Phases of Gastric Secretion Gastric secretion occur in 3 “phases”: Cephalic phase Gastric phase Intestinal phase

Cephalic Phase occurs before food enters the stomach, especially while it is being eaten. It results from the sight, smell, thought, or taste of food signals originate in the cerebral cortex and in the appetite centers of the amygdala and hypothalamus, then through the vagus nerves to the stomach. Accounts for about 30 % of the gastric secretion

Gastric Phase Once food enters the stomach, it excites long vagovagal reflexes from the stomach to the brain and back to the stomach local enteric reflexes gastrin secrertion accounts for about 60 % of the total gastric secretion associated with eating a meal

Intestinal Phase The presence of food in the duodenum will continue to cause stomach secretion, probably because of small amounts of gastrin released by the duodenal mucosa. accounts for about 10 % of the acid response to a meal.

Pancreatic Secretion The pancreatic digestive enzymes are secreted by pancreatic acini & large volumes of sodium bicarbonate solution are secreted by the small ductules and larger ducts leading from the acini. The combined product (enzymes +sodium bicarbonate) flows through a long pancreatic duct that joins the common bile duct.

The pancreatic enzymes for digesting proteins are trypsin, chymotrypsin & carboxypolypeptidase. Trypsin and chymotrypsin split whole and partially digested proteins into peptides of various sizes but do not cause release of individual amino acids. Carboxypolypeptidase splits some peptides into individual amino acids, thus completing digestion of some proteins all the way to the amino acid state.

For digesting carbohydrates there is pancreatic amylase, which hydrolyzes starches, glycogen, and most other carbohydrates to form disaccharides and a few trisaccharides. The main enzymes for fat digestion are pancreatic lipase, which is capable of hydrolyzing neutral fat into fatty acids and monoglycerides cholesterol esterase, which causes hydrolysis of cholesterol esters phospholipase, which splits fatty acids from phospholipids.

Secretion of Bile by the Liver One of the many functions of the liver is to secrete bile, Bile serves two important functions A- bile plays an important role in fat digestion and absorption, because bile acids in the bile do two things: They help to emulsify the large fat particles of the food into many minute particles, which can then be attacked by lipase enzymes secreted in pancreatic juice aid in absorption of the digested fat end products through the intestinal mucosal membrane. B- bile serves as a means for excretion of several important waste products from the blood. These include especially bilirubin, an end product of hemoglobin destruction, and excesses of cholesterol.

Gastrointestinal Disorders Disorders of the Stomach Gastritis- inflammation of the lining of the stomach. Common causes include infection with Helicobacter pylori and use of nonsteroidal anti-inflammatory drugs (NSAIDs).

Pernicious anemia results from vitamin B12 deficiency due to lack of intrinsic factor (IF) Intrinsic factor secreted by the same parietal cells that secrete HCL. is essential for absorption of vitamin B12 from the ileum. Vitamin B12 is necessary for RBCs maturation.

Peptic ulcer It is an injury in the inner lining of the stomach or intestinal mucosa caused by the digestive action of gastric juice or upper small intestinal secretions.

Vomiting is the means by which the upper GI tract rids itself of its contents when any part of the upper tract becomes excessively irritated, overdistended, or even overexcitable. The sensory signals that initiate vomiting originate mainly from the pharynx, esophagus, stomach, and upper portions of the small intestines. nerve impulses are transmitted, By afferent nerve fibers to vomiting center” in the brain stem . From there, motor impulses are transmitted from the vomiting center by way of the 5th, 7th, 9th, 10th, and 12th cranial nerves to the upper GIT, through vagal and sympathetic nerves to the lower tract, to the diaphragm and abdominal muscles.

Reference: Gyton and Hall text book of medical physiology.