Digestive System Chapter

Digestive System Overview

• The digestive system consists of four major organs: esophagus, stomach, small intestine, and large intestine.
• The organs are separated by muscular valves or sphincters, and the digestive tract is lined with a mucosal layer.
• Digestion of carbohydrates begins in the oral cavity through amylase, and the majority of digestion and absorption of carbohydrates and amino acids occurs in the stomach and small intestine.
• Secretions from the exocrine pancreas and liver pass directly into the small intestine.
• Lymphatic vessels are distributed throughout the tract and are involved in intestinal absorption of dietary fat.

Histological Organization of the Digestive Tract

• The digestive tract, except for the oral cavity, has four concentric layers of muscle: mucosa, submucosa, muscularis, and adventitia of the serosa.
• The mucosa consists of superficial epithelium, underlying stroma with vascularized loose connective tissue rich in immunocompetent cells, and a relatively thin layer of smooth muscle (muscularis mucosae).
• The muscularis mucosae is subdivided into an inner circular and outer longitudinal layer, and contractions of this layer throw the mucosa into folds and ridges.
• The submucosa contains large blood vessels, lymphatic vessels, and nerves that send fibers into the mucosa and muscularis layers.
• The muscularis externa consists of two layers of smooth muscle, except in the upper esophagus and anal sphincter, where striated muscle fibers exist.

Digestion and Absorption

• Monosaccharide transport occurs through a Na+-sugar coupled uptake mechanism at the mucosal interface, with energy provided by the Na+/K+-ATPase pump.
• Amino acid transport occurs through specific mucosal and basolateral carriers, with mucosal uptake via co-transport depending on the Na+ gradient.
• Di-, tri-, and tetra-peptide transport occurs through a H+/oligopeptide cotransporter referred to as PepT1.
• The basolateral entry mechanisms are primarily Na+-independent facilitated transport proteins and provide a nutrient supply for epithelial renewal.

Salivary, Gastric, and Pancreatic Secretions

• Salivary glands are compound organs that secrete electrolytes and proteins (e.g., amylase) as a fluid into the oral cavity.
• Saliva lubricates food for swallowing, and glycoproteins secreted from submandibular, sublingual, and buccal glands aid in starch digestion.
• The salivary glandular epithelium is comprised of specialized groups of cells called acinar cells, arranged as endpieces surrounding small central lumina.
• Parasympathetic stimulation mediated via the chorda lingual nerve evokes a marked fluid secretion accompanied by increased blood flow and oxygen consumption.

Gastric Secretion

• The stomach is an exocrine organ that secretes a large acid volume after a meal, containing pepsin, which initiates protein digestion.
• The gastric surface is protected by a thin film of mucous constantly produced by surface epithelial cells.
• Intrinsic factor, a glycoprotein, is also secreted by the stomach and combines with vitamin B12, aiding absorption in the ileum.
• The stomach layers consist of a thin layer of smooth muscle (muscularis mucosa), a dense connective tissue (submucosa), and three primary layers of muscle (muscularis).
• Parietal or oxyntic cells secrete 0.1 N HCl, and chief cells secrete protein in a manner similar to salivary/pancreatic acinar cells.
• Gastrin released by G cells of the antral mucosa and first part of the duodenum stimulates the parietal cells to secrete HCl.