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Human Digestive System

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Published in: Biology
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Detailed notes on Human Digestive System for students of Class XI Biology.(CBSE and ISC)

Mrinmoy G / Kolkata

16 years of teaching experience

Qualification: M.Sc (University of Burdwan, Bardhaman - 2002), B.Ed (University of Burdwan, - 2003)

Teaches: Biology, Botany, Zoology, EVS, NEET

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  1. HUMAN DIGESTIVE SYSTEM The human digestive system consists of the gastroi!ntestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). The activities that are performed by the digestive system include the following activities: 1. Ingestion: the taking of food into the mouth 2. Mastication: chewing food which pulverizes it and mixes it with saliva 3. Deglutination: Swallowing; moving food from the mouth to the pharynx and into the esophagus. 4. Digestion: The mechanical and chemical breakdown of food to prepare it for absorption. 5.Absorption: the passage molecules of food through the mucous membrane of the small intestine and into the blood and lymph for distribution to the cells. 6.PeristaIsis: the rhythmic wavelike contractions of the smooth muscle of the intestines that move food through the Gl tract. 7. Defecation: the discharge of indigestible wastes (feces) from the Gl tract. Anatomically and functionally the digestive system can be divided into a tubular gastrointestinal (Gl) tract and accessory digestive organs. The Gl tract which extends from the mouth to the anus is a continuous tube approximately 30 feet (9m) long. It goes through the thoracic cavity and enters the abdominal cavity through the diaphragm. The organs of the digestive system include the oral cavity (mouth), pharynx, esophagus, stomach, small intestine and large intestine. The accessory organs include teeth, salivary glands, liver, gall bladder and pancreas. It usually takes about 24-48 hours for food to travel the length of the Gl tract. Food travels in an assembly line manner through the tract where it is broken down to the molecular level and transported to the cells. Each region of the Gl tract has a specific function in the process. MEMBRANES OF THE ABDOMINAL Most of the digestive organs are located in the abdominal cavity. These organs are covered by serous membranes that line the cavity and cover the organs within. Serous membranes secrete a lubricating serous fluid that continuously moistens the organs. The parietal membrane lines the wall of the abdominal cavity and the visceral membrane covers the internal organs. The membrane that lines the wall of the abdominal cavity is called the parietal peritoneum. It comes together to form a double layered peritoneal fold called the mesentery. The mesentery supports the Gl tract and at the same time allows the small intestine freedom for peristaltic contractions. It also provides a structure for the passage of blood vessels and nerves. Peritonitis is an inflammation of the peritoneum usually caused by an infection. This can occur due to trauma, rupture of an organ, an ectopic pregnancy or post operative infection. The greater omentum extends from the stomach to the transverse colon forming an apron like covering over most of the small intestine. Function of the omentum includes storage of fat, cushioning visceral organs, supporting lymph nodes and protection against infection. LAYERS OF THE Gl TRACT The Gl tract from the esophagus to the anal canal is comprised of 4 layers or tunics. These layers are : 1
  2. 1. Mucosa- the innermost layer lines the lumen of the Gl tract. It is both absorptive and secretory in function. It contains lymph nodes as well as goblet cells which secrete mucous. 2. Submucosa- this is the second layer, much thicker than the mucosa. It is primarily vascular and nerve containing. Absorbed molecules pass through the mucosa to enter blood or lymph vessels here. The submucosa contain glands and a nerve plexus (Meissner's plexus) which provides autonomic innervation to the muscle layer in the mucosa. 3. Tunica muscularis- This is the primary smooth muscle layer of the Gl tract which is responsible for peristalsis. It has an inner circle and an outer longitudinal layer of muscle. Contraction of this 01 , Serosa Circular muscle muscle Meissner's nerve plexus Mucosa Epithelial lining Mucosal muscle Mucosal gland Myenteric nerve plexus Submucosai gland Mesentery layer causes the movement of food as well as helping to pulverize and churn the food with digestive enzymes. There is a large nerve plexus (Aurebach's plexis ) located between the 2 muscle layers. 4. Serosa- is the outermost layer of the Gl tract wall. It is binding and protective in function. MOUTH AND PHARYNX The functions of the mouth and associated structures is to act as a receptacle for food, to initiate digestion through mastication (chewing), to swallow food and to form words in speech. The pharynx, which is posterior to the mouth serves as a common passageway for the respiratory and digestive systems. The anterior portion of the cheeks terminate in the lips (superior and inferior) that surround the oral orfice. The lips are fleshy highly mobile organs whose primary function in humans is associated with speech. Each lip is attached from it's inner surface to the gum by a midline fold of mucous membrane called the labial frenulum. Between the outer skin and the mucous membrane there is a transition zone called the vermillion. The palate which forms the roof of the oral cavity consists of the bony hard palate and the soft palate posteriorly. The soft palate is a muscular arch covered by mucous membrane and is continuous with the hard palate anteriorly. Suspended from the middle lower border is a projection called the palatine uvula. During swallowing the soft palate Nasal cavity Hard palates-8-...........................--+..=.--....- Vestibule Tongue Lip Vermillion Hyoid bone Facial Region Pharyngeal tonsil Opening of auditory tube Soft palate Nasopharynx Oral cavity alatine uvula Fauces Palatine tonsil Oropharynx Lingual tonsil Epiglottis Larynogopharynx Esophagus Trachea and the uvula are drawn upward to form a seal and close the nasopharynx. This prevents food from entering the nasal cavity. At either side of the soft palate are the palatoglossus muscles which also reach into regions of the tongue. These muscles raise the back of the tongue and also close both sides of the fauces to enable food to be swallowed. 2
  3. Tongue The tongue functions to move food around during mastication and in swallowing .lt is also essential for speech. It is a skeletal muscle covered by mucous membrane. The posterior third of the tongue lies in the pharynx and is attached to the hyoid bone. The tongue is connected along the midline to the floor of the mouth by the lingual frenulum. The surface of the tongue is covered by small elevations called papillae. This gives the tongue a rough surface that aids in manipulating food. Different areas of the tongue have sensory taste receptors (taste buds) that are sensitive to four different tastes. These are • sweet ( tip of tongue) sour ( sides of tongue) bitter (back of tongue) salty (over most of the tongue but more on the sides). Innervation of the tongue comes from 5 cranial nerves. The sense of taste is transmitted by two cranial nerves, the glossopharygeal nerves and the facial nerves. Taste sensations are transmitted to the medulla oblongata and thalamus and then to the parietal lobes where the information is interpreted. Teeth Tongue Root Body Epiglottis Lingual tonsils Palatine tonsil Vallate papillae Fungiform papillae Filiform papillae Humans and other mammals have heterodont dentition. This means that the teeth are varied to handle different types of food. We have in the most anterior position 4 pairs of incisors- chisel shaped teeth for cutting and shearing food. 2 pairs of cuspid ( canine) teeth for holding and tearing. Behind these there are premolars (bicuspids) and molars which are for grinding and crushing food. Human dental formula Deciduous teeth or Milk teeth : 2120/2120; Adult = 2123/2123. Both childhood molars are replaced by adult premolars. Adult total is double the formula = 32. Humans are diphyodont; that is we have two sets of teeth in a lifetime. Twenty deciduous (milk) teeth erupt beginning at 6 months and ending around 2.5 years. Thirty two permanent teeth begin to erupt at around 6 years and continue until around 17. The third molars (wisdom teeth) are the last to emerge. Thecodont dentition is found in human in which root of the tooth is firmly fixed in a socket of the jawbone, making the attachment strong. This is a peg and socket attachment with the help of cementum that surrounds the root portion of the tooth. The dental cusps of the upper and lower premolars and molars occlude for chewing food. The upper incisors normally form an overbite with the incisors of the lower jaw. Masticated food is mixed with saliva containing digestive enzymes. This initiates the digestive process. The tooth consists of an exposed crown supported by a neck and anchored firmly in the jaw by a root. The roots of the teeth fit into sockets ( dental alveoli) in the alveolar processes of the mandible and maxilla. 3
  4. Each socket is lined with a connective tissue periosteum, the periodontal membrane. The root of the tooth is covered by a bonelike material called cementum. Fibers in the periodontal membrane insert into the cementum to fasten the tooth in its dental alveoli. The gingiva ( gum) is the mucous membrane surrounding the alveolar processes in the oral cavity. The bulk of the tooth is composed of dentin/dentine, a substance similar to bone but harder. Covering the dentin on the outside and forming the crown is the enamel. Enamel is composed primarily of calcium phosphate and is the hardest material in the body. Enamel Dentin Dental pulp (in pulp cavity) Neck of tooth Periodontal membrane Root canal Cementum Root Apical foramen Tooth Structure Crown Gingiva Dental alveolus nerve, vein, and artery The center of the tooth contains the pulp cavity. The pulp cavity contains pulp which is composed of connective tissue, blood vessels, lymph vessels and nerves. The root canal, continuous with the pulp cavity opens to the connective tissue surrounding the root through the apical foramen. The tooth receives nourishment via the vessels traversing the apical foramen. Even though enamel is extremely hard it can be destroyed by bacterial activity (dental caries or tooth decay). Salivary glands These are accessory digestive glands that produce a secretion called saliva. Saliva also contains starch dissolving enzymes ( amylase) and lubricating mucous which aids in swallowing. Saliva is secreted continuously in small amounts to keep the mouth moist. There are three Salivary glands in buccal cavitie of human. A. Parotid glands The two parotid glands are major salivary glands wrapped around the mandibular ramus in humans. These are largest of the salivary glands, secreting saliva to facilitate mastication and swallowing, and amylase to begin the digestion of starches. It is the serous type of gland which secretes ptyalin. It enters the oral cavity via the parotid duct (Stensen duct). They produce 20% of the total salivary content in the oral cavity. B. Submandibular glands The submandibular glands (previously known as submaxillary glands) are a pair of major salivary glands located beneath the lower jaws. The secretion produced is Parotid duct Parotid gland Submandibular duct Submandibular gland Sublingual gland a mixture of both serous fluid and mucus, and enters the oral cavity via the submandibular duct or Wharton duct. Approximately 65-70% of saliva in the oral cavity is produced by the submandibular glands. C. Sublingual glands The sublingual glands are a pair of major salivary glands located inferior to the tongue, anterior to the submandibular glands. The secretion produced is mainly mucous in nature; however, it is categorized as a 4
  5. mixed gland. Saliva exits directly from 8-20 excretory ducts known as the Rivinus ducts. Approximately 5% of saliva entering the oral cavity comes from these glands. Pharynx The funnel shaped pharynx is a muscular tube that contains a passageway about 5 inches long that connects the oral cavity and nasal cavity to the esophagus and larynx. The pharynx has both digestive and respiratory functions. The pharynx is divided into 3 regions : the nasopharynx, posterior to the nasal cavity; • the oropharynx, posterior to the oral cavity; • and the laryngopharynx, at the level of the larynx. Esophagus The esophagus is that part of the Gl tract that connects the pharynx to the stomach. It is a collapsible tubular organ about 10 inches long originating at the larynx and lying posterior to the trachea. The esophagus lies within the mediastinum of the thorax and passes through the diaphragm just above the opening to the stomach. The upper third of the esophagus is made up of skeletal muscle, the middle third is a combination of skeletal and smooth muscle. The terminal portion of the esophagus is smooth muscle only. The process of swallowing (deglutination) is a three part process which involves both voluntary and involuntary processes. The first stage which is voluntary involves closing the mouth and interruption of breathing. The tongue is elevated against the roof of the mouth due to contraction of the intrinsic muscles of the tongue. The second stage is the passage of the bolus (food) through the pharynx. This is involuntary and is elicited by sensory receptors located at the opening of the oropharynx. Pressure of the tongue against the transverse palatine folds seals off the nasopharynx from the oral cavity and creates pressure that forces the bolus into the oropharynx. The soft palate and uvula are elevated to close off the nasopharynx as the bolus passes. Sequential constriction of the constrictor muscles moves the bolus from pharynx to esophagus. The third and final stage is involuntary as well. The bolus is moved to the stomach by means of peristalsis. Stomach The stomach - the most distensible part of the Gl tract- is located in the upper left quadrant, immediately below the diaphragm. It is a J shaped organ that is continuous with the esophagus and empties into the duodenal portion of the small intestine inferiorly. In the stomach the food is churned mechanically with gastric secretions to form a pasty substance called chyme. The stomach is divided into four regions: the cardiac, fundus, body and pylorus. The cardiac is the narrow upper region immediately below the lower esophageal sphincter. The fundus is the dome shaped portion to the left of and in direct contact with the diaphragm. The body is the large central portion and the pylorus is the funnel shaped terminal portion. The pyloric sphincter is the modified circular muscle at the end of the pylorus where it joins the small intestine. Pyloric sphincter acts to regulate the flow of chyme into the small intestine. 5
  6. The wall of the stomach is composed of the same 4 tunics found in the other regions of the Gl tract, with 2 principal modifications: an extra oblique muscle layer present in the muscularis, and the mucosa has numerous longitudinal folds called gastric folds or gastric rugae. The mucosa also has microscopic gastric pits and gastric glands. There are 5 types of cells in the gastric glands that secrete specific products: 1. Goblet cells secrete protective mucous. 2. Parietal cells secrete hydrochloric acid 3. Principal cells (chief cells) secrete pepsinoge, an inactive form of the protein digesting enzyme pepsin. 4. Argentaffin cells secrete serotonin, histamine and autocrine regulators 5. Endocrine cells (G cells ) secrete the hormone gastrin into the blood. Stomach Cardia Esophagus Adventitia Longitudinal muscle Lesser curvature Duodenum Pylorus Gastric folds pyloric sphincter Fundus Body Circular muscle Oblique muscle Greater curvature Submucosa Mucosa In adddition to these products the gastric mucosa secretes Castle's intrinsic factor required for the absorption of Vitamin B12 in the small intestine. Small Intestine The small intestine, consisting of the duodenum, jejunum and ileum, is the site where digestion is completed and nutrients are absorbed. Regions of the small intestine 1. Duodenum is a fixed C shaped tube measuring 10 inches from the pyloric sphincter of the stomach to the duodenojejunal flexure. It receives bile secretions from the liver and gall bladder and pancreatic secretions from the pancreatic duct. 2. Jejunum extends from the duodenum to the ileum, is approximately 3 feet long. It has a larger lumen and more internal folds than the ileum. 3. Ileum makes up the remaining 6-7 feet of the small intestine. It empties into the cecum of the large intestine through the ileocecal valve. Lymph nodes called mesentary patches are abundant in the walls of the ileum. Structural modifications of the Small Intestine Digested food are absorbed across the lining of the intestinal mucosa. Absorption occurs mainly in the jejunum, although some occurs in the duodenum and ileum also. Absorption is aided by structures that increase the surface area of the intestine. 1. Plicae circulares are large macroscopic folds of mucosa 2. Villi are finger-like folds of the mucosa that project into the lumen. Villi increase the internal surface area of the intestinal walls making available a greater surface Intestinal Villus Intestinal villus Simple columnar epithelium Lacteal Capillary network Goblet cells Intestinal crypt Lymph vessel Arteriole Venule area for absorption. An increased absorptive area is useful because digested nutrients (including monosaccharide and amino acids) pass into the semi permeable Villi through diffusion. Each villus is 6
  7. supplied with a tuft of blood capillaries and a blind ended lymphatic duct or lacteals.. Proteins and carbohydrates enter the capillaries and fatty acids enter the lacteals. 3. Microvilli are microscopic projections formed by the folding of epithelial cell membranes. The Villi have specialized goblet cells which secrete mucous. Additionally, the center of the Villi contain capillaries and lymphatic vessels called lacteals Large Intestine The large intestine receives food that is undigested or undigestible from the small intestine, absorbs the water and electrolytes from the chyme and passes it as feces out of the Gl tract. The large intestine has little or no digestive function. It absorbs water and electrolytes from the remaining chyme. The large intestine is divided into the cecum, colon rectum and anal canal. The cecum is a dilated pouch positioned Stomach Mucosa Gastric pits Gastric gland Mucous cell Parietal— • cell Principal-..-...—.• cell (b) Mucosa Submucosa slightly below the ileocecal valve. The ileocecal valve is a fold of mucous membrane at the junction of the small and large intestine that prevents back flow of chyme. A finger like projection of the cecum called the appendix is attached to the inferior margin of the cecum. It contains an abundance of lymphatic tissue but it serves no discernible function. It is thought to be a vestigial remnant of an organ that was functional in our ancestors. Because it is a blind pouch and waste material can accumulate within, inflammation and infection can occur. If not treated, rupture will lead to further infection of the peritoneal cavity, resulting in peritonitis. The superior portion of the cecum is continuous with the which consists of the ascending, transverse, descending and sigmoid portions. The end of the line, the last 7.5 inches of the tract is the rectum. The anus is the external opening of the anal canal. Two sphincter muscles are found in this opening: the internal anal sphincter which is smooth muscle and the external anal sphincter which is skeletal muscle. GLANDS OF DIGESTIVE SYSTEM LIVER The liver which consists of 4 lobes secretes bile, which is stored and concentrated in the gall bladder prior to discharge into the duodenum. It has 4 lobes and 2 supporting ligaments. The right lobe, the left lobe, caudate lobe, quadrate lobe. The liver cells, hepatocytes form hepatic plates that are separated by large capillary spaces called liver sinusoids. This allows the liver cells to be in direct contact with blood. The gland do not secrete any digestive enzyme but is useful in the process of digestion, especially of lipids. The primary functions of the liver are: Bile production and excretion Synthesis of urea Excretion of bilirubin, cholesterol, hormones, and drugs Metabolism of fats, proteins, and carbohydrates Enzyme activation Storage of glycogen, vitamins, and minerals Synthesis of plasma proteins, such as albumin, and clotting factors Blood detoxification and purification 7
  8. Bile is produced by the hepatocytes and drains into the bile ducts which in turn drain into the hepatic ducts that carry the bile away from the liver. Gall bladder The gall bladder is a sac like organ attached to the inferior portion of the liver. This organ stores and concentrates bile which is necessary to breakdown, emulsify and absorb ingested fats. A sphincter valve at the neck of the gall bladder controls the storage of bile. Bile is continuously produced by the liver and drains through the hepatic ducts and common bile duct to the duodenum. When the small intestine is empty the sphincter of ampulla constricts and forces the bile up the cystic duct to the gall bladder for storage. Gall stones occur when cholesterol precipitates out of bile and forms solid crystals. Sometimes this leads to Ducts Gallbladder Duodenum Cystic duct Duodenal papilla blockage of the bile duct and surgery is required to remove the gall stones. PANCREAS The pancreas functions as a mixed gland in that it has both endocrine and exocrine functions. The endocrine function occurs at the pancreatic islets (islets of Langerhans). As an exocrine gland the pancreas secretes pancreatic juice through the pancreatic duct which empties into the duodenum. The pancreas is positioned horizontally along the posterior abdominal wall, adjacent to the greater curvature of the stomach. Within the lobules of the pancreas are the Hepatic ducts Cystic duct Gallbladder Duodena papilla Duodenu Pancreas Common bile duct Pancreatic juice Hepatic ducts Common hepatic duct Common bile duct Accessory pancreatic duct Pancreatic duct Pancreatic acinar cells Pancreatic islet cells Pancreatic duct Tail of pancreas Body of pancreas Head of pancreas exocrine secretory units called pancreatic acini and the endocrine secretory units called pancreatic islet cells. Enzymes in the pancreatic juice- Trypsinogen (for proteins) Chymotrypsinogen (for proteins) Procarboxypeptidase (for proteins) Pancreatic amylase (amylopsin) (for carbohydrates) Pancreatic lipase (steapsin) (for fats) PHYSIOLOGY OF DIGESTION A. DIGESTION OF CARBOHYDRATE Buccal Cavity In the mouth cavity, the food is mixed with saliva. It contains an enzyme called salivary amylase or ptyalin. Salivary amylase acts on starch and convert it into maltose, isomaltose and small dextrins or dextrin'(disaccharides). Chewing and mastication of food increases the action of salivary amylase on starch by increasing the surface area of food on which the enzyme acts. About 30 percent of starch present in 8
  9. Since the food gets mixed with the gastric juice the action of amylase ceases due to high acidity. Some of the sucrose present in the food get hydrolysed by the action of HCI in the stomach. No carbohydrate digesting enzyme is present in the stomach. Small Intestine In small intestine both Pancreatic juice and intestinal juice also contain carbohydrates digesting enzymes. Pancreatic juice contains pancreatic amylase that acts on starch to digest it into maltose, isomaltose and dextrin. Pancreatic Amylase food is hydrolysed in the mouth. The action of salivary amylase continues for sometime even in the stomach but soon HCI present in the gastric juice destroys the entire enzyme. Salivary Amylase Maltose + Dextrin ------------> Glucose + Glucose Starch - Stomach: -----------> Maltose + Isomaltose + Dextrin Starch - -- -- -- ------> Maltose + Isomaltose + Dextrin Then maltose and isomaltose along with sucrose, lactose present in the diet are digested by the different disaccharidases present in the intestinal mucosa into their corresponding monosaccharides as shown: Maltase Isomaltase Isomoltose --------> Glucose + Glucose Sucrase Sucrose ------------> Glucose + Fructose Lactase Lactose - ---------> Glucose + Galactose. B. DIGESTION OF PROTEINS The digestion of protein is a multistep enzymatic process which begins in buccal cavity where the physical breakdown of food occurs. But the chemical digestion of protein begins in the stomach. Stomach Gastric juice contains two proteolytic enzymes in inactive form - Pepsinogen and Prorennin. Pepsinogen is activated by HCI and the active form pepsin acts on proteins. Pepsi pepsin (Inactive) (Active) Proteins in the presence of pepsin is converted to proteases and peptones. Pepsin acts in acidic pH (1.5-2). Pepsin is an endopeptide which more deficiently hydrolyses peptide bond between amino acids which are aromatic in nature. Ex. Phenylalanine Tyrosine and Tyrosine Proteins Pepsin Peptone + Proteose pH - 1.5-2 9
  10. Other functions of HCI - Disinfects food by killing bacteria. • Stops the action of salivary enzymes in stomach. Softens the food and aids in digestion. Pepsin also helps in coagulation of milk. It hydrolyses casein to paracasein and whey proteins. Paracasein is precipitated as calcium paracaseinate to form solid curd. Rennin also helps in coagulation of milk. Rennin is secreted in the form of prorennin that is converted to active rennin in the presence of HCI. Prorennin (Inactive) Rennin Casein (Milk Protein) Paracasein + Ca2+ Pepsin Rennin (Active) Paracasein Calcium Paracaseinate Calcium Paracaseinate —...........................................»proteose + Peptone Pepsin can also hydrolyse collagen and other proteins. However, keratin of hair, skin and nails or horns cannot be hydrolysed by pepsin. Small Intestine Most proteins are digested in the duodenum of small intestine. Duodenum receives secretions from pancreas and liver. It also secretes intestinal juice called succus entericus Pancreatic juice contains four proteolytic enzymes called trypsinogen, chymotrypsinogen, procarboxypeptidase and proelastase. A. Trypsinogen is activated to trypsin by enterokinase. Enterokinase Trypsinogen Trypsin (Inactive) (Active) Trypsin is active in alkaline pH of 8 and most efficiently cleaves the peptide bond of basic anime acids like arginine, lysine and Hisidine. Trypsin converts basic proteins to peptides. Trypsin also hydrolyses fibrinogen to fibrin and coagulates milk Trypsin Proteins + Proteose + Peptone pH-8 Peptides B. Trypsin further activates chymotrypsinogen to chymotrypsin and procarboxypeptidase to carboxypeptidase. Trypsin Chymotrypsinogen Chymotrypsin (Inactive) pH—8 (Active) Chymotrypsin converts proteins to peptides. This also helps in coagulation of milk. 10
  11. C. Carboxypeptidase hydrolyse the terminal peptide bonds in a peptide chain acting on the carboxyl group. It is released in inactivated form and activated by trypsin. Trypsin Procarboxypeptidase Carboxypeptidase (Inactive) pH — 7.4 (Active) D. Elasase is released in inactivated form of proelastase and activated by trypsin. It hydrolyses peptide bond between glycine, alanine and serine amino acids. It difests collagen and elastin. Trypsin Elastase Proelastase (Inactive) Collagen + Elastin pH-8 Elastase (Active) Peptides pH—8 Intestinal juice contains protesases like- Aminopeptidase which hydrolyses the terminal peptide bond acting on amino group and releases one amino acid. Peptide Aminopeptidase Amino acid Tripeptidase hydrolyse tripeptides to 1 amino acids and 1 dipeptide. Tripeptidase Tripeptide ...........................................................................................................................+ I Amino acid + IDipeptide Dipeptidase hydrolyse dipeptides to 2 amino acids. Dipeptidase Dipeptide 2 Amino acids C. DIGESTION OF FATS Buccal cavity Besides physical breakdown no chemical digestion of fat occurs in buccal cavity. Stomach The breakdown of dietary triglycerides( most common form of lipid consumed) starts in the stomach by the action of gastric lipase active at acidic pH, with a maximum activity at pH 5.0-5.4. It hydrolyses FA from all three positions on triglycerides]. On the basis of this, the lipolytic activity of the gastric lipase mainly yields one free fatty acid and one diaglyceride. The gastric lipase is responsible for up to 10-30 % of the total lipid breakdown within the stomach. Gastric lipase Triglyceride ............................................................................................+ I Fatty acid + 1 Diglyceride pH-5-5.4 11
  12. Small Intestine The bile contains bile salts and bicarbonates which aids in breaking large fat globules into smaller particles by the process of emulsification. It increases the surface area for the pancreatic lipase for digestion. The pancreatic lipase is the main enzyme responsible for lipolysis in the human gastro intestinal tract, responsible for hydrolysing about 98 % of the remaining dietary triglycerides in healthy humans. The pancreatic lipase hydrolyses triglycerides to yield two fatty acid and 1 monoglyceride. Pancreatic lipase Triglyceride 2 Fatty acid + 1 Monoglyceride pH-8 ABSORPTION AND ASSIMILATION ABSORPTION OF CARBOHYDRATE The monosaccharide units, glucose, galactose and fructose are transported through the wall of the small intestine into the portal vein which then takes them straight to the liver. The mode of transport varies between the three monosaccharides and is described in brief below. Glucose, at low concentrations is transported through the mucosal lining into the epithelial cells of the intestine by active transport, via a sodium dependant transporter. At higher concentrations, a second facilitative transporter becomes involved. From the epithelial cells glucose is moved into the surrounding capillaries by facilitated diffusion. Galactose is transported in the same way as glucose, utilising the same transporters. Fructose moves entirely via facilitated diffusion. The process utilises a different transporter to glucose when entering the enterocytes, however both fructose and glucose utilise the same transporter to exit the enterocyte into the capillaries. Storage Surplus glucose is initially stored as glycogen in the liver or muscles. The liver can store approximately 100g of glycogen which is used to maintain basal blood glucose levels between meals, whilst the muscles typically store 400-500g often used during movement. Once these reserves are saturated, excess glucose is converted to fat for longer term storage. ABSORPTION OF AMINO ACID AND PEPTIDES The mechanism by which amino acids are absorbed is conceptually identical to that of monosaccharides. The plasma membrane of the absorptive cell bears at least four sodium-dependent amino acid transporters - one each for acidic, basic, neutral and amino acids. These transporters bind amino acids only after binding sodium. The fully loaded transporter then undergoes a conformational change that dumps sodium and the amino acid into the cytoplasm, followed by its reorientation back to the original form. Thus, absorption of amino acids is also absolutely dependent on the electrochemical gradient of sodium across the epithelium. Absorption of Peptides There is virtually no absorption of peptides longer than four amino acids. However, there is abundant absorption of di- and tripeptides in the small intestine. These small peptides are absorbed into the small 12
  13. intestinal epithelial cell by cotransport with H+ ions via a transporter called PepT1. Once inside the enterocyte, the bulk of absorbed di- and tripeptides are digested into amino acids by cytoplasmic peptidases and exported from the cell into blood. ABSORPTION OF LIPIDS/FATTY ACIDS Bile acids play their first critical role in lipid assimilation by promoting emulsification. Hydrolysis of triglyceride into monoglyceride and free fatty acids is accomplished predominantly by pancreatic lipase. As monoglycerides and fatty acids are liberated through the action of lipase, they retain their association with bile acids and complex with other lipids to form structures called micelles. Micelles are essentially small aggregates (4-8 nm in diameter) of mixed lipids and bile acids suspended within the lumen of small intestine. As the Chyle is mixed, micelles bump into the brush border of small intestinal enterocytes, and the lipids, including monoglyceride and fatty acids, are taken up into the epithelial cells Once inside the enterocyte, fatty acids and monoglyceride are transported into the endoplasmic reticulum, where they are used to synthesize triglyeride. Beginning in the endoplasmic reticulum and continuing in the Golgi, triglyceride is packaged with cholesterol, lipoproteins and other lipids into particles called chylomicrons. Chylomicrons are extruded from the Golgi into exocytotic vesicles.lnstead of being absorbed directly into capillary blood, chylomicrons are transported first into the lymphatic vessel that penetrates into each villus. Chylomicron-rich lymph then drains into the system lymphatic system, which rapidly flows into blood. Blood-borne chylomicrons are rapidly disassembled and their constitutent lipids utilized throughout the body. REGULATION OF DIGESTION The activities of the digestive system are regulated by both hormones and neural reflexes. Four important hormones and their effects on target cells follow: Gastrin is produced by enteroendocrine cells of the stomach mucosa. Effects include: o Stimulation of gastric juice (especially HCI) secretion by gastric glands. o Stimulation of smooth muscle contraction in the stomach, small intestine, and large intestine, which increases gastric and intestinal motility. Secretin is produced by the enteroendocrine cells of the duodenal mucosa. Effects include: o Stimulation of bile production by the liver. o Inhibition of gastric juice secretions and gastric motility, which in turn slows digestion in the stomach and retards gastric emptying. Cholecystokinin (CCK) is produced by the enteroendocrine cells of the duodenal mucosa. Effects include: o Stimulation of bile release by the gallbladder. o Stimulation of pancreatic juice secretion. Glucose insulinotropic peptide (GIP) is produced and released by the enteroendocrine cells of the duodenal mucosa in response to the presence of the glucose in the small intestine. This hormone stimulates the pancreas to begin releasing insulin. The second regulatory agent of the digestive system is the nervous system. Stimuli that influence digestive activities may originate in the head, the stomach, or the small intestine. Based on these sites, there are three phases of digestive regulation: 13
  14. 1. The cephalic phase comprises those stimuli that originate from the head: sight, smell, taste, or thoughts of food, as well as emotional states. In response, the gastric reflexes are initiated which either stimulates or suppresses the digestive process 2. The gastric phase describes those stimuli that originate from the stomach. In response, the following reflexes are initiated: Neural response: Gastric juice secretion and smooth muscle contraction are promoted. Hormonal response: Gastrin production is promoted. 3. The intestinal phase describes stimuli originating in the small intestine. In response, the following reflexes are initiated: Neural response: Gastric secretion and gastric motility are inhibited (enterogastric reflex). Intestinal secretions, smooth muscle contraction, and bile and pancreatic juice production are promoted. Hormonal response: Production of secretin, CCK, and GIP is promoted. Chyme is a mixture of partly digested food and stomach fluids. It is termed as a 'semifluid mass of partly digested food,' and is also known as chymus Chyle is a milky bodily fluid that forms in the small intestine. The chyle forms specifically during digestion of fatty foods. 14

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