Digestive System


Image Caption : Digestive System of a Male : 3D visualization reconstructed from scanned human data of a lateral oblique view of the male digestive system. An envelope of transparent skin surrounds the digestive organs in the foreground; musculature is cut away in foreground to reveal the organs and their relationship to the skeleton. The digestive system is comprised of an alimentary canal and accessory organs; together they break down complex food stuffs into the simple structures the body can use, absorb the nutrients into the blood stream, and eliminate the leftover waste.

REGIONS OF THE DIGESTIVE SYSTEM

At its simplest, the digestive system is a tube running from mouth to anus. Its chief goal is to break down huge macromolecules (proteins, fats and starch), which cannot be absorbed intact, into smaller molecules (amino acids, fatty acids and glucose) that can be absorbed across the wall of the tube, and into the circulatory system for dissemination throughout the body.

Illustration mapping the locations of the various organs that make up the digestive system

Regions of the digestive system can be divided into two main parts: the alimentary tract and accessory organs. The alimentary tract of the digestive system is composed of the mouth, pharynx, esophagus, stomach, small and large intestines, rectum and anus. Associated with the alimentary tract are the following accessory organs: salivary glands, liver, gallbladder, and pancreas.

GENERAL STRUCTURE OF THE DIGESTIVE SYSTEM

The long continuous tube that is the digestive tract is about 9 meters in length. It opens to the outside at both ends, through the mouth at one end and through the anus at the other. Although there are variations in each region, the basic structure of the wall is the same throughout the entire length of the tube.

The wall of the digestive tract has four layers or tunics:

  • Mucosa
  • Submucosa
  • Muscular layer
  • Serous layer or serosa

The mucosa, or mucous membrane layer, is the innermost tunic of the wall. It lines the lumen of the digestive tract. The mucosa consists of epithelium, an underlying loose connective tissue layer called lamina propria, and a thin layer of smooth muscle called the muscularis mucosa. In certain regions, the mucosa develops folds that increase the surface area. Certain cells in the mucosa secrete mucus, digestive enzymes, and hormones. Ducts from other glands pass through the mucosa to the lumen. In the mouth and anus, where thickness for protection against abrasion is needed, the epithelium is stratified squamous tissue. The stomach and intestines have a thin simple columnar epithelial layer for secretion and absorption.

The submucosa is a thick layer of loose connective tissue that surrounds the mucosa. This layer also contains blood vessels, lymphatic vessels, and nerves. Glands may be embedded in this layer.

The smooth muscle responsible for movements of the digestive tract is arranged in two layers, an inner circular layer and an outer longitudinal layer. The myenteric plexus is between the two muscle layers.

Above the diaphragm, the outermost layer of the digestive tract is a connective tissue called adventitia. Below the diaphragm, it is called serosa.

National Cancer Institute / NIH

The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). In this system, the process of digestion has many stages, the first of which starts in the mouth (oral cavity). Digestion involves the breakdown of food into smaller and smaller components, until they can be absorbed and assimilated into the body. The secretion of saliva helps to produce a bolus which can be swallowed to pass down the esophagus and into the stomach.

Saliva also contains a catalytic enzyme called amylase which starts to act on food in the mouth. Another digestive enzyme called lingual lipase is secreted by some of the lingual papillae on the tongue and also from serous glands in the main salivary glands. Digestion is helped by the mastication of food by the teeth and also by the muscular actions of peristalsis and segmentation contractions. Gastric juice in the stomach is essential for the continuation of digestion as is the production of mucus in the stomach.

Peristalsis is the rhythmic contraction of muscles that begins in the esophagus and continues along the wall of the stomach and the rest of the gastrointestinal tract. This initially results in the production of chyme which when fully broken down in the small intestine is absorbed as chyle into the lymphatic system. Most of the digestion of food takes place in the small intestine. Water and some minerals are reabsorbed back into the blood in the colon of the large intestine. The waste products of digestion (feces) are defecated from the anus via the rectum.



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