ANSC 301 - Digestion + Anatomy

• An alimentary canal is a tube beginning with the mouth and ending with the anus in which digestion and absorption of nutrients occur.
• The gastrointestinal tract is the region of the alimentary canal that runs from the stomach to the colon.
• The layers of the alimentary canal are the mucosa, submucosa, muscularis, and serosa.
• The mucosa is the interface with the lumen and is responsible for absorption and secretion.
• The submucosa consists of structural and immune cells.
• The muscularis contains muscle for the motility of the alimentary canal.
• The serosa is the outermost layer and is the interface with the body.
• The four pregastric functions are prehension, mastication, salivation, and deglutition.
• Prehension is the act of physically getting food into the mouth.
• Mastication is the process of chewing and physical reduction of feed, important for nonruminant herbivores.
• Salivation is important for moistening feed, lubrication, starch digestion via amylase, rumen pH buffering, and other specialized functions.
• The parotid gland produces serous/watery saliva containing water, enzymes, and ions.
• The sublingual gland produces mucous-type saliva containing mucus.
• The submaxillary gland produces mixed saliva containing mucus, ions, water, and enzymes.
• Deglutition is a reflex initiated by the presence of food in the pharynx where food is pushed to the stomach by esophageal peristalsis.
• The functions of gastric digestion are mixing, mechanical breakdown, hydrolytic digestion by acid enzymes, and reservoir for controlled release of digesta.
• The stomach is lined with nonglandular or secretory epithelial cells and functions for storage, mixing, and partial digestion.
• The stomach is divided into four sections based on the types of cells and glands present.
• The esophageal region is a non-glandular portion where no secretions are produced.
• The cardiac region is a secretory top portion where cells produce mucus for lubrication and protection.
• The fundic region is a secretory region where peptic glads are contained within gastric pits.
• Within the fundic region, chief cells produce pepsinogen and parietal cells secrete HCl.
• Pepsinogen is activated by reacting with HCl to create active pepsin for protein digestion.
• The pyloric region is a secretory region containing mucus-secreting cells and endocrine G cells that secrete gastrin.
• Gastrin is a hormone produced by the pyloric region that targets the fundic region to stimulate HCl production by the parietal cells.
• The three phases of gastric digestion are cephalic, gastric, and intestinal.
• The cephalic phase is triggered by a vagal nerve reflex causing increased gastric motility, enzyme production, and HCl secretion.
• The gastric phase is initiated by a local reflex triggered by food presence in the stomach and reinforced by gastrin, causing increased HCl and pepsinogen secretion.
• The intestinal phase is triggered by intestinal distention, low pH, and the presence of nutrients and reinforced by CCK and Secretin, causing decreased HCl secretion and gastric motility.
• Gastric motility aids in mixing, mechanical digestion, and hydrolytic reduction of feed into chyme.
• Gastric emptying is stimulated by distension of the antral wall near the pylorus and the presence of liquid chyme.
• Bile salts are made in the liver and stored in the gallbladder and function to lower the pH of chyme entering the duodenum.
• The pancreas secretes fluid containing enzymes to digest fats, proteins, and nucleic acids, and bicarbonate to neutralize gastric acid.
• The small intestine has three sections and is responsible for almost all of the digestion in the gastrointestinal tract and a large amount of absorption.
• The small intestine is made up of absorptive cells, mucus-secreting cells, endocrine cells, and stem cells.
• The duodenum, the first segment, is the site for many digestive juices including secretions from enterocytes, and pancreatic juice/bile salts entering from the common bile duct.
• The jejunum, the second segment, is the site of the most absorption and contains many folds, villi, and microvilli for increased absorptive surface area.
• The ileum, the third segment, is less absorptive but contains the gut-associated lymphoid tissue (GALT).
• GALT is the intestinal immune system responsible for the creation of peyer's patches that sample the GI tract and monitor for infection.
• The large intestine (hindgut) has three sections, the cecum, colon, and rectum.