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UNIT 5, HUMAN PHYSIOLOGY, Chapter 16, Digestion and Absorption, , The reductionist approach to study of life forms resulted in increasing, use of physico-chemical concepts and techniques. Majority of these, studies employed either surviving tissue model or straightaway cellChapter 17, free systems. An explosion of knowledge resulted in molecular biology., Breathing and Exchange, Molecular physiology became almost synonymous with biochemistry, of Gases, and biophysics. However, it is now being increasingly realised that, neither a purely organismic approach nor a purely reductionistic, Chapter 18, molecular approach would reveal the truth about biological processes, Body Fluids and, or living phenomena. Systems biology makes us believe that all living, Circulation, phenomena are emergent properties due to interaction among, components of the system under study. Regulatory network of molecules,, Chapter 19, supra molecular assemblies, cells, tissues, organisms and indeed,, Excretory Products and, populations and communities, each create emergent properties. In the, their Elimination, chapters under this unit, major human physiological processes like, digestion, exchange of gases, blood circulation, locomotion and, Chapter 20, movement are described in cellular and molecular terms. The last two, Locomotion and Movement chapters point to the coordination and regulation of body events at the, organismic level., Chapter 21, Neural Control and, Coordination, Chapter 22, Chemical Coordination, and Integration, , 2020-21

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ALFONSO CORTI, Italian anatomist, was born in 1822. Corti began, his scientific career studying the cardiovascular systems of, reptiles. Later, he turned his attention to the mammalian, auditory system. In 1851, he published a paper describing a, structure located on the basilar membrane of the cochlea, containing hair cells that convert sound vibrations into nerve, impulses, the organ of Corti. He died in the year 1888., , Alfonso Corti, (1822 – 1888), , 2020-21

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CHAPTER 16, D IGESTION AND ABSORPTION, 16.1 Digestive, System, 16.2 Digestion of, Food, 16.3 Absorption of, Digested, Products, 16.4 Disorders of, Digestive, System, , Food is one of the basic requirements of all living organisms. The major, components of our food are carbohydrates, proteins and fats. Vitamins, and minerals are also required in small quantities. Food provides energy, and organic materials for growth and repair of tissues. The water we take, in, plays an important role in metabolic processes and also prevents, dehydration of the body. Biomacromolecules in food cannot be utilised, by our body in their original form. They have to be broken down and, converted into simple substances in the digestive system. This process of, conversion of complex food substances to simple absorbable forms is, called digestion and is carried out by our digestive system by mechanical, and biochemical methods. General organisation of the human digestive, system is shown in Figure 16.1., , 16.1 D IGESTIVE SYSTEM, The human digestive system consists of the alimentary canal and the, associated glands., , 16.1.1 Alimentary Canal, The alimentary canal begins with an anterior opening – the mouth, and it, opens out posteriorly through the anus. The mouth leads to the buccal, cavity or oral cavity. The oral cavity has a number of teeth and a muscular, tongue. Each tooth is embedded in a socket of jaw bone (Figure16.2)., This type of attachment is called thecodont. Majority of mammals, including human being forms two sets of teeth during their life, a set of, temporary milk or deciduous teeth replaced by a set of permanent or, adult teeth. This type of dentition is called diphyodont. An adult human, , 2020-21

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258, , BIOLOGY, , Sigmoid Colon, , Figure 16.1 The human digestive system, , has 32 permanent teeth which are of four different types (Heterodont, dentition), namely, incisors (I), canine (C), premolars (PM) and molars, (M). Arrangement of teeth in each half of the upper and lower jaw in the, order I, C, PM, M is represented by a dental formula which in human, , 2123, . The hard chewing surface of the teeth, made up of enamel, helps, 2123, in the mastication of food. The tongue is a freely movable muscular organ, attached to the floor of the oral cavity by the frenulum. The upper surface, of the tongue has small projections called papillae, some of which bear, taste buds., The oral cavity leads into a short pharynx which serves as a common, passage for food and air. The oesophagus and the trachea (wind pipe), open into the pharynx. A cartilaginous flap called epiglottis prevents the, entry of food into the glottis – opening of the wind pipe – during swallowing., The oesophagus is a thin, long tube which extends posteriorly passing, through the neck, thorax and diaphragm and leads to a ‘J’ shaped bag, is, , 2020-21

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DIGESTION AND ABSORPTION, , like structure called stomach. A muscular, sphincter (gastro-oesophageal) regulates the, opening of oesophagus into the stomach., The stomach, located in the upper left, portion of the abdominal cavity, has four, major parts – a cardiac portion into which, the oesophagus opens, a fundic region, body, (main central region) and a pyloric portion, which opens into the first part of small, intestine (Figure 16.3). Small intestine is, distinguishable into three regions, a ‘C’, shaped duodenum, a long coiled middle, portion jejunum and a highly coiled ileum., The opening of the stomach into the, duodenum is guarded by the pyloric, sphincter. Ileum opens into the large, intestine. It consists of caecum, colon and, rectum. Caecum is a small blind sac which, hosts some symbiotic micro-organisms. A, narrow finger-like tubular projection, the, vermiform appendix which is a vestigial, organ, arises from the caecum. The caecum, opens into the colon. The colon is divided, into four parts – an ascending, a transverse,, descending part and a sigmoid colon. The, descending part opens into the rectum, which opens out through the anus., The wall of alimentary canal from, oesophagus to rectum possesses four layers, (Figure 16.4) namely serosa, muscularis,, sub-mucosa and mucosa. Serosa is the, outermost layer and is made up of a thin, mesothelium (epithelium of visceral organs), with some connective tissues. Muscularis is, formed by smooth muscles usually, arranged into an inner circular and an outer, longitudinal layer. An oblique muscle layer, may be present in some regions. The submucosal layer is formed of loose connective, tissues containing nerves, blood and lymph, vessels. In duodenum, glands are also, present in sub-mucosa. The innermost, layer lining the lumen of the alimentary, canal is the mucosa. This layer forms, irregular folds (rugae) in the stomach and, small finger-like foldings called villi in the, small intestine (Figure 16.5). The cells lining, the villi produce numerous microscopic, , 259, , Figure 16.2, , Arrangement of different types of, teeth in the jaws on one side and, the sockets on the other side, , Figure 16.3 Anatomical regions of human, stomach, , 2020-21

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260, , BIOLOGY, , Figure 16.4 Diagrammatic representation of transverse section of gut, , Villi, Lacteal, , Capillaries, Artery, Crypts, , projections called microvilli giving a brush border, appearance. These modifications increase the, surface area enormously. Villi are supplied with, a network of capillaries and a large lymph vessel, called the lacteal. Mucosal epithelium has goblet, cells which secrete mucus that help in lubrication., Mucosa also forms glands in the stomach (gastric, glands) and crypts in between the bases of villi in, the intestine (crypts of Lieberkuhn). All the four, layers show modifications in different parts of the, alimentary canal., , 16.1.2, , The digestive glands associated with the, alimentary canal include the salivary glands, the, liver and the pancreas., A section of small intestinal, Saliva is mainly produced by three pairs of, mucosa showing villi, salivary glands, the parotids (cheek), the submaxillary/sub-mandibular (lower jaw) and the, sub- linguals (below the tongue). These glands, situated just outside the buccal cavity secrete, salivary juice into the buccal cavity., Liver is the largest gland of the body weighing about 1.2 to 1.5 kg in, an adult human. It is situated in the abdominal cavity, just below the, diaphragm and has two lobes. The hepatic lobules are the structural and, functional units of liver containing hepatic cells arranged in the form of, cords. Each lobule is covered by a thin connective tissue sheath called, the Glisson’s capsule. The bile secreted by the hepatic cells passes through, the hepatic ducts and is stored and concentrated in a thin muscular sac, called the gall bladder. The duct of gall bladder (cystic duct) along with, Vein, , Figure 16.5, , Digestive Glands, , 2020-21

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DIGESTION AND ABSORPTION, , 261, , the hepatic duct from the liver forms the common bile duct (Figure 16.6)., The bile duct and the pancreatic duct open together into the duodenum, as the common hepato-pancreatic duct which is guarded by a sphincter, called the sphincter of Oddi., The pancreas is a compound (both exocrine and endocrine) elongated, organ situated between the limbs of the ‘C’ shaped duodenum. The, exocrine portion secretes an alkaline pancreatic juice containing enzymes, and the endocrine portion secretes hormones, insulin and glucagon., , Figure 16.6 The duct systems of liver, gall bladder and pancreas, , 16.2 DIGESTION, , OF, , FOOD, , The process of digestion is accomplished by mechanical and chemical, processes., The buccal cavity performs two major functions, mastication of food, and facilitation of swallowing. The teeth and the tongue with the help of, saliva masticate and mix up the food thoroughly. Mucus in saliva helps, in lubricating and adhering the masticated food particles into a bolus., The bolus is then conveyed into the pharynx and then into the oesophagus, by swallowing or deglutition. The bolus further passes down through, the oesophagus by successive waves of muscular contractions called, peristalsis. The gastro-oesophageal sphincter controls the passage of food, into the stomach.The saliva secreted into the oral cavity contains, electrolytes, and enzymes, salivary amylase and, lysozyme. The chemical process of digestion is initiated in the oral cavity, by the hydrolytic action of the carbohydrate splitting enzyme, the salivary, , 2020-21

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262, , BIOLOGY, , amylase. About 30 per cent of starch is hydrolysed here by this enzyme, (optimum pH 6.8) into a disaccharide – maltose. Lysozyme present in, saliva acts as an antibacterial agent that prevents infections., Salivary Amylase, , Starch , → Maltose, pH 6.8, , The mucosa of stomach has gastric glands. Gastric glands have three, major types of cells namely (i) mucus neck cells which secrete mucus;, (ii) peptic or chief cells which secrete the proenzyme pepsinogen; and, (iii) parietal or oxyntic cells which secrete HCl and intrinsic factor, (factor essential for absorption of vitamin B12)., The stomach stores the food for 4-5 hours. The food mixes thoroughly, with the acidic gastric juice of the stomach by the churning movements, of its muscular wall and is called the chyme. The proenzyme pepsinogen,, on exposure to hydrochloric acid gets converted into the active enzyme, pepsin, the proteolytic enzyme of the stomach. Pepsin converts proteins, into proteoses and peptones (peptides). The mucus and bicarbonates, present in the gastric juice play an important role in lubrication and, protection of the mucosal epithelium from excoriation by the highly, concentrated hydrochloric acid. HCl provides the acidic pH (pH 1.8), optimal for pepsins. Rennin is a proteolytic enzyme found in gastric juice, of infants which helps in the digestion of milk proteins. Small amounts of, lipases are also secreted by gastric glands., Various types of movements are generated by the muscularis layer of, the small intestine. These movements help in a thorough mixing up of, the food with various secretions in the intestine and thereby facilitate, digestion. The bile, pancreatic juice and the intestinal juice are the, secretions released into the small intestine. Pancreatic juice and bile are, released through the hepato-pancreatic duct. The pancreatic juice, contains inactive enzymes – trypsinogen, chymotrypsinogen,, procarboxypeptidases, amylases, lipases and nucleases. Trypsinogen is, activated by an enzyme, enterokinase, secreted by the intestinal mucosa, into active trypsin, which in turn activates the other enzymes in the, pancreatic juice. The bile released into the duodenum contains bile, pigments (bilirubin and bili-verdin), bile salts, cholesterol and, phospholipids but no enzymes. Bile helps in emulsification of fats, i.e.,, breaking down of the fats into very small micelles. Bile also activates lipases., The intestinal mucosal epithelium has goblet cells which secrete, mucus. The secretions of the brush border cells of the mucosa alongwith, the secretions of the goblet cells constitute the intestinal juice or, succus entericus. This juice contains a variety of enzymes like, disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc., The mucus alongwith the bicarbonates from the pancreas protects the, intestinal mucosa from acid as well as provide an alkaline medium (pH, 7.8) for enzymatic activities. Sub-mucosal glands (Brunner’s glands) also, help in this., , 2020-21

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264, , BIOLOGY, , No significant digestive activity occurs in the large intestine. The, functions of large intestine are:, (i) absorption of some water, minerals and certain drugs;, (ii) secretion of mucus which helps in adhering the waste (undigested), particles together and lubricating it for an easy passage., The undigested, unabsorbed substances called faeces enters into the caecum, of the large intestine through ileo-caecal valve, which prevents the back flow of, the faecal matter. It is temporarily stored in the rectum till defaecation., The activities of the gastro-intestinal tract are under neural and, hormonal control for proper coordination of different parts. The sight, smell, and/or the presence of food in the oral cavity can stimulate the secretion of, saliva. Gastric and intestinal secretions are also, similarly, stimulated by, neural signals. The muscular activities of different parts of the alimentary, canal can also be moderated by neural mechanisms, both local and through, CNS. Hormonal control of the secretion of digestive juices is carried out by, local hormones produced by the gastric and intestinal mucosa., CALORIFIC VALUE OF PROTEIN, CARBOHYDRATE, (Boxed item – Not for evaluation), , AND, , FAT, , The energy requirements of animals, and the energy content of food, are, expressed in terms of measure of heat energy because heat is the ultimate form, of all energies. This is often measured to as calorie (cal) or joule (J), which is the, amount of heat energy required to raise the temperature of 1 g of water by 1 °C., Since this value is tiny amount of energy, physiologists commonly use kilocalorie, (kcal) or kilo joule (kJ). One kilo calorie is the amount of energy required to raise, the temperature of 1 kg of water by 1 °C. Nutritionists, traditionally refer to kcal, as the Calorie or Joule (always capitalised). The amount of heat liberated from, complete combustion of 1 g food in a bomb calorimeter (a closed metal chamber, filled with O2) is its gross calorific or gross energy value. The actual amount of, energy combustion of 1 g of food is the physiologic value of food. Gross calorific, values of carbohydrates, proteins and fats are 4.1 kcal/g, 5.65 kcal/g and 9.45, kcal/g, respectively, whereas their physiologic values are 4.0 kcal/g, 4.0 kcal/g, and 9.0 kcal/g, respectively., , 16.3 ABSORPTION OF DIGESTED PRODUCTS, Absorption is the process by which the end products of digestion pass, through the intestinal mucosa into the blood or lymph. It is carried out by, passive, active or facilitated transport mechanisms. Small amounts of, monosaccharides like glucose, amino acids and some electrolytes like, chloride ions are generally absorbed by simple diffusion. The passage of, these substances into the blood depends upon the concentration gradients., However, some substances like glucose and amino acids are absorbed with, the help of carrier proteins. This mechanism is called the facilitated transport., Transport of water depends upon the osmotic gradient. Active, transport occurs against the concentration gradient and hence requires, energy. Various nutrients like amino acids, monosaccharides like glucose,, electrolytes like Na+ are absorbed into the blood by this mechanism., Fatty acids and glycerol being insoluble, cannot be absorbed into the, , 2020-21

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DIGESTION AND ABSORPTION, , 265, , blood. They are first incorporated into small droplets called micelles which, move into the intestinal mucosa. They are re-formed into very small protein, coated fat globules called the chylomicrons which are transported into, the lymph vessels (lacteals) in the villi. These lymph vessels ultimately, release the absorbed substances into the blood stream., Absorption of substances takes place in different parts of the alimentary, canal, like mouth, stomach, small intestine and large intestine. However,, maximum absorption occurs in the small intestine. A summary of absorption, (sites of absorption and substances absorbed) is given in Table 16.1., , TABLE 16.1 The Summary of Absorption in Different Parts of Digestive System, Mouth, , Stomach, , Small Intestine, , Large Intestine, , Certain drugs, coming in contact, with the mucosa, of mouth and, lower side of the, tongue are, absorbed into the, blood capillaries, lining them., , Absorption of, water, simple, sugars, and, alcohol etc., takes place., , Principal organ for absorption, of nutrients. The digestion is, completed here and the final, products of digestion such as, glucose, fructose, fatty acids,, glycerol and amino acids are, absorbed through the mucosa, into the blood stream and, lymph., , Absorption of, water, some, minerals and, drugs takes, place., , The absorbed substances finally reach the tissues which utilise them, for their activities. This process is called assimilation., The digestive wastes, solidified into coherent faeces in the rectum, initiate a neural reflex causing an urge or desire for its removal. The, egestion of faeces to the outside through the anal opening (defaecation) is, a voluntary process and is carried out by a mass peristaltic movement., , 16.4 DISORDERS OF DIGESTIVE SYSTEM, The inflammation of the intestinal tract is the most common ailment due, to bacterial or viral infections. The infections are also caused by the, parasites of the intestine like tapeworm, roundworm, threadworm,, hookworm, pin worm, etc., Jaundice: The liver is affected, skin and eyes turn yellow due to the, deposit of bile pigments., Vomiting: It is the ejection of stomach contents through the mouth. This, reflex action is controlled by the vomit centre in the medulla. A feeling of, nausea precedes vomiting., Diarrhoea: The abnormal frequency of bowel movement and increased, liquidity of the faecal discharge is known as diarrhoea. It reduces the, absorption of food., Constipation: In constipation, the faeces are retained within the colon, as the bowel movements occur irregularly., Indigestion: In this condition, the food is not properly digested leading to, a feeling of fullness. The causes of indigestion are inadequate enzyme, secretion, anxiety, food poisoning, over eating, and spicy food., , 2020-21

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266, , BIOLOGY, , PEM, , Dietary deficiencies of proteins and total food calories are widespread in, many underdeveloped countries of South and South-east Asia, South, America, and West and Central Africa. Protein-energy malnutrition, (PEM) may affect large sections of the population during drought, famine, and political turmoil. This happened in Bangladesh during the liberation, war and in Ethiopia during the severe drought in mid-eighties. PEM affects, infants and children to produce Marasmus and Kwashiorkar., Marasmus is produced by a simultaneous deficiency of proteins and, calories. It is found in infants less than a year in age, if mother’s milk is, replaced too early by other foods which are poor in both proteins and, caloric value. This often happens if the mother has second pregnancy or, childbirth when the older infant is still too young. In Marasmus, protein, deficiency impairs growth and replacement of tissue proteins; extreme, emaciation of the body and thinning of limbs results, the skin becomes, dry, thin and wrinkled. Growth rate and body weight decline considerably., Even growth and development of brain and mental faculties are impaired., Kwashiorkar is produced by protein deficiency unaccompanied by calorie, deficiency. It results from the replacement of mother’s milk by a high calorielow protein diet in a child more than one year in age. Like marasmus,, kwashiorkor shows wasting of muscles, thinning of limbs, failure of growth, and brain development. But unlike marasmus, some fat is still left under, the skin; moreover, extensive oedema and swelling of body parts are seen., , SUMMARY, The digestive system of humans consists of an alimentary canal and, associated digestive glands. The alimentary canal consists of the mouth,, buccal cavity, pharynx, oesophagus, stomach, small intestine, large, intestine, rectum and the anus. The accessory digestive glands include the, salivary glands, the liver (with gall bladder) and the pancreas. Inside the, mouth the teeth masticates the food, the tongue tastes the food and, manipulates it for proper mastication by mixing with the saliva. Saliva, contains a starch digestive enzyme, salivary amylase that digests the starch, and converts it into maltose (disaccharide). The food then passes into the, pharynx and enters the oesophagus in the form of bolus, which is further, carried down through the oesophagus by peristalsis into the stomach. In, stomach mainly protein digestion takes place. Absorption of simple sugars,, alcohol and medicines also takes place in the stomach., The chyme (food) enters into the duodenum portion of the small, intestine and is acted on by the pancreatic juice, bile and finally by the, enzymes in the succus entericus, so that the digestion of carbohydrates,, proteins and fats is completed. The food then enters into the jejunum and, ileum portions of the small intestine. Carbohydrates are digested and, converted into monosaccharides like glucose. Proteins are finally broken, down into amino acids. The fats are converted to fatty acids and glycerol., , 2020-21

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DIGESTION AND ABSORPTION, , 267, , The digested end products are absorbed into the body through the epithelial lining, of the intestinal villi. The undigested food (faeces) enters into the caecum of the large, intestine through ileo-caecal valve, which prevents the back flow of the faecal matter., Most of the water is absorbed in the large intestine. The undigested food becomes, semi-solid in nature and then enters into the rectum, anal canal and is finally egested, out through the anus., , EXERCISES, 1., , Choose the correct answer among the following :, (a) Gastric juice contains, (i) pepsin, lipase and rennin, (ii) trypsin, lipase and rennin, (iii) trypsin, pepsin and lipase, (iv) trypsin, pepsin and renin, (b) Succus entericus is the name given to, (i) a junction between ileum and large intestine, (ii) intestinal juice, (iii) swelling in the gut, (iv) appendix, , 2., , Match column I with column II, Column I, (a) Bilirubin and biliverdin, (b) Hydrolysis of starch, (c) Digestion of fat, (d) Salivary gland, , 3., , 4., 5., 6., 7., 8., 9., 10., 11., 12., 13., 14., 15., , Column II, (i) Parotid, (ii) Bile, (iii) Lipases, (iv) Amylases, , Answer briefly:, (a) Why are villi present in the intestine and not in the stomach?, (b) How does pepsinogen change into its active form?, (c) What are the basic layers of the wall of alimentary canal?, (d) How does bile help in the digestion of fats?, State the role of pancreatic juice in digestion of proteins., Describe the process of digestion of protein in stomach., Give the dental formula of human beings., Bile juice contains no digestive enzymes, yet it is important for digestion. Why?, Describe the digestive role of chymotrypsin. Which two other digestive enzymes, of the same category are secreted by its source gland?, How are polysaccharides and disaccharides digested?, What would happen if HCl were not secreted in the stomach?, How does butter in your food get digested and absorbed in the body?, Discuss the main steps in the digestion of proteins as the food passes through, different parts of the alimentary canal., Explain the term thecodont and diphyodont., Name different types of teeth and their number in an adult human., What are the functions of liver?, , 2020-21