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The Digestive System Is the system responsible for: 1. Ingestion of food 2. Digestion of food 3. Absorption of food 4. Egestion (Elimination) of food The Digestive System The digestive organs • • • • • • The mouth The pharynx The esophagus The stomach The small intestine The large intestine The accessory organs • The salivary glands • The liver • The pancreas T H E D I G E S T I V E S Y S T E M Digestion: is the breakdown of the complex food molecules into simpler molecules that could be absorbed by the body. Digestion Mechanical (Physical) Chemical (enzymes) Mechanical digestion: includes the breakdown of the food Without any change in its molecular structure i.e.: smaller particles Chemical digestion: Bigger molecules are dismantled into smaller molecules which are different in structure The optimal functioning of digestive enzymes depends on: 1. The temperature of the medium 2. The pH of the medium The diagram to the right shows the optimal pH values for pepsin, salivary amylase, and trypsin respectively. * In case of an increase in the temperature above a certain limit, the enzyme will be denatured and loses its function. The figure below explains this concept. The Digestive System of Humans: The Mouth (Oral Cavity): The following processes take place inside the mouth: Mastication (chewing) of food Mixing masticated food with saliva Allowing for the sense of taste Initiation of swallowing by the tongue Oral Cavity 2. Saliva: secreted by the salivary glands, it is slightly alkaline and, includes salivary amylase as well as mucus. Functions of saliva: • Moistens the food for easy swallowing • Dissolves the food for discerning taste by the tongue • Contains salivary amylase which digests starch into disaccharides • Act as a natural cleanser for the Parotid gl. teeth (isn’t a substitute for toothpaste and a toothbrush though) Submaxillary gl. Sublingual gl. Three pairs of the salivary glands 3. The Tongue: This diagram shows the structure of a papilla as well as one of the taste buds along the sides of the papilla. The sensory nerve endings end in between the sensory taste cells of the taste bud. The dissolved food enters into the taste bud through the taste pore. The pharynx: The base of the tongue plus the muscular pharynx result in the involuntary reflex of deglutition (swallowing) of food towards the esophagus. Process of deglutition using the base of the tongue and Pharynx (bolus in blue) The Esophagus: • Is a muscular tube that conducts the food to the stomach by peristalsis (slow rythamic contractions) • Its cells also secrete mucous that helps as a lubricant to move the food The Stomach: • Acts as a storage for food • Is made of three muscle layers, from the outside to the inside are the longitudinal muscle layer, the circular muscle layer and the oblique muscle layer • The inner lining of the stomach is called the mucosa it contains the glands of the stomach and is corrugated forming the rugae • Has two sphincters, the cardiac sphincter and the pyloric sphincter • Secretes gastric juice The Functions of the stomach: 1. Mechanical (physical) digestion: through churning food into Chyme 2. Controlling the passage of food: through the two sphincters, contraction of the cardiac sphincter closes the opening to the stomach located near to the heart, while its relaxation allows food to enter. A second sphincter, the pyloric sphincter, regulates the movement of food and stomach acid to the small intestine 3. Production of gastric juice: * HCl → breaks down the food → kills bacteria → converts pepsinogen into pepsin * Pepsinogen HCl Pepsin → incomplete digestion of proteins * Mucus → protects the stomach inner walls from the acid The Small Intestine and Pancreas: • Digestion and absorption take place in the small intestine • The small intestine is a muscular tube extending form the pyloric sphincter to the ileocecal sphincter about 6-7 metres long • It is made of three segments namely, the duodenum, jejunum, and the ileum • The duodenum is only 20-30 cm long • The duodenum receives enzymes secreted from the liver and pancreas Adjusting the small intestine’s pH (function of secretin): The Pancreas: Secretes the following important enzymes: 1. Trypsinogen 2. Erepsin 3. Pancreatic amylase 4. Pancreatic lipase Digestion of proteins: • Starts in the stomach Proteins Pepsin • Continues in the small intestine Digestion of Carbohydrates: • Starts in the mouth Carbohydrates Salivary Amylase Disaccharides • Continues in the small intestine Carbohydrates Pancreatic Amylase Disaccharidases Disaccharides Disaccharides Monosaccharides Digestion of Lipids: • In the small intestine Fats Bile Salts Small fat droplets Pancreatic lipase Small fat droplets glycerol + Fatty acids The liver and the gall bladder: • The liver is the largest gland of the body • The gall bladder is a reservoir for storing Bile secreted by the liver • Bile comprises, bile salts, phospholipids, cholesterol and bilirubin (a pigment that results from the breakdown of RBCs) • The liver has many functions, one of these functions is the emulsification of fat (breaking large fat globules into smaller fat droplets) The hormone cholecystokinin and its role in releasing bile salts: Functions of the liver Liver and Gall Bladder Problems: 1. Gall bladder stones: Cholesterol, an insoluble component of bile, acts as a binding agent for the salt crystals found in bile. The crystals precipitate and form larger crystals called gallstones. Gallstones can block the bile duct, impairing fat digestion and causing considerable pain. 2. Jaundice (Icterus): Bile contains a pigment called bilirubin which results from the breakdown of haemoglobin, any obstruction of the bile duct or accelerated destruction of red blood cells can cause jaundice, turning skin and the eye white yellow in color. 3. Liver Cirrhosis: Alcohol, like many other harmful agents, can destroy liver tissue. Damaged liver cells are replaced by connective tissue and fat, which are not able to carry out normal liver duties. This condition, which can also result from nutritional deprivation or infection, is referred to as cirrhosis of the liver. Digestion in the small intestine: Mechanical digestion in the small intestine: includes what is called segmentation which is the movement of the food back and forth (different than peristalsis which is mainly for the propulsion of the food forwards) Chemical digestion in the small intestine: through the secretions received from the pancreas and the liver as well as other secretions by the glands of the small intestine’s mucosa, such as erepsin. Homeostatic control of digestion: 1. Gastrin (stomach): is produced when partially digested proteins are present in the stomach, gastrin stimulates the release of gastric juices to digest proteins. 2. Secretin (small intestine): stimulates the pancreas to produce HCO3 and adjust the pH from 2 to 9 3. Cholecystokinin (small intestine): stimulates the release of bile salts from the gall bladder 4. Enterogasterone (small intestine): If the meal is fatty, the small intestine secretes a digestive hormone (enterogasterone) that slows peristaltic movements, allowing time for fat digestion and absorption. If the meal is fatty, the small intestine secretes a digestive hormone (Enterogasterone) that slows peristaltic movements, allowing time for fat digestion and absorption. Absorption of Materials: The small intestine: • The absorption of food takes place in the small intestine • Long fingerlike tubes called villi (singular: villus) greatly increase the surface area of the small intestine. The cells that make up the lining of each villus have microvilli, which are fine, threadlike extensions of the membrane that further increase the surface for absorption. • Villi: small fingerlike projections that extend into the small intestine which increase surface area for absorption • Microvilli: microscopic fingerlike outward projections of the cell membrane • Each villus is supplied with a capillary network which intertwines with lymph vessels called lacteals that transport materials. Some nutrients are absorbed by diffusion, but some nutrients are actively transported from the digestive tract. Carbohydrates and amino acids are absorbed into the capillary networks; fats are absorbed into the lacteals. The role of the large intestine in absorption and egestion: • The large intestine consists of the ascending colon, the transverse colon, the descending colon, the sigmoid colon and the rectum • Chemical digestion is complete by the time food reaches the large intestine. • The colon stores wastes long enough to reabsorb water • The large intestine houses bacteria (E.Coli) that use waste materials to synthesize vitamins (B and K). • Most absorption takes place in the small intestine, however reabsorption of water plus the absorption of minerals and vitamins takes place in the large intestine • Defecation reflex takes place when wastes build up in the large intestine, receptors in the wall of the intestine provide information to the central nervous system which, in turn, prompts a bowel movement. The bowel movement ensures the removal of potentially toxic wastes from the body. Individuals who do not eat sufficient amounts of cellulose (roughage or fibre), have fewer bowel movements. Essential Nutrients 1. The essential amino acids are the eight amino acids that cannot be synthesized by your body and must be ingested. • A common source of essential amino acids is animal products. • The essential amino acids can be obtained in vegetarian diets by eating complementary foods. 2. Vitamins are either water soluble or fat soluble. • Water-soluble vitamins dissolve in the blood and are excreted in urine. Thus they cannot be stored and must be consumed daily. • Fat-soluble vitamins can be stored in fatty tissue. Excessive amounts of fat-soluble vitamins can build up in the body and cause problems. 3. Antioxidants can make free radicals less toxic. • Free radicals are created in the body in response to things like sunlight, X-rays, pollution, etc. • Free radicals may contribute to cancer and other disorders.