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| Anatomy and Physiology | |||||||||||||||
| THE DIGESTIVE SYSTEM This information will help you to develop your understanding of the pharmacology of laxatives, drugs for the mouth, and drugs for threadworm, available to the nurse prescriber. The digestive tract is basically a hollow muscular tube, mucosal epithelium lining the inner surface, and circular and longitudinal muscle comprising the walls. The mucosa, forming the inner lining of the tract, is supplied with a rich network of blood vessels, nerve fibres and endocrine cells. Epithelial cells (specialised for both absorption and secretion depending on their location in the digestive tract) cover this mucosal layer. The serosa is a layer of connective tissue covering the tract on the outside. In the mouth, food is broken down to small pieces and worked upon by digestive enzymes in the saliva. After chewing, the food is passed along the digestive tract by muscular action and mixed with further enzymes. These enzymes digest carbohydrates, fats and proteins. The end products of digestion include glucose, amino acids, fats, glycerol and fatty acids. These are absorbed through the gut wall and transported to the cells by the blood stream. The movement and mixing of food in the digestive tract, and its elimination, is brought about primarily by the contractions of smooth muscle. Striated muscle, however, is involved in the mouth, pharynx and upper oesophagus and external anal sphincter. Figure 7 - Anatomy of the large intestine The Oral Cavity Oral mucosa lines the oral cavity. A layer of keratinised cells covers the upper surface of the tongue and the hard palate. The mucosa lining the cheeks, undersurface of the tongue and lips is quite thin and delicate. Nutrient absorption does not occur in the mouth but drugs such as nitroglycerine can be administered via the mucosa inferior to the tongue. Hard and soft palates form the roof of the mouth. The tongue forms the floor. Three pairs of salivary glands secrete into the oral cavity. Saliva is mainly water. It also contains electrolytes, buffers, proteins, antibodies, enzymes and waste products. Saliva has several functions. These include:
The oesophagus The oesophagus is a thin walled tube attaching the pharynx to the stomach. It consists of striated muscle at the top and smooth muscle at the bottom. When food is swallowed, the sphincter in the upper portion of the oesophagus relaxes and peristalsis propels food through the oesophagus. If food particles remain in the oesophagus following this wave of peristalsis, another wave of peristalsis is stimulated which sweeps food through into the stomach. The sphincter at the upper end of the stomach prevents the regurgitation of food from the stomach back into the oesophagus. The final few centimetres of the osesophagus are actually in the abdominal cavity. Therefore, when abdominal pressure increases e.g. when coughing, this terminal section of the oesophagus is compressed and stomach contents are not forced to enter into the oesophagus. Stomach The stomach is divided into three areas:
The fundus and body are quite thin walled and act as a reservoir for ingested food. The pyloric antrum has thick walls and strong waves of contraction occur during the digestion of a meal. The food is mixed with gastric juices and then passes from the stomach to the duodenum via the pyloric canal. A band of smooth muscle circles this canal and is called the pyloric sphincter. Following ingestion of a meal, the body and fundus distend. Ripples of contraction (occurring about three times a minute) then begin in the middle of the stomach forcing the food towards the pyloric antrum and pyloric canal. These peristaltic waves occur approximately three times a minute and become greater in intensity as they reach the pyloric canal. Therefore, every twenty seconds, a portion of the stomach contents is pushed towards the pyloric canal. It is then propelled into the intestine. As the pyloric sphincter contracts, the mixture is pushed back into the body of the stomach. This mixture becomes reduced into chyme, a semi-fluid substance and each minute, 6-10ml of chyme is emptied into the intestine. Small Intestine The small intestine is approximately 6 metres in length and is divided into:
Nearly all the nutrient absorption occurs in the small intestine. Segmentation, rather than peristalsis, is seen in the small intestine. As a result of the contraction of circular muscle at several points along this structure, the small intestine is divided into a number of sacs. The circular muscles then contract at different places and this causes the chyme to be pushed backwards and forwards and mixed with digestive enzymes. Longitudinal muscles also contract and relax and massage the contents of the intestine. The movement of chyme through the small intestine is very slow. This allows digestion and absorption of food. The first food residues reach the end of the small intestine about three to four hours following ingestion. Large Intestine The large intestine is approximately 1.5 m long and 7.5cm wide. It is comprised of three parts:
Figure 7 - Anatomy of the large intestine The ascending colon travels up the right side of the abdomen towards the inferior surface of the liver. It then turns sharply to the left and becomes the transverse colon. As the transverse colon reaches the left side of the body, and nears the spleen, it turns down the left side of the abdomen and becomes the descending colon. This then becomes the pelvic colon, the rectum and finally the anus. Each day approximately 500ml of food material, or chyme, enter the caecum. The longitudinal muscle of the large intestine forms three strips. These muscles are not as long as the colon itself. Therefore, the wall of the intestine becomes puckered and pouches called haustra are formed. Peristaltic movements of the large intestine tend to be slow, and non-propulsive. This aids absorption and storage functions. Haustral contractions, occurring at intervals of approximately thirty minutes, shuffle the contents of the intestine back and forth. Large contractions called mass movements occur three to four times a day. This drives the colonic contents forward for storage in the rectum. The large intestine actively absorbs sodium from the ascending and transverse colon. This is then followed by the passive absorption of chloride and water. About 350ml is absorbed from the 500ml of chyme entering the colon, 150g of faecal material then has to be eliminated. This includes 100g of water and 50g of solids. The length of time the food residue remains in the large intestine will determine the amount of water absorbed. The large intestine also secretes an alkaline mucus. This lubricates the faeces and facilitates their passage through the intestine. The mucus also contains bicarbonate which maintains colonic pH. The mucosa is also protected by the bicarbonate, which neutralises acids produced by bacterial fermentation.. Defaecation Stretch receptors of the rectal walls are stimulated as a result of mass movements. This initiates the defaecation reflex. Defecation occurs when relaxation of the smooth muscle of the internal anal sphincter, and relaxation of the skeletal muscle of the external anal sphincter occurs. Voluntary control of the skeletal muscle of the external sphincter allows an individual to prevent defeacation. Bacterial activity Many bacterial species colonise the large intestine and form a symbiotic relationship with man each deriving some benefit from the other. However, this natural flora can become pathogenic if introduced into other parts of the body. Multiple Choice Question (MCQs) 1. Saliva
has several functions. These include: To see the answers to these questions click here |
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