GIT Lecture Notes 1 PDF

PHYSIOLOGY OF THE GASTROINTESTINAL SYSTEM Gastrointestinal physiology is the branch of human physiology that deals with the physical function of the gastrointestinal tract (GI tract). The function of the GI tract is to process ingested food by mechanical and chemical means, extract...

PHYSIOLOGY OF THE GASTROINTESTINAL SYSTEM Gastrointestinal physiology is the branch of human physiology that deals with the physical function of the gastrointestinal tract (GI tract). The function of the GI tract is to process ingested food by mechanical and chemical means, extract nutrients and waste products. Anatomically and functionally, the gastrointestinal system can be divided into the tubular gastrointestinal (GI) tract, or alimentary canal, and accessory digestive organs. The organs of the GI tract include the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine (fig. 1). The accessory digestive organs include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. Figure 1: The organs of the digestive system. The digestive system includes the gastrointestinal tract and the accessory digestive organs The main functions of the gastrointestinal system are: (1) digestion of nutrients (2) absorption of nutrients from the gut lumen into the bloodstream. In order to perform these functions, the GI system carries out certain activities. These activities can be broadly grouped into: (i) Motility: This refers to the movement of food through the digestive tract through the processes of a. Ingestion: Taking food into the mouth. b. Mastication: Chewing the food and mixing it with saliva. c. Deglutition: Swallowing food. d. Peristalsis: Rhythmic, wavelike contractions that move food through the gastrointestinal tract. (ii) Secretion: This includes both exocrine and endocrine secretions. a. Exocrine secretions: Water, hydrochloric acid, bicarbonate, and many digestive enzymes are secreted into the lumen of the gastrointestinal tract. The stomach alone, for example, secretes 2 to 3 liters of gastric juice a day. b. Endocrine secretions: The stomach and small intestine secrete a number of hormones that help to regulate the digestive system. (iii) Digestion: This refers to the breakdown of food molecules into their smaller subunits, which can be absorbed (iv) Absorption: This refers to the passage of digested end products into the blood or lymph. (v) Storage and elimination: This refers to the temporary storage and subsequent elimination of indigestible food molecules. FUNCTIONAL ANATOMY OF THE GASTROINTESTINAL TRACT The GI tract from the esophagus to the anal canal is composed of four layers, or tunics. Each tunic contains a dominant tissue type that performs specific functions in the digestive process. The four tunics of the GI tract, from the inside out, are the mucosa, submucosa, muscularis, and serosa (fig. 2a). Mucosa The mucosa, which lines the lumen of the GI tract, is the absorptive and major secretory layer. It consists of a simple columnar epithelium supported by the lamina propria, a thin layer of areolar connective tissue containing numerous lymph nodules, which are important in protecting against disease (fig. 2b). External to the lamina propria is a thin layer of smooth muscle called the muscularis mucosae. This is the muscle layer responsible for the numerous small folds in certain portions of the GI tract. These folds greatly increase the absorptive surface area. Specialized goblet cells in the mucosa secrete mucus throughout most of the GI tract. Figure 2: The layers of the digestive tract. (a) An illustration of the major tunics, or layers, of the small intestine. The insert shows how folds of mucosa form projections called villi in the small intestine. (b) An illustration of a cross section of the small intestine showing layers and glands Submucosa The relatively thick submucosa is a highly vascular layer of connective tissue that serves the mucosa. Absorbed molecules that pass through the columnar epithelial cells of the mucosa enter into blood and lymphatic vessels of the submucosa. In addition to blood vessels, the submucosa contains glands and nerve plexuses. The submucosal plexus (Meissner’s plexus) (fig. 2b) provides an autonomic nerve supply to the muscularis mucosae. Muscularis The muscularis (also called the muscularis externa) is responsible for segmental contractions and peristaltic movement through the GI tract. The muscularis has an inner circular and an outer longitudinal layer of smooth muscle. Contractions of these layers move the food through the tract and physically pulverize and mix the food with digestive enzymes. The myenteric plexus (Auerbach’s plexus), located between the two muscle layers, provides the major nerve supply to the GI tract. It includes fibers and ganglia from both the sympathetic and parasympathetic divisions of the autonomic nervous system. Serosa The outer serosa completes the wall of the GI tract. It is a binding and protective layer consisting of areolar connective tissue covered with a layer of simple squamous epithelium. INNERVATION OF THE GASTROINTESTINAL TRACT The digestive system is regulated extrinsically by the autonomic nervous system (sympathetic and parasympathetic nerves), and intrinsically by the enteric nervous system. The wall of the GI tract contains many neurons that are highly interconnected. A dense network of nerve cells in the submucosa forms the submucosal plexus (Meissner's plexus). Between the circular and longitudinal muscle is the myenteric (i.e. gut muscle) plexus (Auerbach's plexus). The submucosal and myenteric plexuses, together with other neurons of the GI tract constitute the enteric nervous system which helps to integrate the motor and secretory activities of the GI system. The GI tract is also supplied by sympathetic and parasympathetic nerves. Most of the sympathetic fibres terminate in the submucosal and myenteric plexuses. Stimulation of the sympathetic nerves usually inhibits the motor and sensory activities of the GI system. Some sypathetic nerves innervate blood vessels of the GI tract causing vasoconstriction when activated while other sympathetic nerves supply glandular structures in the wall of the gut. Parasympathetic nerves supply the GI tract from the mouth to the anus. From the mouth to the transverse colon is supplied by the vagus nerve while the remainder of the colon, the rectum and the anus receive parasympathetic fibres from the pelvic nerves. Stimulation of the parasympathetic nerves usually stimulates the motor and secretory and activity of the gut.