The Digestive System PDF

Topic 10 The Digestive System This prepares food for consumption by the cells through 5 basic activities: 1) Ingestion – taking food in the body (eating) 2) Movement of food along the digestive tract 3) Digestion – the breakdown of food by both chemical and mechanical processes...

Topic 10 The Digestive System This prepares food for consumption by the cells through 5 basic activities: 1) Ingestion – taking food in the body (eating) 2) Movement of food along the digestive tract 3) Digestion – the breakdown of food by both chemical and mechanical processes 4) Absorption – the passage of digested food from the digestive tract into the cardiovascular and lymphatic systems for distribution to cells 5) Defaecation – the elimination of indigestible substances from the body The organs of digestion are traditionally divided into 2 groups: 1. Gastrointestinal Tract: This is a continuous tube running from the mouth to the anus and it consists of the mouth, pharynx, oesophagus, stomach, small intestine, and large intestine. 2. Accessory Structures: These are the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. Muscular contraction happens in the GI tract wall, which breaks down food physically by churning it. Secretions produced by cells along the tract break down the food chemically. The teeth aid in the physical breakdown of food. The other accessory organs produce or store secretions that aid in the chemical breakdown of food. These are released into the tract via ducts. Regulate Food Intake There are 2 centres related to food intake located in the hypothalamus. These act together to regulate the amount of food ingested: 1. Feeding Centre – the hunger centre that is constantly active 2. Safety Centre – a centre that inhibits the hunger centre Blood Glucose Levels: Body temperature: - A cold environment enhances eating. - A warm environment depresses it. Distension of the gastrointestinal tract, especially the stomach and duodenum: When these organs are stretched, a reflex is initiated that activates the satiety centre and depresses the feeding centre.  Cholecystokinin (CCK): when fat enters the small intestine, CCK is released and this inhibits eating. General Histology of the GIT (gastrointestinal tract) 4 coats of the tract from the inside out are the: 1. Mucosa 2. Submucosa 3. Muscularis 4. Serosa or Adventitia There is another layer that is the muscularis mucosae. The Mucosa: - Inner lining of the tract. - It is a mucous membrane attached to a thin layer of visceral muscle. - The membrane is composed of two layers: a lining epithelium – that offers protection and absorption – and nonkeratinized cells that are stratified in the mouth and the oesophagus but are simple throughout the rest of the tract. - The lamina propria is an underlying layer of loose connective tissue that contains many blood and lymph vessels and scattered lymphatic nodules. This layer supports the epithelium, binds it to the muscularis mucosae and provides it with a blood and lymph supply. The Submucosa: - This consists of loose connective tissue that binds the mucosa to the muscularis mucosae. - It is highly vascular and contains a portion of Meissner’s plexus that is important in controlling secretions by the GIT. The Muscularis: - In the mouth, pharynx, and oesophagus the muscularis is in part made up of skeletal muscle that produces voluntary control. - It consists of smooth muscle that is found in the inner ring of circular fibres and the outer sheet of longitudinal fibres. - Also contains the myenteric plexus (Auerbach’s plexus) that controls GI motility. The Serosa: - A serous membrane composed of connective tissue and epithelium. Summary: - 2 major networks of nerve fibres: 1) The myenteric plexus (Auerbach’s plexus) – lies between the outer longitudinal and middle circular muscle layers. 2) The submucous plexus (Meissner’s plexus) – lies between the middle circular layer and the mucosa. - The intestine receives a dual extrinsic innervation from the autonomic nervous system:  Parasympathetic cholinergic activity – increases the activity of intestinal smooth muscle. Sympathetic noradrenergic activity – decreases the activity of intestinal smooth muscle and causes sphincters to contract. Mastication - Breaks up large food particles - Mixes the food with secretions of the salivary glands (wetting and homogenising action aids subsequent digestion). There are 3 pairs of salivary glands: 1) Parotid glands 2) Submandibular glands 3) Sublingual glands Saliva is a fluid that is continuously secreted by glands in or near the mouth. The major portion of saliva is secreted by salivary glands. The secretory (zymogen) granules containing the salivary enzymes are discharged from the acinar cells into the ducts. The pH of saliva is about 7.0, chemically, saliva is 99.5% water and 0.5% solutes. The water is to dissolve the food and among the solutes are salts (chlorides, bicarbonates, phosphates of sodium and potassium), dissolved gases and various organic substances (urea, uric acid, serum albumin, globulin, mucin (protein), lysosome, salivary amylase. Functions of Saliva: - Facilitates swallowing - Keeps the mouth moist - Serves as a solvent for the molecules that stimulate the taste buds - Aids speech by facilitating movements of the lips and tongue - Keeps mouth and teeth clean Control of salivary secretion: Salivary secretion is under neural control Stimulation of the parasympathetic nerve supply - Pronounced vasodilation in the salivary glands - Profuse secretion of watery saliva with a relatively low content of organic material Stimulation of the sympathetic nerve supply - Vasoconstriction in the salivary glands - Secretion of small amounts of saliva rich in organic constituents from the submandibular glands Swallowing (Deglutition) is divided into 3 parts: 1) Voluntary stage – the bolus is moved into the oropharynx - Tongue moves upward and backwards against the palate - Bolus is forced to the back of the mouth cavity and into the oropharynx 2) Pharynegeal stage – the involuntary passage of the bolus through the pharynx into the oesophagus  - The involuntary pharyngeal stage of swallowing begins with the passage of the bolus into the oropharynx o Respiratory airways close and breathing is temporarily interrupted o Bolus-stimulated receptors in the oropharynx, which send impulses to the deglutition centre in the medulla and lower pons of the brainstem o The returning impulses cause the soft palate and uvula to move upward to close the nasopharynx o The larynx is pulled forward and upward after the tongue. As the larynx rises it meets the epiglottis which seals off the glottis o The movement of the larynx also pulls the vocal cords together, further sealing off the respiratory tract, and widens the opening between the laryngopharynx and oesophagus o The respiratory passages then reopen and breathing resumes 3) Oesophageal stage – the involuntary passage of the bolus through the oesophagus into the stomach - During the oesophageal stage of deglutition, food is pushed through the oesophagus by involuntary muscular movements called peristalsis. - Peristalsis is a function of the muscular under the control of the medulla oblongata Oesophagus – the third principal organ involved in swallowing and a muscular, collapsible tube that lies behind the trachea Upper GIT – begins at the end of the laryngopharynx. Passed through the mediastinum anterior to the vertebral column. Pierces the diaphragm. Terminates in the superior portion of the stomach. Functions of Oesophagus: - Transports food to stomach - Does not produce digestive enzyme - Does not carry on absorption - Secrete mucus