PMS1 1/26/05 10:52 AM Page 1 Anatomy and Physiology of the Bowel and 1 Urinary Systems Anthony McGrath INTRODUCTION The aim of this chapter is to increase the reader’s under- standing of the small and large bowel and urinary system as this will enhance their knowledge base and allow them to apply this knowledge when caring for patients who are to undergo stoma formation. LEARNING OBJECTIVES By the end of this chapter the reader will have: ❏ an understanding of the anatomy and physiology of the small and large bowel; ❏ an understanding of the anatomy and physiology of the urinary system. GASTROINTESTINAL TRACT The gastrointestinal (GI) tract (Fig. 1.1) consists of the mouth, pharynx, oesophagus, stomach, duodenum, jejunum, small and large intestines, rectum and anal canal. It is a muscular tube, approximately 9m in length, and it is controlled by the autonomic nervous system. However, while giving a brief outline of the whole system and its makeup, this chapter will focus on the anatomy and physiology of the small and large bowel and the urinary system. The GI tract is responsible for the breakdown, digestion and absorption of food, and the removal of solid waste in the form of faeces from the body. As food is eaten, it passes through each section of the GI tract and is subjected to the action of various 1 PMS1 1/26/05 10:52 AM Page 2 1 Anatomy and Physiology of the Bowel and Urinary Systems Fig. 1.1 The digestive system. Reproduced with kind permission of Coloplast Ltd from An Introduction to Stoma Care 2000 2 PMS1 1/26/05 10:52 AM Page 3 Gastrointestinal Tract 1 digestive fluids and enzymes (Lehne 1998). The salivary glands switch into action as soon as food enters the mouth, and as the food continues on its journey, enzymes found in the stomach, small intestine, the pancreas and the liver continue the process. It is this secretion of fluids that helps maintain the function of the tract (Tortora & Grabowski 2001). Lining of the GI tract Throughout the GI tract, the walls are made up of mucous membrane, constructed in such a way that the various parts can act independently of each other. The walls of the GI tract consist of four layers. These are the: • adventitia; • muscularis; • submucosa; •mucosa. Adventitia The adventitia or outer layer consists of a serous membrane composed of connective tissue and epithelium. In the abdomen it is called the visceral peritoneum. It forms a part of the peritoneum, which is the largest serous membrane of the body (Thibodeau & Patton 2002). Peritoneum The peritoneum is the serous membrane that lines the ab- dominal and pelvic cavities, and covers most abdominal viscera. It is a large closed sac of thin membrane which has two layers: • the parietal peritoneum, which lines the abdominal and pelvic cavities; •the visceral peritoneum which covers the external surfaces of most abdominal organs, including the intestinal tract. The serous membrane is made up of simple squamous epithelium and a supporting layer of connective tissue. The 3 PMS1 1/26/05 10:52 AM Page 4 1 Anatomy and Physiology of the Bowel and Urinary Systems potential space between the visceral and parietal layers is known as the peritoneal cavity and contains serous fluid. In some diseases, such as liver disease, the peritoneal cavity can fill up with serous fluid called ascites. Some organs protrude into the abdominal cavity but are not encased in the visceral peritoneum. The kidneys lie in this type of position and are said to be ‘retroperitoneal’. The folds of the peritoneum bind the organs to the cavity walls and to each other. The folds include the mesentery, the lesser omentum, the greater omentum and the faciform ligament. These folds contain the nerve, blood and lymph supply to the abdominal organs. The mesentery is attached to the posterior abdominal wall and this binds the small intestine to the abdominal wall. The lesser omentum arises from the lesser curvature of the stomach and extends to the liver. The greater omentum is given off from the greater curvature of the stomach, forms a large sheet that lies over the intestines, and then converges into parietal peritoneum. The falciform ligament attaches the liver to the anterior abdominal wall and to the diaphragm (Ross et al. 2001). Muscularis The muscularis mostly consists of two layers of smooth muscle, which contract in a wave-like motion. The exceptions can be found in the mouth, pharynx and the upper oesopha- gus, which are made of skeletal muscle that aids swallowing. The two smooth muscle layers consist of longitudinal fibres in the outer layer and circular fibres in the inner layer. The con- traction of these two layers of muscle assists in breaking down the food, mixing it with the digestive secretions and propelling it forward. This action is referred to as peristalsis. Peristaltic action looks like an ocean wave moving through the muscle. The muscle constricts and then propels the narrowed portion slowly down the length of the organ forcing anything in front of the narrowing to move forward. 4 PMS1 1/26/05 10:52 AM Page 5 Small Intestine 1 Between the two muscle layers the blood vessels, lymph vessels and the major nerve supply to the GI tract can be found. The nerve supply is called the mesenteric or Auerbach’s plexus, and it consists of both sympathetic and parasympa- thetic nerves. It is mostly responsible for GI motility, which is the ability of the GI tract to move spontaneously (Tucker 2002; Martini 2004). Submucosa The submucous layer is highly vascular as it houses plexuses of blood vessels, nerves and lymph vessels, and tissue. It con- sists of connective tissue and elastic fibres. It also contains the submucosal or Meissner’s plexus, which is important in con- trolling the secretions in the GI tract (Martini 2004). Mucosa The mucosa is a layer of mucous membrane that forms the inner lining of the GI tract. It is made up of three layers: •a lining layer of epithelium, which acts as a protective layer in the mouth and oesophagus, and has secretory and absorptive functions throughout the rest of the tract; •the lamina propria, which supports the epithelium by binding it to the muscularis mucosae and is made up of loose connective tissue that contains blood and lymph vessels; •the muscularis mucosae layer, which contains smooth muscle fibres (Siegfried 2002). SMALL INTESTINE The small intestine begins at the pyloric sphinter and coils its way through the central and lower aspects of the abdominal cavity and joins the large intestine (colon) at the ileocaecal valve. The small intestine is divided into three separate seg- ments: the duodenum, jejunum and ileum. The nerve supply for the small bowel is both sympathetic and parasympathetic. 5 PMS1 1/26/05 10:52 AM Page 6 1 Anatomy and Physiology of the Bowel and Urinary Systems It is approximately 6.5m long and has a diameter of approxi- mately 2.5cm. The walls of the small intestine consist of the same four layers as the rest of the GI tract, however, both the mucosal and submucosal layers are modified. The mucosal layer consists of many glands called intestinal glands. These glands are lined with glandular epithelium, and they secrete intestinal juice. The submucosa in the duodenum contains glands that secrete mucus which is alkaline, this is designed to protect the small intestine walls from the acid in chyme and prevent the enzymes from acting on the walls. The small intestine is further modified in that throughout its length the epithelium covering the lining and the mucosa is made up of simple columnar epithelium. This contains both absorptive and goblet cells. If you were to examine the small intestine using a powerful microscope you would note that the absorp- tive cells actually contain projections described as ‘finger-like’. The projections are known as microvilli and allow the small intestine to deal with larger amounts of digested nutrients, having simply increased the surface area for digestion (Thibodeau & Patton 2002; Ellis 2004). The mucosa has a velvety appearance because its surface is made up by a series of villi. There are approximately 20–40 villi per square millimetre and these also increase the absorp- tive and digestive surface of the small intestine. They are about 0.5–1mm long and their walls are made up of columnar epithelial cells with tiny microvilli. The cells enclose a network of blood and lymphatic capillaries. The lymphatic vessels are known as lacteals. Nutrients pass via the blood capillaries and lacteals into the cardiovascular and lymphatic systems (Tortora & Grabowski 2002). The surface area of the small intestine is further increased by the presence of circular folds about 10mm high. These are unlike the rugae in the stomach that flatten out, in that they remain in place. They cause the chyme to twist around as it moves through the small intestine. This assists the digestive and absorptive processes. Throughout the mucous membrane 6 PMS1 1/26/05 10:52 AM Page 7 Small Intestine 1 in the small intestine numerous lymph nodes occur at irregular intervals. The nodes are known as either solitary or aggregated lymphatic follicles (Peyer’s patches) that occur in groups, and they are found mostly in the ileum (Watson 2000; Ross et al. 2001). Thus, the main function of the small intestine is digestion and absorption and its makeup is designed to help this process. The chyme is broken into small molecules that can be transported across the epithelium and into the blood stream. This occurs in the presence of pancreatic enzymes and bile, which are important in the digestive process.