Mouth and Salivary Organs

Digestion starts at the mouth where ingestion and chewing occur to reduce food size. Saliva is secreted to enable taste sensation and foodstuff lubrication.

Digestion of carbohydrate begins, facilitated by salivary amylase enzyme. Carbohydrate is hydrolyzed from polysaccharides into disaccharides. Lysozyme, which acts as an antibiotic agent, is also secreted.

Deglutition turns foodstuff into bolus which enables it to move from the mouth into the stomach after passing the pharynx and esophagus.

Stomach

Receptive relaxation and contractions begins as the food bolus enters the lower portion of the esophagus. These are facilitated by the smooth muscle tissues. Peristalsis and segmentation motility enables the bolus to be moved, broken-down and mixed with hydrochloric acid (HCl). HCl, produced by the parietal cells of the gastric mucosa, turns bolus into chyme.

Carbohydrate digestion continues in the body of stomach where polysaccharides are hydrolyzed to disaccharides.

Protein digestion begins in the antrum. Pepsin is secreted by the stomach chief cells to hydrolyze protein into peptide fragments.

Only a few of lipid-soluble substances, such as alcohol and aspirin, are absorbed in the stomach.

The chyme is then continually pushed towards the pyloric sphincter by peristaltic waves and then injected into the duodenum every 20 seconds (Thibodeau & Patton, 2007).

Small Intestine

Further protein and carbohydrate digestion occurs here. In the lumen, carbohydrate and protein digestion continues and fat digestion is accomplished.

Carbohydrate and protein digestions are completed in the small-intestine brush border.

Trypsin, chymotrypsin, and carboxypeptidase are pancreatic digestive enzymes that attack different peptide fragments of protein in the small-intestine lumen.

Aminopeptidases are secreted by the small-intestine epithelial cells to hydrolyze peptide fragments to amino acids inside the small-intestine brush border.

Pancreatic amylase hydrolyzes starch, e.g. amylopectin, into glucose and maltose.

Disaccharidases include maltase, sucrase and lactase. They are secreted by the small-intestine epithelial cells to hydrolyze disaccharides to monosaccharides in the small-intestine lumen.

Bile salts, produced by the liver, facilitate fat digestion and absorption in duodenal lumen. Bile salts emulsify large fat globules to prepare further fat digestion by the pancreatic enzyme, lipase. Lipase hydrolyzes triglycerides to fatty acids and monoglycerides.

Small-intestinal motility includes both peristaltic contractions and segmentation. Segmentation in the duodenum and upper jejunum mixes the incoming chyme with digestive juices from the pancreas, liver and intestinal mucosa. This enables the absorption of nutrients, most electrolytes and water as the chyme comes in contact with the intestinal mucosa (Thibodeau & Patton, 2007).

Segmentation occurs between 9 to 12 times per minute (Sherwood, 2007).

Peristalsis moves the chyme nears the end of the jejunum, enabling movement of digested food into the large intestine.

Large Intestine

The function of the large intestine is to absorb salt and water and store feces. Because most digestion and absorption occurs in the small intestine, the contents delivered consist of indigestible food residues, unabsorbed biliary components and remaining fluid (Sherwood, 2007).

Sacs called haustra actively change locations which result in contraction of the circular smooth muscle layer. Haustral contraction is similar to the small-intestine contraction however occurs only at about every 30 minutes. Its back-and-forth mixing movement exposes the chyme to the absorptive mucosa (Sherwood, 2007).

Haustral contractions delay foodstuff transit and enhance absorption (Thomson & Shaffer, 2000)

Mass movements drive the colonic contents toward the sigmoid colon. During mass movements, the ascending and transverse colon contract simultaneously. It is triggered by the gastrocolic reflex which triggers defecation reflex when colonic contents is pushed into the rectum. Defecation reflex causes the internal anal sphincter to relax and the rectum and sigmoid colon to contract more vigorously. When the external anal sphincter relaxes, defecation occurs. Feces elimination is assisted by the reflex as well as voluntary straining which increases intra-abdominal pressure and helps feces expulsion through the anus canal (Sherwood, 2007).