Digestion and Absorption PDF

Human Physiology: Digestive system © 2022, Aakash BYJU'S. All rights reserved reserved. Key Takeaway Nutrients 1 2 Mouth Pharynx Digestive system 3 4 Buccal cavity 5 Tongue Teeth © 2022, Aakash BYJU'S. All rights reserved. Oesophagus 6 7 Stomach Muscles Small intestine 8 9 Large intestine Functions...

Human Physiology: Digestive system © 2022, Aakash BYJU'S. All rights reserved reserved. Key Takeaway Nutrients 1 2 Mouth Pharynx Digestive system 3 4 Buccal cavity 5 Tongue Teeth © 2022, Aakash BYJU'S. All rights reserved. Oesophagus 6 7 Stomach Muscles Small intestine 8 9 Large intestine Functions Layers of the alimentary canal © 2022, Aakash BYJU'S. All rights reserved. 10 Digestive glands Salivary glands 11 12 Liver Digestion In oral cavity Pancreas In stomach Gastric glands In small intestine Intestinal glands Digestive hormones 13 14 © 2022, Aakash BYJU'S. All rights reserved. Absorption of nutrients Assimilation of nutrients 15 16 Balanced diet 17 18 Summary​ © 2022, Aakash BYJU'S. All rights reserved. Egestion Disorders of the digestive system Nutrients  The word ‘nutrient’ is derived from the Latin word ‘nutritio’, which means ‘to nourish’.  Nutrients are the chemical substances present in food.  They are required by living organisms for the growth and maintenance of bodily functions.  Nutrition is the process by which a living organism obtains nutrients that are necessary for their growth and maintenance. © 2022, Aakash BYJU'S. All rights reserved. Vitamins Water Carbohydrates Food Lipids Minerals Proteins Nutrients Types of nutrients Based on constituents Organic   These are the compounds that contain carbon. These include carbohydrates, lipids, proteins, and vitamins. Inorganic   These are the compounds that contain metal and other elements. These include minerals and water. © 2022, Aakash BYJU'S. All rights reserved. Nutrients Based on amount required Macronutrients    These are required by the body in large quantities. These are involved in the synthesis of various compounds required by the body. Examples: Carbohydrates, fats, and proteins. Micronutrients    These are required by the body in less or minute quantities. These are involved in the regulation of metabolism. Examples: Vitamins and minerals. © 2022, Aakash BYJU'S. All rights reserved. Digestive System Digestion is the breakdown of large food molecules into small food molecules. Alimentary canal Associated glands Mouth Buccal cavity Pharynx Oesophagus Stomach Small intestine Large intestine © 2022, Aakash BYJU'S. All rights reserved. Mouth & Buccal Cavity  It is a transverse opening to the alimentary canal present below the nose and above the chin.  It is bound by two movable lips , the upper and the lower lips.  Mouth leads to the buccal cavity or the oral cavity. Parts of the buccal cavity Palate Roof of the buccal cavity © 2022, Aakash BYJU'S. All rights reserved. Tongue Teeth Buccal Cavity Tongue  It is a freely movable muscular organ that occupies the floor of the mouth.  It is flat, voluntary, and sensory in function. Frenulum  The tongue is attached to the floor of the mouth by the lingual frenulum. Papillae Taste buds  They are projections  They are the receptors of taste  Some of them bear  They are capable of sensing all on the upper surface of the tongue. taste buds. that are present within some papillae. four tastes (bitter, sour, salty, and sweet) and sending messages to the brain to generate the sensation of taste.  An average person has about 10,000 taste buds. © 2022, Aakash BYJU'S. All rights reserved. Buccal Cavity Tongue Functions      It helps in chewing the food. It aids in swallowing the food. It is an organ of taste. It acts as a brush to clean the teeth. It plays an important role in speech. Myth buster box  The common assumption that the human tongue is divided into different taste areas is a myth.  The ability to sense different tastes - sweet, salty, sour and bitter - is not allocated to different parts of the tongue.  The taste receptors that pick up these tastes are actually distributed all over the tongue. © 2022, Aakash BYJU'S. All rights reserved. Buccal Cavity Teeth  They are hard structures that are present in the form of two semicircles embedded in the socket of the jaw bone.  They are ecto- mesodermal in origin. Crown  Exposed part of the tooth  Projects above the gums Enamel Parts of a tooth  They have several uses such as gripping, cutting, gnawing, tearing, and crushing. © 2022, Aakash BYJU'S. All rights reserved.  Hardest part of the human body  Covers the crown  Characteristics: Acellular, avascular (not supplied by blood vessels) and non-regenerable  Helps in the mastication of food Neck  Middle part of the tooth  Surrounded by the gums Root  Basal part of the tooth  Embedded in the jaw bone Buccal Cavity Teeth Types of dentitions Heterodont Homodont Presence of different types of teeth in the oral cavity Presence of a single type of teeth in the oral cavity Example: Human beings, cheetah. Example: Dolphins Note: In elephants, the upper incisors are modified into curved tusks. © 2022, Aakash BYJU'S. All rights reserved. Types of human teeth Incisors  Eight in number Premolars  Shaped for cutting  Eight in number  Bicuspid ( two cusps at the crown)  Shaped for chewing and crushing Canines Molars  Chisel shaped  Four in number  Dagger shaped  Shaped for ripping and shredding  Twelve in number  Upper jaw has 3 roots and lower jaw has 2 roots  Shaped for chewing, crushing, and grinding Buccal Cavity Teeth Types of teeth based on placement in jaw Thecodont Pleurodont Acrodont Teeth in bony socket of jawbone. E.g.: Man, crocodile. Teeth on the lateral side of jawbone. E.g.: Reptiles. Teeth on the terminal part of jawbone. E.g.: Amphibians, fish. © 2022, Aakash BYJU'S. All rights reserved. Buccal Cavity Teeth Monophyodont   Teeth appear only once in life E.g. Last molar in man (wisdom teeth). Diphyodont  Bear two sets of teeth during the life, one set of temporary teeth which are replaced by permanent teeth. Temporary/Milk/Deciduous teeth    Fully developed by the age of two to three years. 20 in number, eight incisors, four canines and eight molars. Premolars are absent. © 2022, Aakash BYJU'S. All rights reserved. Polyphyodont  Teeth which appear more than twice in life.  E.g. – Sharks. Permanent teeth     Replace the temporary teeth 32 in number with eight incisors, four canines, eight premolars and 12 molars Once broken, cannot be replaced naturally. Start forming at the age of six and completely replace the temporary teeth by the age of 12. Pharynx  The oesophagus and trachea open into the pharynx.  Functions o It is the common passage for food and air. o It helps in swallowing. ▪ Muscles of the pharynx contract, raising and expanding the pharynx, thus helping in swallowing. o It has the tonsils. ▪ Tonsils are the first line of defense against illness.  Epiglottis o It is a cartilaginous flap located in the throat. o It prevents the food from entering the glottis (opening of the windpipe) while swallowing. © 2022, Aakash BYJU'S. All rights reserved. Pharynx Oesophagus  It is a narrow muscular tube.  It extends posteriorly through the neck, thorax, and the diaphragm.  It is also known as the food pipe, or the food tube.  It transfers food from the pharynx to the stomach by peristalsis. o Peristalsis is a series of wave-like muscle contractions that move food.  Muscles in the oesophagus prevent entry of air into the digestive tract.  Muscular gastro-oesophageal sphincter regulates its opening into stomach. © 2022, Aakash BYJU'S. All rights reserved. Oesophagus Stomach  It is a J-shaped bag located in the upper-left part of the abdomen.  It is a hollow organ that holds the food.  It is also known as ‘gaster’ and plays an important role in digestion. Cardiac    The oesophagus opens into the stomach in this region. It is present near the heart, hence it is known as the cardiac region. The opening is guarded by cardiac/gastrooesophageal sphincter. Fundus   © 2022, Aakash BYJU'S. All rights reserved. This region is filled with air and gas. It is also known as the fundic region. Body  This is the main part of the stomach where the food is broken down into smaller particles, as the enzymes act here. Pylorus    It is also known as the pyloric part. It opens into the intestine, i.e., duodenum. The opening is guarded by pyloric sphincter. Stomach Muscles of stomach  The churning action in the stomach is brought in by the muscles of the stomach. o Physical breakdown of food in the stomach. o Formation of chyme. Oblique muscle layer Longitudinal muscle  Runs lengthwise.   Unique muscle present only in the stomach Aids in digestion Circular muscle  Present in a ring-like arrangement. © 2022, Aakash BYJU'S. All rights reserved. Rugae  Irregular folds present in the innermost layer which disappear when distended. Small Intestine Duodenum    It is the shortest and widest part of the small intestine. The opening of stomach into duodenum is regulated by pyloric sphincter. It is C shaped. Jejunum   It is the middle part of the small intestine It is highly coiled. Ileum Parts of small intestine © 2022, Aakash BYJU'S. All rights reserved.   It is the longest part of the small intestine. The wall of ileum is thinner than that of jejunum. Small Intestine Other parts of small intestine Villi  Finger-like projection. present in the small intestine  Composed of mucosa layer  Increase the surface area for absorption  Covered by epithelium, and have lacteal (lymph capillary) and blood capillaries. © 2022, Aakash BYJU'S. All rights reserved. Mucosa Small Intestine Other parts of small intestine Microvilli  Microscopic projections present in a single villus  Give a brush border appearance  Increases the surface area for absorption of food Villi Microvilli Entrocyte Crypt © 2022, Aakash BYJU'S. All rights reserved. Structuring of microvilli Large Intestine It is known as the large intestine because of its large diameter. It is about 1.5 m long. Caecum  Pouch-like structure into which ileum opens  Host to symbiotic microbes  Small blind sac that opens into the colon.  Has narrow finger-like projection known as vermiform appendix o o o o Vestigial organ Residual part from ancestors with no known function Well- developed in herbivores Inflammation of appendix is known as appendicitis © 2022, Aakash BYJU'S. All rights reserved. Colon  Reabsorbs fluids  Processes waste products and prepares for elimination from the body.  Has the following four parts: o Ascending colon o Transverse colon o Descending colon o Sigmoid colon Rectum  Last part of the digestive tract  Descending colon open into rectum  Ends in anal canal that opens through anus  Anus has two sphincters that control bowel movement: o Internal anal sphincter o External anal sphincter  Voluntary Large Intestine Functions    Absorption of water and electrolytes o In large intestine, digestion does not occur rather absorption occurs. o Colon absorbs water, minerals and some drugs. Elimination of solid waste o Rectum eliminates the food that is not digested as solid waste. o Most of the water is reabsorbed by osmosis. Secretion of mucus Mucus helps in adhering the waste particles together and lubricating for an easy passage. Transverse colon Ascending colon LEFT RIGHT Descending colon Caecum Sigmoid colon Rectum Large intestine structure © 2022, Aakash BYJU'S. All rights reserved. Layers of the Alimentary Canal Serosa Free surface Serosa Muscularis Alimentary canal Muscularis Submucosa Submucosa Mucosa Lumen Mucosa © 2022, Aakash BYJU'S. All rights reserved. Layers of the Alimentary Canal Serosa Muscularis  Outermost layer of the stomach.  Often known as visceral peritoneum.   It is made up of thin squamous epithelium also called mesothelium.  Formed by smooth muscles Arranged into inner circular and outer longitudinal layers Mucosa Submucosa  Made of loose connective tissues  Contains the following: o Nerves o Blood vessels o Lymph vessels  In duodenum, glands are present in submucosa as well. © 2022, Aakash BYJU'S. All rights reserved.  Innermost lining of the lumen of alimentary canal  Contains secretory and absorptive cells  Forms irregular folds known as rugae in stomach  Also forms villi and crypts of Lieberkühn in small intestine.  Has mucus-secreting goblet cells.  Mucus helps in lubrication.  Villi have microscopic projection called ‘microvilli’.  Mucosal epithelium has goblet cells which secrete mucus for lubrication. Digestive Glands Types of digestive glands Accessory digestive glands Salivary glands Digestive glands within alimentary canal Gastric glands Liver Pancreas © 2022, Aakash BYJU'S. All rights reserved. Intestinal glands Salivary Glands Components of saliva  These glands secrete saliva into the oral cavity. o Saliva is a watery substance produced in the mouth of certain animals, including humans. o It is slightly acidic (6.8 pH) and 1-1.5 L is secreted per day. Water  Prevents drying of mouth  Taste buds can sense the taste only when the food is moist Electrolytes  Act as buffering agents Lysozyme  Has antimicrobial properties Salivary amylase  Helps in the digestion of starch © 2022, Aakash BYJU'S. All rights reserved. Salivary Glands There are three pairs of salivary glands at different locations. Parotid glands  Located just in front of the ear, in cheeks Submandibular/Submaxillary glands  Located in the lower jaw Sublingual glands  ’Located below the tongue © 2022, Aakash BYJU'S. All rights reserved. Liver    It is the largest gland of the human body (weight: 1.2-1.5 kg in adults). It plays a crucial role in digestion. It is located in the upper right side of the abdominal cavity. Falciform ligament Structure of the liver Left lobe Divided into two lobes Lobes are separated by falciform ligament Right lobe Liver © 2022, Aakash BYJU'S. All rights reserved. Lobule consists of cells known as hepatocytes  Hepatocytes are arranged in cords - hepatic cords  Space between hepatic lobule - hepatic sinusoid  Blood flows through the sinusoids  Lined by endothelial cells and macrophages known as Kupffer cells Each lobe is divided into lobules  Structural and functional units of the liver  Lobules are connected by a thin sheath of connective tissue - Glisson’s capsule (characteristic feature of the mammalian liver)  Cells present in it - hepatocytes Liver Liver converts toxic substances into harmless substances. Process by which amino group is removed from amino acids. Kupffer cells engulf disease causing microbes and dead cells. Detoxification Deamination Haemopoiesis Process of blood formation. Liver produces RBC in embryo. Storage Stores fat in the form of glycogen. It also stores bile in the gall bladder. Functions of liver Phagocytosis Carbohydrate Metabolism Glycogenesis, glycogenolysis, gluconeogenesis, glyconeogenesis are involved in carbohydrate metabolism. © 2022, Aakash BYJU'S. All rights reserved. Bile  It is secreted by the liver cells or hepatocytes.  It is stored in the gallbladder. o It is a pear-shaped organ attached to the posterior surface of the liver by the connective tissues. o It acts as a concentrating reservoir for bile. © 2022, Aakash BYJU'S. All rights reserved. Gallbladder Liver Pancreas  The pancreas secretes enzymes and hormones that help in digestion.  The pancreas is a compound organ, i.e., it functions as both an exocrine and endocrine gland.  The exocrine pancreas ○ The exocrine part of the pancreas secretes an alkaline pancreatic juice. ○ Composition of the pancreatic juice Pancreatic lipase Sodium bicarbonate Trypsinogen DNase Chymotrypsinogen RNase Procarboxypeptidase © 2022, Aakash BYJU'S. All rights reserved. Ducts of the Liver and the Pancreas 1. The bile stored in the gallbladder is transported into a common bile duct that is formed by the joining of hepatic duct (duct of the liver) and cystic duct (duct of the gallbladder). 2. The common bile duct and the pancreatic duct join together to form the hepato-pancreatic duct. It is guarded by sphincter of Oddi. Liver Hepatic duct Cystic duct Common Bile duct Gallbladder Pancreatic duct Small Intestine 3. The bile flows through the hepatopancreatic duct into the duodenum. Hepatopancreatic duct Pancreas Ducts of the bile and pancreatic juice © 2022, Aakash BYJU'S. All rights reserved. Gastric Glands Mucosa  They are part of the alimentary canal.  They are present in the mucosal regions. Gastric pit  They are tubular glands formed by the epithelium of the stomach. Gastric gland  These are the glands that secrete enzymes necessary for digestion. Parietal cell Sub mucosa Muscularis externa Serosa © 2022, Aakash BYJU'S. All rights reserved. Chief cell Enteroendocrin e cell Gastric Glands Goblet/ Mucus neck cells  Present throughout the epithelium  Secrete mucus  Mucus protects the stomach lining from the strong acids in the stomach Cells of the mucosa Chief/ Peptic cells  Secrete the enzyme pepsinogen  Secretions of the chief cells aid in the digestion of proteins Oxyntic/ Parietal cells  Secrete hydrochloric acid (HCl)  HCl provides the acidic environment required for the activation of pepsinogen  Oxyntic cells also secrete the Castle’s intrinsic factor  The intrinsic factor helps in the absorption of vitamin B12 Enteroendocrine cells © 2022, Aakash BYJU'S. All rights reserved. Pepsinogen  Pepsinogen is an inactive enzyme.  The common term given to the inactive precursor of an enzyme is zymogen and pepsinogen is a zymogen released by the chief cells.  Pepsinogen is activated by the acidic medium provided by HCl. Gastric juice = Mucus + Pepsinogen + Hydrochloric acid Intestinal Glands Epithelial cells of the small intestine are modified into intestinal glands Crypts of Lieberkuhn Brunner’s glands  Simple tubular structures   Occur throughout the small intestine between the villi Found only in the submucosal layer of the duodenum  Synthesise and secrete the nonenzymatic alkaline secretion of the intestinal juice  Also secrete enzymes and mucus  Mucus protects the duodenal wall from the effects of digestion  Open into the crypts of Lieberkuhn  Cells found in the lining i. Enteroendocrine cells (Produce hormones) ii. Goblet cells (Secrete mucus) iii. Paneth cells (Unicellular cells found in the mucosal layer of the crypts of Lieberkuhn of the jejunum and they secrete antimicrobial proteins) © 2022, Aakash BYJU'S. All rights reserved. Digestion  The process of mechanical and chemical breakdown of the complex nutrients into simpler nutrients is known as digestion. Types of digestion Chemical digestion Involves 4 classes of digestive enzymes Carbohydrases Proteinases Mechanical digestion  The digestion that involves the mechanical breakdown of food is known as mechanical digestion.  It does not involve enzymes.  It helps to reduce the size of the food particles. This aids in the smooth movement of the particles through the alimentary canal.  Mouth and the stomach aids in the mechanical digestion. Lipases Nucleases © 2022, Aakash BYJU'S. All rights reserved. Digestion in Oral Cavity Mechanical digestion Food Food is chewed and broken down into smaller pieces. It is mixed with saliva with the help of the tongue. Mastication- It is the process of chewing food by teeth, and mixing it with the help of tongue and saliva. Deglutition- It is the Bolus Saliva holds the chewed food together and forms the bolus. © 2022, Aakash BYJU'S. All rights reserved. process that allows the bolus to pass from the mouth to the pharynx and into the oesophagus. Bolus consists of water, food, electrolytes, enzymes and bacteria Food + Saliva Bolus Pharynx Oesophagus Digestion in Oral Cavity Digestion in oesophagus   The process of contraction and relaxation of the oesophagus to move the bolus down the oesophagus is known as peristalsis. Area above the bolus constricts Food bolus moves down Breakdown of food does not occur in the oesophagus. Process of peristalsis © 2022, Aakash BYJU'S. All rights reserved. Digestion in Oral Cavity Chemical digestion Chemical digestion in the oral cavity Carbohydrates     Carbohydrates are long chains of sugar molecules (polysaccharides). Saliva contains enzymes like salivary amylase. It breaks down polysaccharides into simpler forms like monosaccharides. 30% of ingested starch is hydrolysed to disaccharide maltose in this step. Proteins   Proteins are chains of amino acids. They are not digested in the oral cavity. Lipids   Lipids are made up of glycerol and fatty acids. They are not digested in the oral cavity. Nucleic acids Nucleic acids are made up of nitrogenous bases and phosphate groups held by hydrogen bonds. They are not digested in the oral cavity. © 2022, Aakash BYJU'S. All rights reserved. Digestion in Stomach Mechanical digestion  The stomach stores food for about 4–5 hours.  The inner wall of the stomach is convoluted o Stomach folds up when empty. o It swells up like a balloon as it fills with food. o The infolded rugae of the stomach allow it to expand to many times its empty size. Gastric glands have 3 types of cells Mucus cells or neck cells Chief cells or peptic cells Parietal cells or oxyntic cells © 2022, Aakash BYJU'S. All rights reserved. Mechanical digestion Bolus enters the stomach It mixes with gastric juices with the help of the churning of the stomach wall. This results in the formation of chyme. Digestion in Stomach Chemical digestion Carbohydrates  Carbohydrates are not digested in the stomach. Proteins   Proteins are digested by the action of the enzyme known as pepsin. Pepsin converts some proteins into proteases and peptides. Lipids    Lipases are secreted by the gastric glands that bring about little digestion of lipids. Lipases act on lipids and break it down into fatty acids and glycerol. Not all lipids are broken down in stomach. Nucleic acids  They are not digested in the stomach. Milk proteins  Soluble milk proteins present in the stomach are digested by an enzyme known as rennin/chymosin. It acts on soluble milk protein and converts it into insoluble milk protein. Further, the insoluble milk proteins are digested by pepsin. Rennin is present in the gastric juice of infants. As the individual grows up, the quantity of rennin is reduced and pepsin functions as the major enzyme that digests milk proteins.     © 2022, Aakash BYJU'S. All rights reserved. Digestion in Small intestine Mechanical digestion  Chyme enters the small intestine through the pyloric sphincter.  The movement of the walls of the small intestine allows the chyme to mix thoroughly with the secretions of the intestine Chemical digestion  Pancreatic juice and bile are released into intestine via hepatopancreatic duct. © 2022, Aakash BYJU'S. All rights reserved. Digestion in Small intestine Components of pancreatic juice Trypsinogen Lipases Components of the pancreatic juice Inactive enzymes Chymotrypsinogen Nucleases Amylases Procarboxypeptidases © 2022, Aakash BYJU'S. All rights reserved. Digestion in Small intestine Activation of pancreatic juice Step 1: Enterokinase Trypsinogen Trypsin  It converts inactive trypsinogen into active trypsin.  It is secreted by intestinal mucosa.  It is component of intestinal juice. Step 2: Chymotrypsinogen Procarboxypeptidase Chymotrypsin Trypsin Inactive amylase Inactive lipase Carboxypeptidase Active amylase It activates inactive pancreatic enzymes. Active lipase Inactive nuclease © 2022, Aakash BYJU'S. All rights reserved. Active nuclease Digestion in Small intestine Bile juice  The bile juice is the other component of the hepatopancreatic juice.  Bile It does not contain any pigments enzymes but it plays an important role in the digestion of fats.  Bile juice does not contain any enzymes.  Bile helps in emulsification of fats.  It also activates lipases. © 2022, Aakash BYJU'S. All rights reserved. Components of bile juice Water Fats Bile salts Bilirubin Orangish yellow in colour Biliverdin Green in colour and is oxidised form of bilirubin Cholesterol Phospholipids Digestion in Small intestine Chemical digestion by hepatopancreatic juices Carbohydrates   Remaining 70% of the polysaccharides are broken down into disaccharides Achieved with the help of Pancreatic amylase Proteins  Trypsin, chymotrypsin, carboxypeptidase digests proteins, proteoses and peptones Lipids  Digestion of lipids by the hepatopancreatic juices occurs in two stages. Step 1 : Emulsification Step 2 : Action of hepatopancreatic juices   Nucleic acids    © 2022, Aakash BYJU'S. All rights reserved. Nucleic acids are broken down by nucleases in the pancreatic juice. They act on nucleic acids to form nucleotides. Some nucleotides are broken down into nucleosides Digestion in Small intestine Action of intestinal juice  Intestinal juice/succus entericus is secreted by the goblet cells and the brush border cells of the intestinal mucosa. Disaccharidases Dipeptidases Intestinal juice Lipases Enterokinase Nucleotidases Nucleosidases © 2022, Aakash BYJU'S. All rights reserved. Digestion in Small intestine Chemical digestion by intestinal juices Maltose Carbohydrates Lactose Sucrose Maltase Lactase Sucrase Dipeptidase Glucose + Glucose Glucose + Galactose Glucose + Fructose Amino acids Proteins Dipeptides Lipids Di and monoglycerides Nucleic acids Nucleosidases Nucleotidases Nucleotides Nucleosides Sugar+ bases © 2022, Aakash BYJU'S. All rights reserved. Lipases Glucose + fatty acids Digestive Hormones  Hormones are chemical messengers that are produced in small amounts in our body.  They control and coordinate various activities in the body.  The process of digestion is also coordinated by several digestive hormones.  Hormones are chemical messengers which act on specific targets.  They can have stimulatory or inhibitory effect on target. Source Hormone Effect Stimulatory © 2022, Aakash BYJU'S. All rights reserved. Target Inhibitory Digestive Hormones Source Hormone Target Action Stomach Gastrin Stomach  Stimulates secretion of gastric juice  Stimulates gastric mobility Duodenum Enterogastrone/ Gastric inhibitory peptide Stomach  Inhibits secretion of gastric juice  Inhibits gastric motility Duodenum Secretin Pancreas  Stimulates release of bicarbonates into pancreatic juice Duodenum Secretin Liver  Stimulates secretion of bile Duodenum Secretin Stomach  Inhibits gastric secretion and motility Small intestine Cholecystokinin/ Pancreozymin Pancreas Gallbladder  Stimulates release of pancreatic juice  Stimulates release of bile © 2022, Aakash BYJU'S. All rights reserved. Digestive Hormones Source Hormone Target Action Stomach Serotonin Stomach  Causes contraction of stomach muscle Stomach, Duodenum Somatostatin Stomach Small intestine Gallbladder and liver    Inhibits gastric secretion Inhibits intestinal absorption Inhibits contraction and bile release Stomach Histamine Stomach  Stimulates parietal cells to release HCl © 2022, Aakash BYJU'S. All rights reserved. Absorption of Nutrients  It is the process by which the products of digestion are passed on from the mucosa of the digestive organs to the blood or lymph vessels.  The small intestine is the principal organ for the absorption of nutrients.  The nutrients absorbed in the small intestine are as follows: o Glycerol o Fatty acids o Fructose o Glucose o Amino acids © 2022, Aakash BYJU'S. All rights reserved. Absorption of Nutrients Villi are small finger-like projections that increase the surface area for the absorption of nutrients. Intestinal villi These are small projections on the surface of the villi that further help in increasing the surface area of absorption. Microvilli Blood vessels Lacteals Internal structure of villus © 2022, Aakash BYJU'S. All rights reserved. Blood vessels and lacteals (lymph vessel) are located close to the microvilli. The nutrients from the microvilli are transported to the blood vessels and lacteals. Absorption of Nutrients Mechanism of absorption in the small intestine Transportation from the intestinal lumen to the epithelial cells Transportation from the intestinal cells to the blood vessels/lacteal Fructose Facilitated diffusion Facilitated diffusion into the blood vessels Glucose Galactose Active transport coupled with transport of sodium ions. Facilitated diffusion into the blood vessels Active transport Simple diffusion Nutrients Carbohydrates Amino acids Fatty acids and glycerol © 2022, Aakash BYJU'S. All rights reserved.  In the lumen, lipid droplets (containing fatty acids and glycerol) combine with bile salts to form micelles.  Micelles enter epithelial cells by simple diffusion.  Inside the cells, the micelles form small protein-coated fat globules known as chylomicrons, which are released into the lacteals. Absorption of Nutrients Buccal cavity  Not much absorption occurs here.  Certain drugs are absorbed into the blood capillaries, lining the mucosa of mouth and the lower side of the tongue. Stomach Alcohol, water and simple sugars Large intestine Water, minerals and some drugs Mucosa of the mouth Lower side of the tongue © 2022, Aakash BYJU'S. All rights reserved. Assimilation of Nutrients  It is the process by which absorbed substances reach tissues in the body that utilise them in carrying out various functions. Assimilation of nutrients Proteins and amino acids  Amino acids are used to form proteins o These proteins contribute to growth and repair.  Majority of enzymes are also protein in nature.  Amino acids can also be converted to glucose by deamination. o The deamination occurs only during fasting conditions when there is shortage of sources of glucose. © 2022, Aakash BYJU'S. All rights reserved. Fats  Used as reserve material  Fats stored in adipose tissue provide thermal insulation  Acts as packaging material Carbohydrates  Stored in liver and muscles in the form of glycogen  Produce energy Egestion  It is the process by which undigested wastes are removed from the body.  As chyme passes through the large intestine, it gets converted into the semi-solid faeces that enter the rectum through the sigmoid colon. Sigmoid colon Rectum Internal anal sphincter Anus © 2022, Aakash BYJU'S. All rights reserved. External anal sphincter Egestion Large intestine  The rectum opens into the anus, which is guarded by the internal and external anal sphincters.  As the faeces reach the rectum, they cause a movement of the rectal wall.  This initiates a neural reflex known as defecation reflex, causing an urge or desire for its removal.  The involuntary relaxation of the internal anal sphincter and a voluntary relaxation of the external anal sphincter causes defecation.  It occurs with the help of a mass peristaltic movement. © 2022, Aakash BYJU'S. All rights reserved. Calorific Value  The calorific value of food is the total amount of energy that the body could generate during the metabolism of food.  Calorific value is expressed in the units of calorie (cal) or joule (J).  One calorie or one joule is the amount of heat energy required to raise the temperature of 1 g of water by 1o C.  Calorific value may also be expressed in kilocalorie (kcal/calorie) or kilojoule (kJ/joule).  One kilocalorie is the amount of energy required to raise the temperature of 1 kg of water by 1°C. 1000 cal/J  = 1 kcal/1kJ = 1 calorie/joule The actual amount of energy of combustion of 1 g of food is the physiologic value of food. © 2022, Aakash BYJU'S. All rights reserved. Calorific Value Proteins Carbohydrates Fats Gross calorific/energy 5.65 kcal/g value (Amount of heat liberated from the complete combustion of 1 g of food in a bomb calorimeter) 4.1 kcal/g 9.45 kcal/g Physiological calorific/energy value (Actual amount of heat liberated from the complete combustion of 1 g of food) 4.0 kcal/g 9.0 kcal/g © 2022, Aakash BYJU'S. All rights reserved. 4.0 kcal/g Balanced Diet  A balanced diet is a diet that fulfils all of an individual’s nutritional needs. Components of a balanced diet Nutrients Source Carbohydrates Grains like wheat and rice, fruits, and vegetables. Fats Cooking oils, nuts, milk, egg yolk. Proteins Milk, eggs, meat, fish, pulses. Vitamins and minerals Fruits, vegetables, milk, pulses, eggs, meat. © 2022, Aakash BYJU'S. All rights reserved. Disorders of the Digestive System  Digestive system disorders include:  Inflammation of the intestinal tract that may be caused due to bacterial, viral, or parasitic infections (tapeworm, roundworm, threadworm, hookworm, pinworm, etc.)  Liver dysfunction  The deposition of bile pigments causes the yellowing of the skin and eyes Disorders of digestive system Vomiting    © 2022, Aakash BYJU'S. All rights reserved.  It is the emptying of the contents of the stomach through the mouth. It may be preceded by the feeling of nausea (uneasiness in the stomach). The reflex action of vomiting is controlled by the vomit centre in the medulla of the brain. Causes: Food poisoning, infections, overeating etc. Disorders of the Digestive System Disorders of digestive system   Jaundice Liver dysfunction Yellowing of skin and eyes observed due to deposition of bile pigments Diarrhoea     It is the abnormal frequency of bowel movement. Faecal matter appears watery Reduces absorption of food Causes: Food poisoning, imbalance in microbiota etc. Constipation    This is a condition when hard faeces are retained in the colon. Bowel movements occur irregularly Causes: Dehydration, lack of exercise etc Indigestion    © 2022, Aakash BYJU'S. All rights reserved. It is the improper digestion of food. It may also generate the feeling of fullness. Causes: Inadequate enzyme secretion, anxiety, food poisoning, overeating, spicy food etc. Protein-Energy Malnutrition Protein-energy malnutrition Dietary deficiency of proteins and calories Prevalent in underdeveloped countries   Effects of PEM in children Marasmus   Characteristics Characteristics Impaired replacement of tissue proteins. Wasting of muscles, thinning of limbs. Skin: Dry, thin and wrinkled. Failure of growth and brain development. Decline of growth rate and body weight. However, some fats is still left under the skin. Impaired development of brain and mental facilities. Extensive oedema and swelling. Simultaneous deficiency of proteins and calories. Cause: Substitution of mother’s milk early in the child’s life with foods that are poor in both proteins and calories. © 2022, Aakash BYJU'S. All rights reserved.   Kwashiorkor Deficiency of proteins unaccompanied by the deficiency of calories. Cause: Substitution of mother’s milk with a high calorie and low protein diet in a child of more than one year in age. Summary Organic Macronutrients   Required in large quantities Includes Carbohydrates, fats and proteins Micronutrients   Required in less or minute quantities Includes vitamins, minerals © 2022, Aakash BYJU'S. All rights reserved.  Based on constituent  Nutrients Carbon-containing compounds Includes carbohydrates, lipids, proteins and vitamins. Inorganic   Compounds containing metal and other elements Includes water and minerals Summary Digestive system Oesophagus  Narrow muscular tube  Transfers food from pharynx to stomach by peristalsis Alimentary canal Pharynx  Common passage for food and air  Helps in swallowing Mouth (Opening to the alimentary canal) Palate  Roof of the buccal cavity © 2022, Aakash BYJU'S. All rights reserved. Tongue  Muscular organ  Attached to the floor of the mouth by the frenulum  Has papillae that bear taste buds Characteristics of human teeth  Heterodont  Thecodont  Diphyodont  Temporary teeth (dentition - 2102)  Permanent teeth (dentition - 2123) Teeth  Hard structure  Tooth has three parts: crown, neck and root  Types of human teeth: incisors, canines, premolars and molars Summary Duodenum  Shortest and widest part of the small intestine  C shaped Jejunum  Middle part of the small intestine and is coiled Parts of small intestine © 2022, Aakash BYJU'S. All rights reserved. Ileum  Longest part of the small intestine Summary Layers of alimentary canal Serosa Inner-circular Outerlongitudinal Sub-mucosa Lumen Mucosa © 2022, Aakash BYJU'S. All rights reserved. Muscularis Summary Salivary glands Secrete saliva(It has salivary amylase that converts starch to glucose) Accessory digestive gland (Not part of the alimentary canal) Digestive glands (Exocrine gland) Digestive glands within the alimentary canal Liver Secretes bile that is stored in the gallbladder Pancreas Secretes pancreatic juice Submandibular gland Sublingual gland It helps in deamination, detoxification, haemopoiesis, storage, carbohydrate metabolism and phagocytosis (a) Chief cells: Secrete pepsinogen that helps in the digestion of proteins (b) Goblet cells: Secrete mucus that protects stomach lining from strong acids (c) Oxyntic cells: Secrete HCl that provides acidic environment Gastric glands (a) Brunner’s gland (b) Crypts of Lieberkuhn Intestinal glands © 2022, Aakash BYJU'S. All rights reserved. Parotid gland Summary Mechanical digestion Carbohydrases Polysaccharides Types of digestion  Reduction in size of food particles  No use of enzymes in digestion  Occurs in mouth and stomach Carbohydrases Monosaccharides Proteinases Proteins Proteolytic Enzymes Aminoacids Lipases Chemical digestion  Chemical breakdown of complex substances to simpler substances  Enzymes are involved © 2022, Aakash BYJU'S. All rights reserved. Fats/Lipids Lipases Fatty acids + Glycerol Nucleases Nucelic Acids Nucleases Sugar + Nitrogenous Bases Summary Carbohydrates Proteins Lipids Some proteins are broken down into polypeptides by pepsin. Some lipids are broken down into glycerol and fatty acids by lipase. Small intestine (Hepatopancreatic juice) Remaining 70% of carbohydrates are digested into disaccharides by pancreatic amylase. Remaining proteins, proteoses, and peptones are broken down into dipeptides by trypsin, chymotrypsin and carboxypeptidase. Water-insoluble fats are emulsified with the help of bile salts. They are digested into di and monoglycerides by the action of lipase. Nucleic acids are broken down into nucleotides and nucleosides by the action of nuclease. Small intestine (Intestinal juice) Disaccharides are broken down into monosaccharides by disaccharidase. Dipeptides are broken down into monopeptides by the action of dipeptidase. Di and monoglycerides are broken down into glycerol and fatty acids by lipase. Nucleotides broken down into nucleosides by nucleotidases which are broken down into sugars and bases by nucleosidases. Mouth Nucleic acids 30% of polysaccharides are digested into monosaccharides by salivary amylase. Oesophagus Stomach © 2022, Aakash BYJU'S. All rights reserved. Summary Buccal cavity Few drugs  Assimilation: Process by which absorbed substances reach tissues in the body  Egestion: Process by which undigested wastes are removed from the body  Calorific value: Total amount of energy that the body can generate during the metabolism of food. o One calorie or one joule is the amount of heat energy required to raise the temperature of 1 g of water by 1oC. Stomach Alcohol, water, and simple sugars Large intestine Water, minerals, and some drugs Small intestine Glycerol, fatty acids, fructose, glucose, amino acids © 2022, Aakash BYJU'S. All rights reserved. Summary Disorders of digestive system Vomiting  Emptying of the contents of the stomach through the mouth Jaundice  Liver dysfunction Diarrhoea  Abnormal frequency of bowel movement Constipation  Hard faeces being retained in the colon Indigestion  Improper digestion of food © 2022, Aakash BYJU'S. All rights reserved.