Anatomy and Physiology of Gastrointestinal Tract Structure of the GI Tract Wall The digestive tract, from the esophagus to the anus, is characterized by a wall with four layers, or tunics. Here are the layers, from the inside of the tract to the outside: The mucosa is a mucous membrane that lines the inside of the digestive tract from mouth to anus. Depending upon the section of the digestive tract, it protects the GI tract wall, secretes substances, and absorbs the end products of digestion. It is composed of three layers: The epithelium is the innermost layer of the mucosa. It is composed of simple columnar epithelium or stratified squamous epithelium. Also present are goblet cells that secrete mucus that protects the epithelium from digestion and endocrine cells that secrete hormones into the blood. The lamina propria lies outside the epithelium. It is composed of areolar connective tissue. Blood vessels and lymphatic vessels present in this layer provide nutrients to the epithelial layer, distribute hormones produced in the epithelium, and absorb end products of digestion from the lumen. The lamina propria also contains the mucosa-associated lymphoid tissue (MALT), nodules of lymphatic tissue bearing lymphocytes and macrophages that protect the GI tract wall from bacteria and other pathogens that may be mixed with food. The muscularis mucosae, the outer layer of the mucosa, is a thin layer of smooth muscle responsible for generating local movements. In the stomach and small intestine, the smooth muscle generates folds that increase the absorptive surface area of the mucosa. The submucosa lies outside the mucosa. It consists of areolar connective tissue containing blood vessels, lymphatic vessels, and nerve fibers. The muscularis (muscularis externa) is a layer of muscle. In the mouth and pharynx, it consists of skeletal muscle that aids in swallowing. In the rest of the GI tract, it consists of smooth muscle (three layers in the stomach, two layers in the small and large intestines) and associated nerve fibers. The smooth muscle is responsible for movement of food by peristalsis and mechanical digestion by segmentation. In some regions, the circular layer of smooth muscle enlarges to form sphincters, circular muscles that control the opening and closing of the lumen (such as between the stomach and small intestine). The serosa is a serous membrane that lines the outside of an organ. The following serosae are associated with the digestive tract: The adventitia is the serous membrane that lines the esophagus. The visceral peritoneum is the serous membrane that lines the stomach, large intestine, and small intestine. The mesentery is an extension of the visceral peritoneum that attaches the small intestine to the rear abdominal wall. The mesocolon is an extension of the visceral peritoneum that attaches the large intestine to the rear of the abdominal wall. The parietal peritoneum lines the abdominopelvic cavity (abdominal and pelvic cavities). The abdominal cavity contains the stomach, small intestine, large intestine, liver, spleen, and pancreas. The pelvic cavity contains the urinary bladder, rectum, and internal reproductive organs. Gastrointestinal Tract : form and function The human gastrointestinal (GI) tract, or alimentary system, is a single tube approximately nine metres long when relaxed, though shorter in life because of the tone of the muscles within it (Marieb, 2005). It is open to the outside world at each end - the mouth and the anus - and includes several regions: the oral cavity; pharynx; oesophagus; stomach; small and large intestines; rectum; and anal canal (Fig 1). Various accessory digestive organs open into the tract and there are a number of sphincters and valves. Its healthy functioning is vital because it breaks down food and converts it into components to build and provide energy for all the cells of the body. Food that is in the GI tract is not really inside the body. To enter the body, food must be broken down and enter the blood or lymphatic system. General structure The same basic four-layered structure (Fig 2) is found throughout the GI tract, though different parts are adapted for different functions. Sphincters and valves ensure that food usually moves in one direction only and help to separate the different parts of the canal. The mucosal layer This is the innermost layer of the GI tube and is subject to a good deal of wear and tear as ingested food is in direct contact with it. It is particularly thick, with several layers of cells, in the mouth, oesophagus and rectum. In other areas the mucosa is thinner with a single layer of columnar cells. Throughout the mucosa are scattered specialised goblet cells, which secrete mucus to lubricate and protect the gut lining. In much of the GI tract, the mucosal layer is folded to provide a larger surface area for digestion and absorption. A thin layer of muscle beneath the mucosa is responsible for movements in these folds (Godfrey, 2005). The submucosal layer This layer of loose connective tissue supports the structures needed to supply and drain the cells of the tract. It contains: - Blood vessels, which bring oxygen and nutrients to the cells, and remove waste and digestion products; - The lymphatic system, which helps to drain excess and unwanted substances and defends the body against disease; - Nerve fibres from the autonomic nervous system to control reflexes such as smooth muscle contraction. The muscularis layer This consists of muscle fibres arranged in two distinct sheets: a thicker, inner sheet made of two layers of circular fibres and an outer sheet of longitudinal fibres. The muscle fibres are smooth and are not under voluntary control except at the two ends of the tract. The two layers contract together but, because their fibres lie in different directions, they produce slow, wave-like contractions that mix the contents of the tube - 'segmentation' - and propel it along - 'peristalsis'. Peristalsis is continuous and rhythmic but can be influenced by hormones and by the many nerves that enter the muscularis layer. At intervals throughout the GI tract, the circular muscle layer is modified to form rings of tissue called sphincters. These help to separate one section of the tract from another and control the speed of contents through the tract. The adventitia or serosa This outermost, protective layer is made of loose connective tissue and squamous (flat, scale-like) cells and carries nerves and blood vessels to supply the inner layers of the tract. This layer forms the inner, or visceral, part of a much larger organ - the peritoneum. Blood and nerve supply The gut requires a rich blood supply to support its digestive activities. Arterial blood is supplied mainly by the coeliac artery to the stomach, pancreas, spleen and liver and by the mesenteric arteries to the intestines. Venous blood drains from the stomach, pancreas and spleen via the hepatic portal vein into the liver, where the products of digestion undergo further processing and detoxification. Blood from the oesophagus and rectum does not go through the liver but drains directly into the venous system. There are two types of nerve supply to the GI tract. The enteric system, found within the walls of the GI tract, is sometimes known as the 'gut brain' and controls movement and secretion within the gut. Nerves from the autonomic nervous system also supply the GI tract. The sympathetic nerves generally reduce blood flow to the gut and decrease secretions, motility and contractions, while stimulation of the parasympathetic nerves leads to an increase in motility and secretion within the tract and relaxation of the gut sphincters. The vagus nerve (Xth cranial) supplies the oesophagus, stomach, pancreas, bile duct, small intestine and upper colon. General functions Marieb (2005) describes the digestive tract as a 'disassembly line' carrying food from one stage to the next so that it can be broken down and absorbed into the body. The stages involved in this breakdown process are ingestion, mastication, mechanical digestion (chewing, churning and pulverising), chemical digestion (by digestive enzymes), absorption into the blood and lymph, assimilation of useful components into the body by the liver and, finally, elimination in the faeces of non- usable residues such as animal and plant fibre (Godfrey, 2005; Smith, 2005). Conditions in the lumen of the GI tract are controlled to maximise its efficiency. Mechanical digestion is mainly controlled by reflexes of the parasympathetic nerves. The receptors that stimulate this activity are found in the walls of the alimentary canal and respond to stretch, the acidity of the contents and the presence of certain breakdown products. When stimulated they set off reflexes that activate or inhibit the glands that secrete digestive juices and the smooth muscle of the muscularis layer that propels food along the tract (Marieb, 2005). Chemical digestion - the process by which food molecules are broken down to their building blocks - is achieved by digestive enzymes secreted from the attached glands (salivary, liver, gall bladder, pancreas) into the lumen of the GI tract. The food needs to be soft and moist to enhance this process so water is needed in the diet. Carbohydrates are broken down to the simple sugars glucose, fructose and galactose, proteins are digested to amino acids while fats or lipids become fatty acids and an alcohol called glycerol (Godfrey, 2005; Marieb, 2005). The Stomach Form The stomach (Fig 1) is a J-shaped area of the gastrointestinal (GI) tract that sits in the upper left side of the abdomen. It joins with the oesophagus above it and the small intestine beyond it. It is the most dilated area of the tract and has several regions - the fundus (the expanded part of the stomach), the cardia, the body, and the funnel-shaped antrum. The shorter, inner curve of the stomach is called the lesser curvature and the longer, outer one, the greater curvature. Food enters the stomach through the cardiac or gastroesophageal sphincter and leaves it via the pyloric sphincter. The stomach is approximately 25cm long and can expand to hold up to 4L of food and drink, although its empty volume is only 50ml (Marieb, 2005). The total interior surface area of the stomach is about 800cm2. It has an extra muscle layer in addition to those lining the rest of the GI tract - an oblique layer that allows the stomach to distend in order to store food (Smith, 2005). The stomach collapses in on itself when it is empty and forms folds or rugae. Functions of the stomach The stomach performs a number of important functions including: 1. Food reservoir. We are able to eat large meals spaced many hours apart because of the stomach's ability to expand and hold food. The contents are released slowly due to the action of the strong pyloric sphincter; 2. Absorption. Foodstuffs are only partially broken down by the time they reach the stomach and the molecules are too big to cross the gastric wall. Most of the digestive activity takes place in the pyloric region but only a small amount of absorption occurs in the stomach - some water is absorbed, about 20 per cent of the alcohol we drink and some drugs, especially aspirin and other non-steroidal anti-inflammatory drugs, which are mildly acidic. These drugs can cause gastric irritation and bleeding; 3. Mucus secretion. This is particularly important in the stomach as it prevents the stomach digesting itself. The enzyme pepsin, which digests protein, is produced in the stomach and would erode the stomach walls if it came into contact with them. The stomach mucus is like a gel. It is made of a protein (mucin) and glycoproteins, and is spread in a layer about 1mm thick that adheres to the rugae of the stomach. Mucus in the stomach also contains some bicarbonate, which helps to neutralise the stomach acids. Mucus also helps to lubricate food in the stomach; 4. Gastric juice secretion. Gastric juice is a mixture of the secretions from two types of cell within the stomach. The billion or so parietal cells in the adult stomach secrete intrinsic factor (see below) and hydrochloric acid (HCl), while the chief cells secrete an enzyme, pepsinogen (Fig 2). Together they produce 2- 3L of gastric juice a day, which is highly acidic (pH 1.2-3.0). The acid has a number of functions: - It stops the proliferation of bacteria in the stomach; - It inactivates salivary amylase, mixed with the food in the mouth; - It curdles milk to prepare it for digestion; - It tenderises proteins (by denaturing them); - It converts the pepsinogen produced by chief cells into pepsin, which starts to digest protein; 5. Churning food. Food that enters the stomach is mixed with and diluted by the gastric secretions into a thick soup-like substance called chyme. The chyme is churned by waves of peristalsis. Each wave lasts about half a minute and 'flows' from the top of the stomach to the bottom; 6. Production of intrinsic factor. The parietal cells in the stomach also produce intrinsic factor, which is essential for the absorption of vitamin B12 from the ileum in the small intestine. Vitamin B12 is necessary for the healthy functioning of nerve fibres in the body, for the formation of myelin sheaths on the nerves in the spinal cord and for the formation of red blood cells. Nausea and vomiting Nausea is the unpleasant sensation that often precedes vomiting. Sufferers may look pale and sweaty and may experience waterbrash (a sudden and profuse secretion of saliva into the mouth) and antiperistalsis (reverse waves of peristalsis in the stomach from pylorus to cardia and sometimes also in the first part of the small intestine). Vomiting can be defined as the forceful expulsion of the gastric and intestinal contents through the mouth (Marieb, 2005). It occurs as the result of a reflux and can be stimulated by a number of different factors: - Irritation of any part of the GI tract - this is a protective mechanism against the ingestion of substances dangerous to the body; - Impulses from the semicircular canals of the ear, namely motion sickness; - Brain tumours or anything else that causes a rise in intracranial pressure; - Impulses from the higher cerebral centres in response to heightened emotions such as anxiety, fear and unpleasant smells and sights; - Some drugs such as opiates, digoxin and the emetic substance ipecacuanha. Movement and emptying Foodstuffs leave the stomach at different rates. Emptying starts and food begins to enter the small intestine about 30 minutes after a meal and is usually completed within 4-5 hours. The strength of the peristaltic movement of the stomach is altered by a number of factors. Generally, stronger movements are linked with more rapid emptying. Gastric emptying is slowed down when the sympathetic nervous system is stimulated, for example if we experience fear or anxiety, during heavy exercise or following blood loss. During a meal the stomach distends due to the presence of food and activity increases in the parasympathetic nervous system. These factors, combined with the presence of the hormone gastrin, which is produced in the G cells of the mucosal layer of the stomach, act together to increase motility in the stomach and to speed up gastric emptying. The lower parts of the stomach, the antrum and pylorus, act together with the first part of the small intestine, the duodenal cap, and squirt the chyme through the pyloric valve into the small intestine - the antrum contracts first, followed by the pylorus and finally the duodenal cap (Smith, 2005). The pylorus holds about 30ml of chyme but each contraction of the stomach squirts 3ml or less through the pyloric sphincter into the small intestine (Marieb, 2005). The presence of fat or acid in the duodenum of the small intestine slows down gastric emptying and allows time for the acidity to be neutralised and for fats to be absorbed in the small intestine. This effect may be brought about by hormones produced by the small intestine in response to chyme. Once the duodenum is filled with chyme and its wall is stretched, the enterogastric reflex occurs and slows down gastric emptying by inhibiting the parasympathetic nerves and tightening the pyloric sphincter (Marieb, 2005).
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