BOWEL ELIMINATION

Chapter 33 BOWEL ELIMINATION




KEY TERMS/CONCEPTS


























THE DIGESTIVE TRACT


Every cell in the body requires energy to carry out its normal functions. Cellular energy is produced when nutrients in food are broken down and absorbed. Solid wastes that accumulate during the digestive process must be eliminated. The digestive system is the means by which food is ingested, digested and eliminated. In the digestive tract, food is digested to its elemental components — nutrients, fluid and electrolytes. The digested elements are absorbed into the bloodstream for transport to all body cells, and the solid wastes that accumulate during digestion are excreted from the body.



ANATOMY


The digestive tract is a muscular tube about 9–10 metres in length, which extends from the mouth to the anus (Figure 33.1). The structure of the alimentary canal is similar for most of its length and consists of an outer covering, middle layers of involuntary muscle and connective tissue, and an inner mucous membrane lining (Figure 33.2).




The outer covering of the alimentary canal consists of fibrous tissue (the serosa) or, in the abdomen, peritoneum. The peritoneum is a double-layer serous membrane that secretes serous fluid to prevent friction between the abdominal organs. The inner layer is called the visceral serous membrane and the outer layer the parietal serous membrane. The two layers of the peritoneum are kept proximate and separated by peritoneal fluid. The peritoneum forms a lining for the abdominal cavity (parietal layer) and a covering for most of the abdominal organs (visceral layer). The mesentery, which is formed by the peritoneum, covers the intestines and attaches them to the posterior abdominal wall. Ligaments are formed by folds of peritoneum to attach some organs to each other or to the abdominal wall. The greater omentum is attached to the lower border of the stomach and hangs down like an apron and loops up to be attached to the transverse colon. The lesser omentum extends from the lower border of the liver to the lesser curvature of the stomach.


The middle layer of the digestive tract contains smooth involuntary muscle with both circular and longitudinal fibres.


The innermost layer of the digestive tract is composed of connective tissue (the submucosa), which contains many large blood and lymph vessels, and an inner lining of mucous membrane (the mucosa), which secretes mucus into the digestive tract. The mucosa also contains a small amount of loose connective tissue and a thin layer of smooth muscle.


The organs that make up the digestive tract are the mouth, the oropharynx, the oesophagus, the stomach, the small intestine and the large intestine.



The mouth


The mouth, or oral cavity, has boundaries of muscle and bone and is lined by mucous membrane. The lips protect its anterior opening, the cheeks form the lateral walls, the hard palate forms its anterior roof, and the soft palate forms its posterior roof. The uvula is a finger-like muscular projection hanging down from the midline of the soft palate. It helps prevent the entry of food and fluids into the nasal cavities.



Components of the mouth


The mouth contains the tongue and the teeth. The tongue is a muscular organ that occupies the floor of the mouth. It is attached to the hyoid bone and also to the floor of the mouth by folds of mucous membrane called the frenulum. The tongue consists of a mass of voluntary muscle and is covered by squamous epithelium. On the upper surface of the tongue there are many small projections called papillae, which contain tastebuds, the sensory endings of the nerve that perceives taste. The tongue is a very mobile organ which is important in the chewing (mastication) of food, assists in swallowing and is essential for speech.


The teeth are embedded in the maxillae and the mandible. All teeth have the same basic structural organisation but differ in shape and size. Each tooth has one or more roots embedded in the maxilla or mandible, a portion (the crown) above the gum, and a neck, which joins the root and the crown and which is surrounded by the gum. Each tooth is made up of an ivory-like substance called dentine; a central pulp cavity containing blood and lymphatic vessels, nerves and connective tissue; and a thin layer of enamel covering the crown. In children there are 20 deciduous (milk) teeth, consisting of 10 in each jaw; in adults there are 16 permanent teeth in each jaw. The teeth are named according to their shape and function. In each jaw there are four incisors, used for biting; two canines, used for tearing; four premolars, used for crushing; and six molars, used for grinding. A wisdom tooth is the third molar tooth and it is the last tooth to erupt. Wisdom teeth usually erupt from age 18 to 25.


The salivary glands, of which there are three pairs, pour their secretions into the mouth. The salivary glands are:





The salivary glands are accessory digestive organs.





The stomach


The stomach is a hollow muscular organ that lies primarily in the upper left quadrant of the abdomen, beneath the diaphragm (Figure 33.3). It is commonly described as being ‘J shaped’, but the size and shape of the stomach varies according to its contents. The stomach is divided into four areas: the fundus (the upper portion); the cardia, where the oesophagus joins the stomach; the body, or main part of the stomach; and the pylorus (the narrowed lower portion).



The ‘curvatures’ of the stomach are the lesser curvature, which is the medial border; and the greater curvature, which is the lateral border.


The opening of the oesophagus into the stomach is called the cardiac orifice, and is surrounded by a functional sphincter called the cardiac sphincter. The pyloric orifice is the opening between the stomach and the small intestine and is surrounded by the pyloric sphincter, which consists of a thickened layer of circular muscle and is normally partly open. Peristaltic waves in the stomach push some of the gastric contents through the orifice and into the duodenum. The orifice then closes.


The stomach wall consists of four layers:









The large intestine


The large intestine is a muscular tube about 1.5 m in length and 6 cm in diameter, and extends from the end of the ileum to the anus (Figure 33.5). Lying in the abdominal and pelvic cavities, the large intestine may be divided into regions that are distinguished by their anatomical structure and position:










The wall of the large intestine has the same basic structure as that of the small intestine (the serosa, muscular layer, submucosa and mucosa).




THE ACCESSORY DIGESTIVE ORGANS


The accessory organs secrete enzymes into the alimentary canal, secretions (enzymes) that are actively involved in the process of digestion. An enzyme is a substance, usually protein in nature, that initiates and accelerates a chemical reaction. The accessory organs are the salivary glands, the pancreas, the liver and the biliary tract.





The liver


The liver is an organ situated in the upper part of the abdominal cavity, immediately beneath the diaphragm. The greater part of the liver lies in the right upper abdomen but the organ extends across to the left upper abdomen (Figure 33.7). The liver is divided into two parts, a large right lobe and a much smaller left lobe. Like the alimentary canal, the liver is almost entirely covered by a layer of peritoneum. Beneath this is a fibrous capsule, which is continuous with areolar connective tissue situated within the liver. The areolar tissue forms a tree-like structure, which carries branches of the hepatic artery, hepatic portal vein, bile ducts and lymphatic vessels. These vessels enter and leave the liver through the porta hepatis, a short transverse fissure on the inferior surface of the liver.



The hepatic artery carries oxygenated blood to the liver. The portal vein carries deoxygenated blood, rich in nutrients from the small intestine, to the liver. Three hepatic veins carry deoxygenated blood from the liver to the inferior vena cava. The right and left hepatic ducts carry bile, secreted by the liver, to the common hepatic duct. The latter combines with the cystic duct from the gall bladder to form the common bile duct, which drains into the duodenum. The biliary tract, which transports bile from the liver to the duodenum, consists of the left and right hepatic ducts, the common hepatic duct, the cystic duct, the gall bladder and the common bile duct (Figure 33.8).




The gall bladder


The gall bladder is a small muscular sac that lies on the inferior surface beneath the right lobe of the liver.


Functions of the liver and gall bladder include:










PHYSIOLOGY OF DIGESTION



DIGESTION OF FOOD


During the process of digestion, food is reduced to its simplest chemical form so that it can be absorbed into the bloodstream and used by the tissues. Digestion occurs through both mechanical and chemical actions. Mechanical action involves the physical process of liquefying the food, mixing it with digestive juices and moving it through the alimentary canal. Chemical action occurs when the digestive juices mix with the food, resulting in complex chemical substances being split into simple substances.



Digestion in the mouth


In the mouth food is broken down physically by the process of chewing (mastication), and mixed with saliva to bring about the formation of a moist ball, or bolus. Chewing softens the food so that it passes more easily through the alimentary canal. The presence of food in the mouth, together with its taste and smell, stimulates the secretion of saliva, gastric and pancreatic juices and bile (by means of parasympathetic pathways). The enzyme salivary beta-amylase in saliva begins to digest starches.


After the food has been formed into a bolus it is passed through the pharynx and down the oesophagus into the stomach by the act of swallowing. Swallowing is a complex reflex regulated by a ‘swallowing centre’ in the medulla oblongata of the brain. Swallowing is initiated when the tongue muscles push the bolus upwards and backwards into the oropharynx. The soft palate is elevated and comes into contact with the posterior wall of the pharynx, thereby closing off the nasopharynx. The larynx is pulled upwards and forwards, and the bolus pushes the epiglottis back over the glottis to prevent food from entering the respiratory tract. The oesophageal sphincter opens and the bolus enters the oesophagus.


Peristaltic waves carry the bolus through the oesophagus, and the cardiac sphincter relaxes to allow food and fluids to enter the stomach. The cardiac sphincter contracts to close the cardiac orifice at the end of each wave of contraction of the oesophagus, then relaxes to open the orifice when the next wave of contraction begins.



Digestion in the stomach


The stomach stores the food and later releases it at a rate that is optimal for digestion. Food is mixed with gastric juice, thereby changing its consistency so that it will be more easily transported along the alimentary canal. The food is exposed to enzymes (pepsins) which begin the digestion of proteins, and the gastric juice converts ferric iron (Fe3+) to ferrous iron (Fe2+). When the stomach muscles are stretched by swallowed food, peristaltic contractions are stimulated, which results in a churning movement. When the food is mixed with gastric juice it develops a pasty consistency and becomes known as chyme. The rate of emptying of the stomach depends on the:






The average time for the stomach to empty after a meal is 4–6 hours. When the food has been well mixed in the stomach, peristalsis begins in the lower half of the stomach and forces the chyme through the pyloric sphincter. Because this sphincter is only partially opened, only small amounts of chyme enter it at one time. When the duodenum is filled with chyme a nervous reflex (the enterogastric reflex) occurs, which inhibits the vagus nerves from stimulating the stomach muscles, and slows the emptying of the stomach. This mechanism ensures that food does not enter the small intestine too rapidly, to enable its digestion.





EXCRETION


Chyme enters the caecum through the ileo-caecal valve, which is normally closed but opens briefly to allow a small amount of chyme through with each peristaltic wave. Movement of chyme through the large intestine is a slow process. Various types of movement occur in the colon, including peristalsis, segmentation, and mass movements. Mass movements are brought about as a result of distension of the stomach or duodenum by ingested food. This sudden movement of colonic contents can push large amounts of faeces into the rectum and initiate the desire to defecate.


Feb 12, 2017 | Posted by in NURSING | Comments Off on BOWEL ELIMINATION
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