Digestion is the process by which food is broken down mechanically and chemically in the gastro-intestinal tract (digestive tract/alimentary canal).Alimentary canal is about 7.5-9 m (30 ft) long. The tract releases hormones and enzymes to help regulate the digestion. It also helps in mechanical digestion of food via paristalsis. The walls of entire gastrointestinal tract is made up of four layers: serosa, mucularis externa, submucosa and mucosa.Upper alimentary canal consists of oesophagus (esophagus), stomach and duodenum. Lower tract consists of intestines and anal canal.Mechanical and Chemical Digestion in Mouth: Most of mechanical digestion occurs in mouth. It contains modified teeth for mastication (chewing)of food. Tongue and saliva play an important role in mastication and deglutition (swallowing). Human teeth are Diphyodont. Teeth of 1st succession are called deciduous, lacteals or milk teeth.Saliva contains enzymes and ions that help in chemical digestion in mouth and fighting against germs in food. It is normally tasteless, odorless and weekly alkaline. It is secreted by salivary gland (3 pairs in human buccal cavity). Its secretion is stimulated by sight, smell and taste of food and inhibited by Atropine. Secretion of saliva is controlled by both, sympathetic and parasympathetic nervous system. About 1500 ml of saliva is secreted in 24 hours. Destruction of bacteria in food is brought about by ions of thiocyanate and lysozyme in saliva. Lactoferrin and musin also help inhibit pathogens.The enzyme Salivary amylase (ptyalin) digests starch. Its optimum pH is 6.9.Deglutition(Swallowing): Swallowing process is divisible into two phases_Oral phase and Pharygeal phase. Oral phase: Masticated food is rolled into bolus and squeezed posteriorly into the pharyngeal region by pressure of tongue, moving upwards and downwards against the palate. Saliva miostens the food. Pharyngeal phase: After oral phase, swallowing of bolus becomes an involuntary and reflex action triggered by tactile stimulation of palate and pharyngeal wall, controlled by swallowing center located in medulla and pons. Swallowing takes about six seconds. Pharynx is soft cavity behind soft palate (4.5 inches long). From here food is pushed towards oesophagus (gullet).Oesophgus: Oesophagus or food pipe is a muscular tube extending towards stomach. It is about 10 inches or 25 cm long. It lies between trachea and vertebral column (at level of C6). It passes through hole in diaphragm at level of T10 (tenth thoracic vertebra).From here food travels via paristalsis to the stomach.Stomach: Stomach is an elastic, muscular, thick walled J-shaped bag. It is the widest part of digestive system, about 10 inches in length. It is situated below the diaphragm on the left side of abdominal cavity.Folds in the stomach mucosa are called rugae. Muscular rings that control entry and exit of food from stomach are called sphincters. Stomach is composed of cardiac region (nearest to heart), a fundus (round part), a body or corpus (middle portion) and pyrolic region which is smallest distal end.Mucosa layer of stomach is consists of epithelium, lamina propria and mucularis mucosae. Meissner's plexus is in submucosa layer of stomach.The muscularis externa layer of stomach wall is made of three layers: outer layer of longitudinal muscles (Auerbach's plexus is in this layer) middle layer of smooth muscles (circular muscle layer) and inner oblique muscle layer which contains many tubuler gastric glands. Gastric glands secrete gastric juice. Gastric juice is a thin, colourless fluid containing pepsin, hydrochloric acid, mucin, small amount of inorganic salts and intrinsic factor. Its pH is 0.9-2. Gastric juice of infants contains another enzyme "renin" which curdles the milk. The curd is then digested by the pepsin.Pepsin converts protein into peptides. H+ ions from HCl converts inactive pepsinogen into active enzyme pepsin. HCl also helps in killing bacteria in food.Parietal cells (present in body of gastric glands) secrete HCl and intrinsic factor, cheif cells (zymogenic cells) present in base of gastric glands, secrete pepsinogen and renin. Goblet cells secrete musin. A layer of mucus along the inner wall of stomach protects its lining from acidic environment within it. Intrinsic factor in gastric juice makes the absorption of vitamin B12 possible.Pyloric region of stomach secrete hormones like gastrin, somatostatin, cholecystokinin, serotonin and endorphins. Gastrin stimulates gastric juice secretion and induces pancreatic secretion and gallbladder empting. Another hormone called epinephrine (from adrenal gland) stimulates secretion of gastrin.Gastric inhibitory peptide (GIP), calcitonin, somatostatin, secretin and glucagon inhibit secretion of Gastrin. Gastric juice changes the solid food into semifluid chyme. Chyme leaves stomach in 3-4 hours. Small intestine: Stomach opens into small intestine. The small intestine is longer (5.5-6m) than large intestine but its diameter is smaller than that of large intestine (2.5-3 cm).It is elongated narrow and coiled tube.Glands lining the small intestinal wall secrete intestinal juice (succus entericus) It is divided into three parts: Duodenum: It is 12 inches long and curved like "C". It recieves chyme from stomach.Muscularis externa layer of duodenum (like that of ilium and jejunum) contains Auerbach's plexus. Submucosa layer of doudenum contains Meissner's plexus and Brunner's glands. Brunner's glands secrete mucus rich alkaline secretion. Epithelium of mucosa layer has Goblet cells and Paneth cells. In duodenum chyme meets bile (from liver) and pancreatic juice. Liver:Liver is lrge reddish brown gland. It is located in right upper quadrant of abdominal cavity resting below the diadiaphragm. It weighs 1.4-1.6 kg (3.2-3.7 lb). It is covered with sheath called Glisson's capsule.The liver produces green alkaline liquid (pH:8), bile. The hormone called Secretin stimulates secretion of bile. The bile contains water, NaHCO3, calcium and bilirubin. Bilirubin is orage/yellow in color but color of bile is green because of oxidised bilirubin.Bile is secreted by hepatocyst cells of liver and is stored in gallbldder. The hepatic duct (of liver) and cystic duct (of gallbladder) form common bile duct. The common bile duct joins pancreatic duct and forms ampulla of vanter (hepatopancreatic ampulla). Finally,the bile from liver and pancreatic juice from pancreas enter doudenum through shpincter of oddi if it is opened. Opening of sphincter of oddi is controlled by hormone, Cholecystokinin. Cholecystokinin causes contraction of gallbladder and relaxation of sphincter of oddi. Somatostatin inhibits release of cholecyctokinin when gallbladder empting is not required.Bile neuteralizes acidity of chyme. It kills germs in food. It emulsifies fats. Molecules of bile have dual nature (one end is attracted towards water molecule while other is attracted towards fat molecule). Thus its molecules remain in between the fat molecules and water molecules and inhibits formation of large masses of fats. If bile molecules were not present between fats and water molecules, the fat molecules would be repelled by water molecules and pushed towards each other. The fat molecules would merge together making it difficult for lipase to digest fats. Thus bile is necessary for lipase activity.About 1000 ml of bile is secreted by liver each day. Pancreas: A leaf like narrow gland that lies in the epigastrium and left hypochondrium areas of the abdomin. It secretes pancreatic juice. Pancreatic juice is alkaline (pH 8.8) thus reduces acidity of chyme. Pancreatic juice secretion is regulated by the hormone sacretin and cholecystokinin. Somatostatin reduces its secretion. Pancreatic juice contains variety of enzymes including trypsinogen, chymotrypsinogen, elastase, carboxypeptidasse, lipase and pancreatic amylase (amylopsin). Inactive enzyme trypsinogen is converted int o active trypsin by duodenal enzyme Enterokinase(also called eneropeptidase). Inactive chymotrypsinogen enters duodenum. On cleavage by trypsin it becomes chymotrypsin. Enzyme elastase breaks down elastin, an elastic fiber together with collagen. Carboxypeptidase hydrolyzes peptide bond of amino acid residue at carboxy terminal.Trypsin, chymotrypsin, elastase and carboxypeptidase are proteolytic enzymes and act on proteins which have escaped the action of stomach.Lipase acts on emulsified fats and converts them into fatty acids and glycerol. Amylase acts on sugars which have escaped from the action of salivary amylase.Pancretic duct (aslo called duct of wirsung) joins common bile duct forming ampulla of vanter, then pancreatic juice leaves ampulla of vanter through sphincter of oddi.Some people have additional (accessary) pancreatic duct called duct of Santorini. Jejunum: It is middle portion(2.5m) of small intestine. Jejunum contains very few Peyer's patches(found in ileum) and brunner's glands (found in duodenum). pH in jejunum is between 7 and 9. If the jejunum is impacted by blunt force, emesis (vommiting)reflex will be initiated. Ileum (distal intestine or posterior intestine): Ileum is final section of small intestine. It is 2.4m long. Lamin propria of mucosa contains unencapsulated lymphoid nodules, the Peyer's patches (or aggregated lymphoid nodules). Intestinal glands of ileum produce enzyme including erepsin, maltase, sucrase and lactase. Erepsin splits peptides into amino acids.Maltase converts maltose into glucose. Sucrase converts sucrose into fructose and glucose. Lactase reduces lactose into glucose and glactose. Digestion of food is completed in small intestine.Absorption of Food: Absorption is the passage of simple sugars, fatty acids, glycerol, amino acids, vitamins, minerals and water into the circulatory system. Small intestine is the site where most of absorption occurss. Mucosa of small intestine is covered in wrinkles or folds called plicae circulares (are circular folds also called valves of kerckring or valvulae conniventes). The circular folds slow the passage of food along the intestine and afford an increased surface for absorption.They are covered with small fingerlike projections called villi (singlur:villus). Each villus, in turn, is covered with microvilli. Microvilli absorb nutrients from chyme.Each villus has a network of capillaries and fine lymphatic vessels called lacteals. These capillaries join up to form hepatic portal vein that carries blood along with nutrients to the liver. Simple sugars, amino acids, vitamins, minerals and water enter capillaries while fatty acids and glycerol are absorbed into the lymph vessels or lacteals which pour them into the blood stream. Iron is absobed in duodenum. Vitamin B12 and bile salts are absorbed in terminal ileum. Water and lipids are absorbed by passage diffusion throughout the small intestine. Sodium bicarbonate is absorbed by active transport. Glucose and amino acids are absorbed by co-transport. Fructose is absorbed by facilitated diffusion.Large intestine: