Session 20. The Digestive Organs. Session Outline. Introduction. Unit 6 Digestive System

Transcription

1 Unit 6 Digestive System Session 20 The Digestive Organs Session Outline Introduction 20.1Alimentary tract 20.2 Mouth 20.3Tongue 20.4Teeth 20.5Oesophagus 20.6 Stomach 20.7Small intestine 20.8 large intestine 20.9 Rectum Summary Learning Outcomes Review Questions References Introduction In developed countries gastrointestinal symptoms are a common reason for seeking medical care. Many of these are for symptoms with no serious clinical significance. It must be remembered that almost 20% of all cancers occur in the gastrointestinal tract. In developing countries, malnutrition and poor hygiene make infections in the gastrointestinal tract a more probable diagnosis. As health care personnel it is important to recognize and treat these infections promptly and also help with prevention of these diseases by encouraging good hygiene. The organs of the gastrointestinal tract are specially modified to perform the function of ingestion, mechanical and chemical digestion, absorption, propulsion and defaecation ( figure 1

2 20.1). Sessions 20, 21 and 22 will explain the structure and function of each of these organs and their main clinical disorders will be explained in session 23. Figure 20.2 shows the organs of the gastrointestinal tract and the main structure and functions of each of these organs are described in this session. Figure 20.1 main actions of the gastrointestinal system Figure 20.2 Organs of the gastrointestinal system 20.1 Alimentary tract This is a long tube through which food passes. It commences at the mouth and terminates at the anus. Each of the organs are given separate names to describe their functional importance better. The main parts of the alimentary tract are listed below ( figure 20.1) Mouth pharynx oesophagus stomach small intestine large intestine 2

3 rectum and anal canal 20.2 The mouth It is a cavity lined by the mucosa, and is also called the oral cavity(buccal cavity). Its boundaries are as follows: -the lips anteriorly, - cheeks laterally - palate superiorly - tongue inferiorly It has an anterior opening, the oral orifice. The oral cavity is continuous posteriorly, with the oropharynx. The walls of the mouth are lined with stratified squamous epithelium hence can withstand a lot of friction. The epithelium on the gums, hard palate, and dorsum of the tongue is slightly keratinized for extra protection against abrasions (during eating). The oral mucosa secretes several substances and their main actions are listed below. Mucus- for lubrication and protection of the mucosa Secretary immunoglobulins- immunity against invading organisms Lysozymes- destruction of bacteria. These substances protect the mouth, against invading organisms. The palate forms the roof of the mouth and is divided into the anterior hard palate and the posterior soft palate. The uvula is a curved fold of muscle curved with mucous membrane, hanging down from the middle of the free border of the palate. Originating from the upper end of the uvula there are folds of mucous membrane. In between these folds of mucosa lie the group of lymphoid tissue known as the tonsils. Self Assessment Questions What are the boundaries of the mouth? What factors protect the mucosa of the oral cavity? 20.3 Tongue The tongue helps in breaking food into small pieces and helps in chewing and helping to move food from the mouth down into the throat. The tongue is composed mostly of skeletal muscle and is covered by a smooth film of mucous membrane on the underside. The irregular dorsal (top) surface contains papillae taste buds and other structures to sense taste by the 7 th and 9 th cranial nerves. The front of the tongue is used to manipulate the food during chewing, and the base of the tongue helps in swallowing. It is also a sensitive touch sensitive organ and plays an 3

4 important role in speech. (In the session on sensory nerves we will describe the sensory functions of the tongue in tasting food). The mucous membrane covering the tongue is divided into two sections, the anterior two thirds (2/3rds) and the posterior third (1/3). The sensation of taste (sensory supply) is recognised in the anterior 2/3rds by the facial nerve and in the posterior 1/3 by the glossopharyngeal nerve. The muscles of the tongue are arranged so movement in any direction is possible. The muscles ( motor supply) are supplied by the hypoglossal cranial nerve(x11). Functions of the tongue are listed below. - Mastication (chewing) - Deglutition (swallowing) - Production of Speech - Taste The soft palate is posterior to the hard palate and is made of skeletal muscle. It functions to close off the nasopharynx during swallowing, preventing food from being forced into the nasal cavity. Self Assessment Questions Explain the boundaries of the mouth List the functions of the tongue What is the function of the soft palate? 20.4 Teeth The teeth are embedded in the alveoli or sockets of the mandible and the maxilla. Each individual has two sets of dentitions; the temporary or deciduous teeth and the permanent teeth. There are 20 temporary teeth 10 in each jaw. They begin to erupt when the child is about 6 months old, and should all be present after 24 months. The permanent teeth begin to replace the deciduous teeth in the 6 th year of age and this dentition consisting of 32 teeth is usually complete by the 24 th year. Functions of the teeth are in the cutting and biting off pieces of food. Self Assessment Questions To what bones of the skull are teeth attached? How many teeth does a child have? At what age does the adult teeth growth end? 4

5 20.5 Oesophagus The oesophagus is a a muscular tube about 25 cm (10 inches) long. It has an unique ability to carry food. It is collapsed when not involved in passing food through it. The process of swallowing is known as deglutition. Deglutition (swallowing). The process of swallowing consists of a voluntary (buccal) phase (a) and involuntary (pharyngeal phase and oesophageal phase) phases. (a) During the buccal phase: the tongue rises and presses against the hard palate, forcing the food bolus into the oropharynx. Once food enters the oropharynx the involuntary phase of swallowing begins. (b) The uvula and larynx rise to prevent food from entering respiratory passageways. Relaxation of the upper oesophageal sphincter allows food to enter the oesophagus. (c) The constrictor muscles of the pharynx contract, forcing food into the oesophagus inferiorly, and the upper esophageal sphincter contracts after entry. (d) Food is moved through the oesophagus to the stomach by peristalsis. (e) The gastroesophageal sphincter opens, and food enters the stomach. 5

6 Figure 20.3 the process of deglutition Source : E N Marieb and K Hohen, Human Anatomy and Physiology Self Assessment Questions List the phases of swallowing? Can you explain the process of degluttition? 20.6 Stomach The stomach is a large muscular organ in the abdomen. It is divided into regions namely the cardiac region, fundus, body, and pyloric region. The major regions of the stomach are shown in Figure The food enters the stomach from the esophagus at the cardia. The fundus is the dome-shaped part, and is located beneath the diaphragm. The body, the midportion of the stomach, is continuous inferiorly with the pylorus. The pylorus is continuous with the duodenum (the first part of the small intestine) through the pyloric sphincter. The pyloric sphincter controls emptying of the stomach. Food in stomach empties into the small intestine through the pyloric sphincter. 6

7 Functions of stomach are listed below. - The stomach stores large quantities food - It mixes food using a churning action to mix food into a soupy mixture called chyme. - It secretes gastric juices that initiate protein digestion with then enzyme pepsin, acidifies chyme with HCl. Milk protein digestion is initiated with renin. - It releases the chyme, in regular spurts into the small intestine. The muscularies externa of the stomach has three layers of smooth muscle, the outermost longitudinal layer, the middle circular layer and the innermost oblique layer. These act to break up food into a semi liquid chyme. Gastric juice is composed of hydrochloric acid, mucus, and enzymes including pepsinogen (a precursor of the active enzyme pepsin). The major function of gastric juice is to maintain an acidic environment for the digestive actions of the stomach. The control of gastric juice secretion occurs in three overlapping phases, These phases are - cephalic phase - initiated by the sight or taste of food; - gastric phase - initiated in response to distension of the stomach; - intestinal phase- initiated, with an inhibitory component to prevent over secretion of gastric juice. The glands of the stomach secrete hydrochloric acid, pepsinogen, mucin and intrinsic factor. The different cells of these glands are listed below: i. Chief cells or pepsinogen cells, which secrete pepsinogen, ii. Parietal cells or oxyntic cells, which secrete hydrochloric acid ( HCl) and intrinsic factor. iii. Mucous cells, which secrete mucin iv. Columnar epithelial cells secrete mucin v. G cells secrete gastrin. 7

8 Figure Stomach Self Assessment Questions Can you draw a labelled diagram of the stomach? Can you list the functions of the stomach? 20.7 Small intestine The, small intestine is subdivided into the duodenum,( where most of the remaining digestion takes place), and the jejunum and ileum, (where most of the absorption of nutrients and water into the blood and lymph occurs). The absorptive surface of the small intestine is increased by mucosal protrusions called villi; which contain additional protrusions called microvilli and circular folds called plicae circulars.( figure 20. ) Mucosal and submucosal glands secrete intestinal juice and enzymes that aid the digestion of carbohydrates, proteins, and lipids. Large lymphatic nodules combat microorganisms and provide immunity. Muscular activity in the small intestine includes segmenting contractions and peristaltic contractions which move the chyme forwards. These movements aid digestion. Intestinal juice contains water, salts, and mucus. The digesting enzymes of the small intestine includes enterokinase, lactase, lipase, maltase, peptidase and sucrose. The products of carbohydrate, protein lipids etc are, digested as well as absorbed by the small intestine. Fig.20.5 A highly magnified view of one complete villus in the small intestine 8

9 The absorption of lipids is a more complex process than absorption of carbohydrates and proteins. It involves the breakdown is water- insoluble triglyceride droplets into water- soluble particles called micelles. Micelles are absorbed by cells. Once in-side the cells the breakdown products of lipids are resynthesized into triglycerides and are packaged into tiny droplets called chylomicrons. The chylomicrons move from the cells into lymphatics and blood vessels for distribution throughout the body. Intestinal juice About 1500 ml of intestinal juice are secreted daily by the glands of the small intestine. It consists of water, mucus, mineral salts, enzyme: enterokinase (enteropeptidases).the ph of intestinal juice is usually between 7.8 and 8.0. Functions of the small intestine The main functions are listed below. onward movement of intestinal contents by peristalsis secretion of intestinal juice by secretory cells of the mucosa completion of chemical digestion of carbohydrates,protein and fats in the enterocytes of the intestinal villi protection against infection by microbes by the solitary and aggregated lymph follicles secretion of the hormones cholecystokinin (CCK) and secretin absorption of nutrients. Chemical digestion of food in the small intestine When acid chyme passes into the small intestine it is mixed with pancreatic juice, bile and intestinal juice, and is in contact with the enterocytes of the villi. In the small intestine the digestion of all the nutrients is completed: carbohydrates are broken down to monosaccharides proteins are broken down to amino acids 9

10 fats are broken down to fatty acids and glycerol. Self Assessment Questions Describe the main structure of an intestinal villus List the functions of the duodenum List the functions of the small intestine What is the importance of intestinal juice in digestion and absorption 20.8 Large intestine RGE INTESTINE (COLON), The large intestine is about 1.5 metres long, it begins at the caecum in the right iliac fossa and terminates at the rectum and anal canal deep in the pelvis. It has a larger lumen than the small intestine. It forms an arch round the coiled-up small intestine. it is divided into the caecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum and anal canal. They are briefly described below. The caecum. This is the first part of the colon. It is a dilated region which has a blind end inferiorly and is continuous with the ascending colon superiorly. Just below the junction of the two the ileocaecal valve opens from the ileum. The vermiform appendix is a fine tube, closed at one end, which leads from the caecum. It is usually about 13 cm long and has a similar structure as the walls of the colon but contains more lymphoid tissue (Fig ). The ascending colon. This passes upwards from the caecum to the level of the liver where it curves acutely to the left at the hepatic flexure to become the transverse colon. The transverse colon. This is a loop of colon which extends across the abdominal cavity in front of the duodenum and the stomach to the area of the spleen where it forms the splenic flexure and curves acutely downwards to become the descending colon. The descending colon. This passes down the left side of the abdominal cavity then curves towards the midline. After it enters the true pelvis it is known as the sigmoid colon. The sigmoid colon. This part describes an S-shaped curve in the pelvis then continues downwards to become the rectum The rectum. 10

11 This is a slightly dilated section of the colon about 13 cm long. It leads from the sigmoid colon and terminates in the anal canal. Fig.20.6 Arrangement of muscle fibres in the colon,rectum and anus The anal canal. This is a short passage about 3.8 cm long in the adult and leads from the rectum to the exterior. Two sphincter muscles control the anus; the internal sphincter, consisting of smooth muscle fibres, is under the control of the autonomic nervous system and hence involuntary and the external sphincter, formed by skeletal muscle, is under voluntary control.the four layers of tissue described in the basic structure of the gastrointestinal tract are present in the colon, the rectum and the anal canal. The arrangement of the longitudinal muscle fibres is modified in the colon. Figure Large intestine 11

12 Self Assessment Questions List the parts of the large intestine List the functions of the large intestine Summary In this session you learnt about the organs of the digestive tract their main structural features. You learnt about the main functions of those organs and how these organs coordinate with each other for completion of digestion and promote optimal absorption of nutrients. You learnt about how the secretion of bile and pancreatic juice into the duodenum structure se organs helped in digestion. Learning Outcomes After studying this section, you should be able to List the main organs of the alimentary tract. List the accessory organs of digestion Discuss the digestive functions of the small intestine and its secretions Explain how nutrients are absorbed in the small intestine Identify the different sections of the large intestine Describe the structure and functions of the large intestine, the rectum and the anal canal. 12

13 13