histamine into their surroundings (labeled hexagon) are found near parietal cells. These cells are targeted by gastrin (solid arrow) thus causing the release of histamine. This 'local hormone' diffuses (solid arrow drawn inside stomach) to target nearby parietal cells making them more responsive to neural stimuli.

When food passes to the small intestine, fatty acids and hydrogen ions,H+ cause secretin to be released into the blood stream . This hormone inhibits (dashed arrows) both the chief and parietal cells.

Secretions into the Small Intestine

The hormone secretin (labeled hexagon)--the first hormone discovered, 1902-- is released into the blood stream when fatty acids and hydrogen ions, H+ enter the small intestine. This hormone stimulates (solid arrow) Brunner's glands, in the first section of the small intestine, to secrete (solid-headed dashed arrow) mucus into the intestinal tract. This mucus coats the intestinal walls to protect them from the increasing acidity.

Secretin was named because it stimulates (solid arrows) the duct cells in the pancreas and the liver to 'secrete' sodium bicarbonate. The bicarbonate ion reduces the acidity in the small intestine by combining with hydrogen ions.

Cholecystokinin release is stimulated (solid arrows) by the presence of peptides, fatty acids & monoglycerides in the small intestine. It relaxes (dashed arrow) the sphincter muscle (donut) thus opening the ducts leading from the gallbladder and pancreas. It stimulates (solid arrow) the exocrine regions of the pancreas to produce pancreatic juice and the pancreatic ducts to secrete sodium bicarbonate. It was these activities that earned this enzyme its initial name, 'pancreozymin.'

CCK also stimulates (solid arrow) the smooth muscle of the gallbladder to contract thus releasing bile into the small intestine.

Last Updated: 7/19/2005