Anatomy of the Digestive System

Oral Cavity

• Consists of lips, tongue, teeth, salivary glands (mandibular, parotid, sublingual), hard palate, soft palate, and oropharynx.

Occlusal Surface

• The area where teeth touch.

Esophagus

• Has a cardia, a sphincter between esophagus and stomach, which is strong in horses and rabbits to prevent vomiting.

Stomach

• Divided into cardia, fundus, body, pyloric antrum, and pylorus.
• Fundus is a distensible pouch with gastric glands (parietal, chief, and mucous cells).
• Gastric pits are lined by mucous cells.
• Parietal cells secrete hydrochloric acid.
• Chief cells secrete pepsinogen and chymosin.
• Mucous cells secrete mucus.
• Pyloric antrum contains G cells that secrete gastrin.
• Pylorus is a sphincter regulating chyme movement to the duodenum and preventing backflow.
• Food mixing occurs through fundus and body relaxation, followed by body contraction to create chyme.
• Stomach movements include propulsion, grinding, and retropulsion.
• Stomach layers consist of mucosa, submucosa, muscular (longitudinal and circular), and outer serosa.
• Parasympathetic stimulation causes fundus relaxation and antrum contraction increases.
• Sympathetic stimulation decreases motility.

Ruminant Digestive System

• Reticulum has a honeycomb texture.
• Rumen has papillae that absorb volatile fatty acids.
• Omasum has lamina that absorbs volatile fatty acids.
• Abomasum in newborn calves produces renin to clot milk and slow transit.

Small Intestine

• Divided into duodenum, jejunum, and ileum.
• Layers consist of mucosa, submucosa, muscular, and serosa.
• Mucosa contains villi with microvilli and crypts that produce new cells and mucus (Goblet cells).
• Crypts of Lieberkühn are found in the duodenum.

Digestion and Absorption

• Peristalsis involves circular muscle contractions pushing food, controlled by the myoenteric plexus.
• Segmental contraction mixes contents with enzymes.
• Absorption involves water, electrolytes, and vitamins absorbed intact.
• Starch is converted to disaccharides by pancreatic amylase and further broken into monosaccharides by microvilli enzymes.
• Protein digestion involves:
• Pepsin partially breaking into polypeptides.
• Pancreatic proteases (trypsin, chymotrypsin, elastase, aminopeptidase, carboxypeptidase) digesting polypeptides.
• Microvilli proteases breaking down peptides into amino acids/dipeptides for absorption.
• Fat digestion involves:
• Bile acids coating fat droplets.
• Pancreatic lipase converting to glycerol, fatty acids, and monoglycerides.
• Micelles transporting to the lymphatic system via chylomicrons.

Gastrointestinal System

• Ileocecal sphincter separates the ileum from the cecum.