Ch 13 Digestion

G Cells and Gastric Secretion

• G cells secrete a hormone called gastrin, which stimulates parietal cells and overall gastric secretion.

Chemical Digestion and Absorption in the Stomach

• Carbohydrate digestion is continued with gastric amylase, resulting in disaccharides.
• Protein digestion begins with pepsin (activation of pepsinogen by HCl), resulting in peptides.
• Lipid digestion begins with gastric lipases, which can only break down certain lipids such as butterfat, resulting in fatty acids.
• Absorption in the stomach is limited, where only small and fat-soluble substances can be absorbed (water, alcohol, aspirin, and certain drugs).
• The result of all these mixing, chemical digestion, secretion, and absorption is a yellowish paste called chyme, which will be passed on to the small intestine.

Regulation of Gastric Secretion

• Regulation of gastric secretion and activities is by both nervous and hormonal mechanisms.
• Food moving along the oral cavity and esophagus stimulates the parasympathetic nerves to activate the secretion in gastric glands.
• Gastrin hormone from G cells stimulates the gastric glands for more activities ("positive feedback").
• When food is emptying from the stomach, sympathetic nerves inhibit the gastric glands and gastrin, and a hormone called intestinal gastrin (released by small intestine) inhibits other gastric activities.

Phases of Gastric Regulation

• Cephalic Phase: involves special senses detecting food and using parasympathetic nerves in the vagus nerve to stimulate gastric activities.
• Gastric Phase: involves the distension of the stomach and stimulates its own activities by the vagus nerve.
• Intestinal Phase: involves acidic chyme passing into the small intestine, which secretes intestinal gastrin hormone to inhibit gastric activities.

Pancreas

• Pancreas produces most pancreatic enzymes as inactivate molecules, or zymogens, to minimize self-digestion.
• More than 98% of the pancreas mass is devoted to its exocrine function: the secretion of pancreatic juice by the pancreatic acini and their ductile cells.
• Ductile cells produce sodium bicarbonate, which helps neutralize the acidic gastric contents.
• Acinar cells of the exocrine pancreas produce a variety of digestive enzymes to break down food substances into smaller absorbable molecules.

Pancreatic Enzymes

• Pancreatic amylase: digests polysaccharides into disaccharides.
• Pancreatic lipases: digest triglycerides into fatty acids.
• Pancreatic nucleases: digest nucleic acids into nucleotides.
• Pancreatic proteinases: digest peptides into amino acids, including trypsin, chymotrypsin, aminopeptidase, and carboxypeptidase.

Pancreatic Secretion

• The parasympathetic nervous system increases pancreatic secretion.
• Sight, smell, and taste of food cause stimulation of vagus nuclei in the brain, which stimulates acid secretion.
• Vagus stimulates acid secretion directly and indirectly through gastrin secretion.

Regulation of Gastric Emptying

• Distension of the stomach and small intestine stimulates the vagus nerve, which inhibits gastric activity.
• Amino acids and peptides in the stomach lumen stimulate acid secretion.
• Intestinal gastrin hormone inhibits gastric activity.
• Secretin, a hormone secreted by the duodenum, inhibits gastric acid secretion in response to fat in chyme.
• The enterogastric reflex: a neural reflex that inhibits gastric activity when the duodenum is filled with chyme.