The process of digestion begins immediately after food is ingested with both chemical and mechanical digestion taking place. Food is masticated by the teeth with help from the tongue and cheeks to form a bolus. Three pairs of salivary glands secrete a fluid consisting of water, electrolytes, mucus and enzymes. Salivary amylase begins the breakdown of carbohydrates into maltose or simple sugars and mucin is secreted which makes the bolus lubricated facilitating its transfer into the pharynx, bypassing the trachea by the closed epiglottis and subsequently into the oesophagus. As the bolus enters the oesophagus, peristalsis contractions carry the food to the stomach in four to five seconds. The oesophagus is exposed to rough food stuffs so is lined with a stratified squamous epithelium to resist trauma. Once the bolus has traversed the oesophagus, the cardiac sphincter opens to allow food to pass into the stomach and closes to prevent reflux of acid into the oesophagus (Bowen, 2010).
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The stomach is an extendable sac comprised of three layers and is responsible for the secretion of acids and enzymes to continue breaking down food and killing any bacteria present. The mucosa is where the stomach acid is produced, sub mucosa is a layer of connective tissue and the muscularis interna is made up of three layers of muscle; inner oblique, middle circular and outer longitudinal. These muscles provide the rhythmic contractions needed to churn food and secretions. The stomach comprises of four regions: cardiac region where contents of the oesophagus empty; the fundus which forms the upper curvature of the stomach; the body which is the largest section and the pylorus which is the narrower end of the stomach where it joins the small intestine. There are four major secretions within the stomach which come from different cells contained in gastric glands. Mucous cells are the most predominant and cover the entire lumen. Mucus cells extend down the glands as mucous neck cells and secrete bicarbonate rich mucus protecting the gastric surface from the highly acidic hydrochloric acid which is secreted from parietal cells. Parietal cells also secrete intrinsic factor which is specific in the absorption of vitamin B12. Hydrochloric acid activates pepsinogen which is secreted from mucous cells and chief cells into a protein digesting enzyme pepsin, which then begins the breakdown of proteins into polypeptides. The actions inside the stomach churn the ingested food into chyme which is then released into the duodenum through the pyloric sphincter in small amounts (Kashefipour, 2011).
When chyme reaches the duodenum, the pancreas releases a battery of enzymes which breakdown specific molecules through catabolic activity. The duodenum is the first section of the small intestine which receives secretions from the pancreas and the gall bladder. Pancreatic juice entering the duodenum is alkaline due to the high number of bicarbonate ions which neutralises the acidic chyme coming from the stomach. Enzymes are biological catalysts which work on specific molecules. The primary substrate molecules in the digestive system are proteins, starches and lipids. Each specific enzyme has an active site where the particular molecule fits, an anabolic reaction takes place resulting in smaller product molecules (Clancy and McVicar, 2009).
Trypsin and chymotrypsin are released from the pancreas as inactive proenzymes, trypsinogen and chymotrypsinogen. Trypsinogen is activated within the lumen of the small intestine which then activates the chymotrypsinogen. These enzymes then break down the polypeptides into peptides and peptides into amino acids ready for absorption. Lipids which are mainly in the form of triglycerides need to be broken down into smaller particles in order for lipase to work effectively. Emulsification into monoglycerides and free fatty acids is accomplished by the secretion of bile salts from the liver and gall bladder. Fat soluble vitamins, A, D, E and K which entered the duodenum in fat droplets are also broken down during this process. Pancreatic amylase continues to hydrolyse starches into maltose within the duodenum (Bowen, 2010).
Peristalsis then carries food to the second part of the small intestine, the jejunum, where the majority of absorption occurs. The mucosa of the jejunum is covered in folds and lined with columnar epithelial tissue which has finger like projections called villi (see appendix ?). Each villus has its own network of microvilli which all contribute to extending the surface area for optimum absorption of nutrients. The villi each have their own network of capillaries and lymphatic vessels called lacteals and mucus secreting goblet cells. At the base of these glands are the Crypts of Lieberkuhn which are tubular invaginations lined with younger epithelial cells. They are primarily involved with secreting intestinal juices to advance the simplifying of molecules and provide a source of cells for the villus. Maltase, sucrase and lactase are secreted to break down disaccharides into glucose, fructose and galactose. Lipase continues to break down fats into fatty acids and glycerol and erepsin further breaks down peptides into vast quantities of different amino acids. Diffusion of these amino acids and monosaccharides now takes place through the single celled epithelial wall of the villi into the blood stream along with water soluble vitamins and minerals. Fatty acids along with fat soluble vitamins diffuse through the lacteal into the lymphatic system (Whitney & Rolfes, 2002).
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The final stage of absorption takes place in the ileum where specific receptors absorb vitamin B12, bile salts, fluid and electrolytes. As in the jejunum, the ileum walls are lined with villae and secrete enzymes to further break down approximately 5% remaining matter which has not been absorbed. Peristalsis contractions move these unabsorbed remains towards the large intestine. A mixture of undigested material, water, some vitamins and minerals or salts enter the large intestine through ileocaecal valve entering a short pouch, the caecum. It then moves towards the colon where the reabsorption of water to maintain fluid balance takes place along with the absorption of some vitamins and processing of undigested material such as fibre. Bacteria in the colon chemically break down some of the fibre to produce nutrients which nourish the cells lining the colon and also produce vitamin K which is then absorbed into the blood. The rectum is the terminal segment of the digestive tract where faeces moves into the anus through the internal sphincter and is held in by the external sphincter until defecation occurs (Collins, 2009).
When the process of diffusion has taken place through the epithelial walls of the small intestine, molecules are then transported in the blood, through the hepatic portal vein to the liver for processing and assimilation. The liver is responsible for the synthesising of amino acids to carry out regenerating functions within cells and transforms certain amino acids into ones required through an anabolic process called deamination. Any unused amino acid is then broken down into urea, glucose and energy which are consumed during vast processing activities. Monosaccharides are also received by the liver through the hepatic portal vein where it is transformed by cell respiration into glucose to produce energy. Any unused glucose is stored in the liver as glycogen or turned to fat and stored under skin. The lacteal in the villi transport fatty acids and glycerol into the lymphatic system which then carries them to the liver. Fat is metabolised as a primary source of energy and is transported round the body in high and low density lipoproteins where they are taken up by cells and tissues, metabolised for energy and used to produce substances such as cholesterol (Innvista, 2010).