Digestion and Metabolism

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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 tr...

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... secrete enzymes pepsinogen and renin, which break down the unfolded proteins. (Nam et al, 2010).
Enteroendocrine cells. Enteroendocrine cells secrete the hormone gastrin, which promotes secretion of hydrochloric acid and pepsinogen by parietal cells and chief cells, respectively. Gastrin also initiates gastric contractions, which aid in food-mixing. This hormone is secreted by G cell, a type of enteroendocrine cell. (Sternini, Anselmi & Rozengurt, 2008).
Mucus cells. Mucus cells produce large amounts of mucus to form a lining throughout the stomach. This mucus lining protects the stomach from the activities of the digestive juices. (Saladin, 2007).
Gastric Stem cells. Gastric stem cells differentiate into parietal cells, chief cells and other types of gastric cells to replace the worn out gastric cells. (Modlin, Kidd, Lye & Wright, 2002; Mills & Shivdasani, 2011).

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