Digestion occurs in the gastrointestinal system, which is composed of the gastrointestinal tract (GI tract) and the associated accessory organs. The process of digestion occurs through six steps, namely ingestion, secretion, mixing, digestion, absorption and defecation. (Tortora & Derrickson, 2008).
Anatomy of the digestive system
The GI tract is also called as the alimentary canal, which is essentially a long winding tube of approximately 30 feet (9 meters). The mouth, esophagus, pharynx, stomach, small and large intestines and the anus, make up the GI tract. The salivary glands, tongue, teeth, gall bladder, pancreas and the liver form the associated accessory organs (Tortora & Derrickson, 2008).
The mouth. The mouth starts with the oral orifice and extends up to the opening of the pharynx. The palate forms the roof of the mouth. It helps separating mouth cavity from the nasal passage. The mouth consists of the teeth and the tongue. The teeth help breaking down the food into smaller pieces. The tongue helps pushing the food down the esophagus. (Tortora & Derrickson, 2008).
The salivary glands. There are three extrinsic salivary glands in the mouth, namely, the parotid (located beneath the skin of the oral cavity, behind the jaw and anterior to the ear lobe), the submandibular (located along the jaw) and the sublingual glands (located under the tongue). The acidic saliva (pH 6.35-6.85) secreted by these glands is composed of water, salivary α- amylase (ptyalin), chloride ions, buffer in the form of bicarbonate and phosphate, IgA and lysozyme. Saliva breaks down carbohydrates and lipids in the mouth (de Almeida et al, 2008).
The pharynx. The pharynx or the throat forms a common passage for food and air. The epiglottis closes the trachea so that food enters only through the esophagus. (Saladin, 2007).
The esophagus. The esophagus is a long tube lined with mucus secreting cells. The masticated food passes down through peristalsis. Upper esophageal sphincter controls the top part of the tube and acts similar to the epiglottis, preventing air from entering the stomach, while the lower esophageal sphincter prevents acid reflux. Esophagus runs through the diaphragm to bulge into a sac like stomach (Saladin, 2007).
The stomach. The stomach is present on the left of the alimentary canal in the form of a bulged J. The smaller curved portion is superior and medial, while the larger curvature is inferior and lateral. It starts with cardiac orifice and ends in pylorus (opening of the small intestine). There are four regions in the stomach: 1) cardia, which is a small area within the cardiac orifice; 2) fundus, which is superior to the cardiac orifice; 3) the body, which is inferior to the cardiac orifice; and 4) the pylorus. The stomach is lined with gastric mucosa, which folds when the stomach is empty. These folds are called gastric rugae. Gastric mucosa is composed of five different types of cells: stem cells, chief, parietal, enteroendocrine and mucus...