Digestive System • The digestive system or the GI tract starts with the mouth and proceeds to the phyrynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), and then to the large intestine (colon), rectum, and terminates at the anus. • Accessory Organs includes: Salivary glands, Tongue, Teeth, Liver, Gallbladder. • The digestive System must accomplish the following tasks: • Ingestion • Digestion: Physical or Chemical • Absorption • Egestion Oral cavity • Teeth: grind and break down food to expose food to a large surface area for enzyme action. • Tongue: Rolls food into bolus (a ball of solid mass of food) before swallowing. • Salivary gland: secrets saliva in the buccal cavity. • Saliva: A clear, tasteless, odorless, slightly acid (pH 6.8) viscid fluid, consisting of the secretion from the parotid, sublingual, and submandibular salivary glands and the mucous glands of the oral cavity. Function of saliva 1- Preparation of food for swallowing Mucin acts as a good lubricant for swallowing. Mucin softens the mass of food. Saliva moistens the mouth and facilitates chewing & mixing of food and thus in bolus formation & deglutition. 2- Solvent actions Taste is a chemical sensation. Unless the substance be in solution the taste buds cannot be stimulated. Saliva acts as a solvent. 3- Antibacterial or cleansing action Constant flow of saliva removes bacteria from any area where bacteria can grow. Saliva contains several factors that can destroy bacteria like several proteolytic enzymes like lysozyme, thiocyanate ions. Saliva often contains significant amount of protein antibodies that can destroy oral bacteria including those that cause dental caries. 4- Speech Moistening of lips and mucous membranes by saliva aids in speech. Decrease in salivary secretion cause impairment of speech. 5- Digestive function : Saliva contain digestive enzymes such as salivary amylase which aids a chemical breakdown of polysaccharides to disaccharides. ex. starch to maltose Also saliva contain mucous , a glycoprotein , which help in make a bolus. Presence of lingual lipase in very small amount. 6- Excretory function: Many substances both organic and inorganic are secreted to saliva. In diabetes mellitus, glucose is secreted. In Nephritis, urea is secreted. Viruses of rabies and poliomyelitis. Alkaloids like morphine and antibiotics. Alcohol content of saliva has been used for medicolegal purpose (alcohol test). 7- Regulation of water balance When liquid is lost from the body, salivary glands are subjected to dehydration effect. This arouses a sense of thirst. 8- Helps in heat loss Frothing found in animals (dog, sheep, etc.). 9- Removes irritating factor Saliva helps to remove the irritating factor by diluting or neutralising it. It thus prevents injury to mucous membranes. Applied Decrease or absence of salivary secretion Xerostomia Excessive salivary secretion Sialorrhea (seen in patients with cancer of esophagus). Pharynx Divided into three regions; nasopharynx, the oropharynx, and the laryngopharynx. The pharynx plays a role in both respiratory as well as digestive system. Function of the Pharynx: • The pharynx is a common passageway for air and food, which is why pharynx has a dual role. • The pharynx then opens into two pathways, one that leads to the esophagus or food passage and the other trachea or air passage. Pharynx Function in Digestive System • The moistened food bolus is moved to the back of the mouth by the tongue and enters the pharynx. • Here muscle contractions (circular muscle constrictions) take place and the swallowing reflex is triggered. • Part of the swallowing action takes place as a reflex action, while part of it is under voluntary control. • The food bolus is then pushed toward the esophagus, which is a muscular tube extending from the esophagus. This swallowing reflex prevents the food from entering in the wind pipe or trachea. • The flap like epiglottis covers the larynx which prevent food from enters the trachea. The contraction of the longitudinal muscles in the walls of the pharynx lifts the walls of the pharynx during swallowing. Pharynx Functions • • Vocalization The process by which humans are able to make vocal sounds and speak is called vocalization. When air passes through the pharynx and then into larynx, it causes the vocal cords in the larynx to vibrate, thereby helping in production of sound, which is used for speech in humans. • Equalizing Air Pressure in Middle Ear Auditory tubes connect the middle ear to the pharynx at an opening in the nasopharynx region. This opening opens and closes, thereby equalizing air pressure in the middle ear to that of the outside atmosphere, so that sound conduction takes place properly. Esophagus • The esophagus is a narrow muscular tube about 20-30 cm long which starts from the end of pharynx, passes through the thorax, and ends at the beginning of stomach. • There are a number of glands lining the inner wall of the esophagus that help in keeping the passage moist and also facilitate in swallowing. • The esophagus is playing a major role in transporting food, saliva and liquids to the stomach. This function is carried out by a muscle layers, lining the wall of the esophagus, called sphincters. • There are two parts of the sphincters, the upper esophageal sphincter and lower esophageal sphincter. The upper sphincter is usually closed, but opens when food or liquid is swallowed. • Food in the esophagus is carried to the stomach by peristalsis, a process where muscles contract to push food through the esophagus to the stomach. • Sphincters also release certain enzymes that help in the semi digestion of food in the esophagus. • Even if the person is lying down or is upside down, sphincters enable the food to be pushed into the stomach. Abnormalities of Esophagus Function over consumption of carbonated drinks, spicy food, etc. can lead to esophagus pain. Symptoms that appear during abnormal esophagus function are chest pain, dysphagia or difficulty in swallowing and heartburn. Another disorder that affects the esophagus function is acid reflux disease. The stomach releases certain acids during digestion. When the acid content increases, due to over consumption of spicy or hot food stuff, it irritates the lining of the esophagus. This occurs when the lower esophageal sphincter that separates the stomach from the lower esophagus does not function properly. Stomach • The human stomach is located on the upper left region of the abdominal cavity and is a vital part of the digestive system. • It is a muscular and hollow structure. It is preceded by the esophagus and followed by duodenum. • Two valves control the entrances of food into and out of the stomach 1- Esophageal Sphincter - opening between esophagus and stomach. It stops the acid in the stomach from flowing back up into the esophagus. 2- Pyloric Sphincter – when relaxes, it opens the entrance to the small intestine. • A layer of mucus prevents the stomach from digesting itself. • Food in the stomach is converted into a thick acidic liquid called chyme, which then moves into the small intestine. Stomach Structure • Parts of the Stomach: The stomach has five parts, which are named as cardia, fundus, corpus (body), antrum and pylorus. • Stomach Secretions: • The inner surface of the stomach has different types of epithelial cells that produce various secretions which aid the process of digestion. • This includes mucus cells that produce mucus, which is alkaline in nature. This mucus prevents harm to the inner layer of the stomach from acids. • There are chief cells that produce an enzyme called pepsin (aids in the stomach function of breaking down proteins) • parietal cells that produce hydrochloric acid (fights microorganisms and digests food). • G cells also produce a hormone called gastrin. What is the Function of the Stomach? • The main function of the stomach is to breakdown the large molecules into small parts to be easily absorbed. • The chewed food reaches the stomach and gets mixed with the acids and enzymes produced in the organ. This mixture is called chyme, which is stored in the stomach and is released to the small intestine in smaller amounts. • The stomach is the organ that is an important part of the digestive system. This is the organ that is responsible for digesting the food that is carried from the mouth to the stomach through the esophagus. • The enzymes that are present in the stomach which digest the food require an acidic pH to function properly. This is where the hydrochloric acid in stomach comes into play. • The main function of hydrochloric acid in stomach is to provide an optimum pH for normal functioning of the enzymes present there. • For example, hydrochloric acid helps convert pepsinogen to pepsin, (which is responsible for breaking down proteins in the stomach). • How Does the Stomach Prevent Itself from Digestion? • There is a thick mucus protective layer which prevents HCl from harming the cells of the stomach. Furthermore, sodium bicarbonate buffering aids in preventing damage to the cells of the stomach. Movements in Stomach Small Intestine • Has a tiny projections called villi which absorb the digested food into the capillaries. • Participate in digestion of proteins, lipids and carbohydrates partially. • Divided into three parts: 1. Duodenum – upper part; connected to the stomach. The duodenum is also where the bile and pancreatic juices enter the intestine. 2. Jejunum. 3. Ileum. Movement in small intestine: • • • • • Mixing: Segmental contraction that occurs in small intestine Secretion: Lubricate, liquefy, digest Digestion: Mechanical and chemical Absorption: Movement from tract into circulation or lymph Elimination: Waste products removed from body Large Intestine • The large intestine (colon) extends from the end of the ileum to the anus. • divided into the cecum, colon, rectum, and anal canal. • There are different kinds of bacteria in the colon, most of them are helpful, where they can synthesis vitamins as B12 and K. also they can breakdown toxins and prevent the proliferation of harmfull organisms. They also stimulate the immune system to produce short chain fatty acids which can prevent the incidence of colon cancer. • Water is absorbed from the undigested food making the waste harder until it becomes solid. Accessory Organs Pancreas • It has a dual function: 1. Exocrine: where it produce enzymes that break down all categories of digestible foods (trypsinogen, chymotrypsinogen, pancreatic lipase, and amylase ). 2. Endocrine: where it secrete hormones by islets of Langerhans as follow: • Beta cells--- producing insulin. • Alpha cells--- producing glucagon. • Delta cells--- producing somatostatin. • PP cells--- producing pancreatic polypeptide. • Gall bladder • It is connected to the liver and the duodenum by biliary tract. • Stores bile, which is released when food containing fat enters the digestive tract, stimulating the secretion of cholecystokinin (CCK). • Bile is an emulsifier NOT an enzyme. Emulsifier – dissolves fat into the watery contents of the intestine. Functions of liver 1. The liver produces and excretes bile requires for dissolving fats. Some of the bile drains directly into the duodenum, and some is stored in the gallbladder. 2. The liver performs several roles in carbohydrate, protein and lipid metabolism. 3. The liver produces coagulation factors I (fibrinogen), II (prothrombin), V, VII, IX, X and XI, as well as protein C, Protein S and antithrombin. 4. The liver breaks down hemoglobin, creating metabolites that are added to bile as pigment. 5. The liver breaks down toxic substances and most medicinal products in a process called drug metabolism. 6. The liver converts ammonia to urea. 7. The liver stores a multitude of substances, including glucose in the form of glycogen, vitamin B12, iron, and copper. 8. In the first trimester fetus, the liver is the main site of red blood cell production. 9. The liver is responsible for immunological effects the reticuloendothelial system if the liver contains many immunologically active cells, acting as a 'sieve' for antigens carried to it via the portal system. Path of Digestion • • • • • • • Mouth Pharynx Esophagus Stomach Small Intestine Large Intestine Anus ABSORPTION AND TRANSPORT OF NUTRIENTS • Digested molecules of carbohydrates, proteins, lipids and vitamins as well as water and minerals from the diet, are absorbed from the cavity of the upper small intestine. Most absorbed materials cross the mucosa into the blood and are carried off in the bloodstream to other parts of the body for storage or further chemical change. How is the digestive process controlled? The hormones that control digestion are: •Gastrin causes the stomach to produce an acid for dissolving and digesting some foods. It is also necessary for the normal growth of the lining of the stomach, small intestine, and colon. •Secretin causes the pancreas to send out a digestive juice that is rich in bicarbonate. It stimulates the stomach to produce pepsin, an enzyme that digests protein, and it also stimulates the liver to produce bile. •Cholecystokinin (CCK) causes the pancreas to grow and to produce the enzymes of pancreatic juice, and it causes the gallbladder to empty. Nerve regulations • Two types of nerves help to control the action of the digestive system: 1. Extrinsic nerves cause muscle contractions that push food along the digestive tract. They also cause the stomach and pancreas to produce more digestive juice. 2. Intrinsic nerves within the walls of the esophagus, stomach, small intestine, and large intestine react when their walls are stretched by food. They cause the release of substances that speed up or slow down the movement of food, and the production of juices, by the digestive organs.
