Anatomy and Physiology of the Bowel and Urinary Systems

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           Anatomy and Physiology
           of the Bowel and
           Urinary Systems
                                                                       1
           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 9 m 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

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   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




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                                                       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

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   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.

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                                                             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
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