NIT 5 - NCERT

1 UNIT 5 The reductionist approach to study of life forms resulted in increasinguse of physico-chemical concepts and techniques. Majority of thesestudies employed either surviving tissue model or straightaway cell-free systems. An explosion of knowledge resulted in molecular physiology became almost synonymous with biochemistryand biophysics. However, it is now being increasingly realised thatneither a purely organismic approach nor a purely reductionisticmolecular approach would reveal the truth about biological processesor living phenomena. Systems biology makes us believe that all livingphenomena are emergent properties due to interaction amongcomponents of the system under study. Regulatory network of molecules,supra molecular assemblies, cells, tissues, organisms and indeed,populations and communities, each create emergent properties. In thechapters under this unit, major human physiological processes likedigestion, exchange of gases, blood circulation, locomotion andmovement are described in cellular and molecular terms.

2 The last twochapters point to the coordination and regulation of body events at theorganismic PHYSIOLOGYC hapter 16 Digestion and AbsorptionChapter 17 Breathing and Exchangeof GasesChapter 18 Body Fluids andCirculationChapter 19 Excretory Products andtheir EliminationChapter 20 Locomotion and MovementChapter 21 Neural Control andCoordinationChapter 22 Chemical Coordinationand Integration2022-23 ALFONSO CORTI, Italian anatomist, was born in 1822. Corti beganhis scientific career studying the cardiovascular systems ofreptiles. Later, he turned his attention to the mammalianauditory system . In 1851, he published a paper describing astructure located on the basilar membrane of the cochleacontaining hair cells that convert sound vibrations into nerveimpulses, the organ of Corti. He died in the year Corti(1822 1888)2022-23 Food is one of the basic requirements of all living organisms. The majorcomponents of our food are carbohydrates, proteins and fats.

3 Vitaminsand minerals are also required in small quantities. Food provides energyand organic materials for growth and repair of tissues. The water we takein, plays an important role in metabolic processes and also preventsdehydration of the body. Biomacromolecules in food cannot be utilisedby our body in their original form. They have to be broken down andconverted into simple substances in the digestive system . This process ofconversion of complex food substances to simple absorbable forms iscalled digestion and is carried out by our digestive system by mechanicaland biochemical methods. General organisation of the human digestivesystem is shown in Figure SYSTEMThe human digestive system consists of the alimentary canal and theassociated CanalThe alimentary canal begins with an anterior opening the mouth, and itopens out posteriorly through the anus. The mouth leads to the buccalcavity or oral cavity. The oral cavity has a number of teeth and a musculartongue.

4 Each tooth is embedded in a socket of jaw bone ( ).This type of attachment is called thecodont. Majority of mammalsincluding human being forms two sets of teeth during their life, a set oftemporary milk or deciduous teeth replaced by a set of permanent oradult teeth. This type of dentition is called diphyodont. An adult humanDIGESTION AND ABSORPTIONCHAPTER ofDigestiveSystem2022-23258 BIOLOGYS igmoid ColonFigure The human digestive systemhas 32 permanent teeth which are of four different types (Heterodontdentition), namely, incisors (I), canine (C), premolars (PM) and molars(M). Arrangement of teeth in each half of the upper and lower jaw in theorder I, C, PM, M is represented by a dental formula which in humanis 21232123. The hard chewing surface of the teeth, made up of enamel, helpsin the mastication of food. The tongue is a freely movable muscular organattached to the floor of the oral cavity by the frenulum.

5 The upper surfaceof the tongue has small projections called papillae, some of which beartaste oral cavity leads into a short pharynx which serves as a commonpassage for food and air. The oesophagus and the trachea (wind pipe)open into the pharynx. A cartilaginous flap called epiglottis prevents theentry of food into the glottis opening of the wind pipe during oesophagus is a thin, long tube which extends posteriorly passingthrough the neck, thorax and diaphragm and leads to a J shaped bag2022-23 DIGESTION AND ABSORPTION259like structure called stomach. A muscularsphincter (gastro-oesophageal) regulates theopening of oesophagus into the stomach, located in the upper leftportion of the abdominal cavity, has fourmajor parts a cardiac portion into whichthe oesophagus opens, a fundic region, body(main central region) and a pyloric portionwhich opens into the first part of smallintestine (Figure ).

6 Small intestine isdistinguishable into three regions, a C shaped duodenum, a long coiled middleportion jejunum and a highly coiled opening of the stomach into theduodenum is guarded by the pyloricsphincter. Ileum opens into the largeintestine. It consists of caecum, colon andrectum. Caecum is a small blind sac whichhosts some symbiotic micro-organisms. Anarrow finger-like tubular projection, thevermiform appendix which is a vestigialorgan, arises from the caecum. The caecumopens into the colon. The colon is dividedinto four parts an ascending, a transverse,descending part and a sigmoid colon. Thedescending part opens into the rectumwhich opens out through the wall of alimentary canal fromoesophagus to rectum possesses four layers(Figure ) namely serosa, muscularis,sub-mucosa and mucosa. Serosa is theoutermost layer and is made up of a thinmesothelium (epithelium of visceral organs)with some connective tissues.

7 Muscularis isformed by smooth muscles usuallyarranged into an inner circular and an outerlongitudinal layer. An oblique muscle layermay be present in some regions. The sub-mucosal layer is formed of loose connectivetissues containing nerves, blood and lymphvessels. In duodenum, glands are alsopresent in sub-mucosa. The innermostlayer lining the lumen of the alimentarycanal is the mucosa. This layer formsirregular folds (rugae) in the stomach andsmall finger-like foldings called villi in thesmall intestine (Figure ). The cells liningthe villi produce numerous microscopicFigure of different types ofteeth in the jaws on one side andthe sockets on the other sideFigure regions of humanstomach2022-23260 BIOLOGYF igure Diagrammatic representation of transverse section of gutprojections called microvilli giving a brush borderappearance. These modifications increase thesurface area enormously.

8 Villi are supplied witha network of capillaries and a large lymph vesselcalled the lacteal. Mucosal epithelium has gobletcells which secrete mucus that help in also forms glands in the stomach (gastricglands) and crypts in between the bases of villi inthe intestine (crypts of Lieberkuhn). All the fourlayers show modifications in different parts of thealimentary GlandsThe digestive glands associated with thealimentary canal include the salivary glands, theliver and the is mainly produced by three pairs ofsalivary glands, the parotids (cheek), the sub-maxillary/sub-mandibular (lower jaw) and thesub- linguals (below the tongue). These glandssituated just outside the buccal cavity secretesalivary juice into the buccal is the largest gland of the body weighing about to kg inan adult human. It is situated in the abdominal cavity, just below thediaphragm and has two lobes. The hepatic lobules are the structural andfunctional units of liver containing hepatic cells arranged in the form ofcords.

9 Each lobule is covered by a thin connective tissue sheath calledthe Glisson s capsule. The bile secreted by the hepatic cells passes throughthe hepatic ducts and is stored and concentrated in a thin muscular saccalled the gall bladder. The duct of gall bladder (cystic duct) along withFigure section of small intestinalmucosa showing villiVilliLactealCapillariesCryptsArtery Vein2022-23 DIGESTION AND ABSORPTION261the hepatic duct from the liver forms the common bile duct (Figure ).The bile duct and the pancreatic duct open together into the duodenumas the common hepato-pancreatic duct which is guarded by a sphinctercalled the sphincter of pancreas is a compound (both exocrine and endocrine) elongatedorgan situated between the limbs of the C shaped duodenum. Theexocrine portion secretes an alkaline pancreatic juice containing enzymesand the endocrine portion secretes hormones, insulin and The duct systems of liver, gall bladder and OF FOODThe process of digestion is accomplished by mechanical and buccal cavity performs two major functions, mastication of foodand facilitation of swallowing.

10 The teeth and the tongue with the help ofsaliva masticate and mix up the food thoroughly. Mucus in saliva helpsin lubricating and adhering the masticated food particles into a bolus is then conveyed into the pharynx and then into the oesophagusby swallowing or deglutition. The bolus further passes down throughthe oesophagus by successive waves of muscular contractions calledperistalsis. The gastro-oesophageal sphincter controls the passage of foodinto the saliva secreted into the oral cavity containselectrolytes and enzymes, salivary amylase andlysozyme. The chemical process of digestion is initiated in the oral cavityby the hydrolytic action of the carbohydrate splitting enzyme, the salivary2022-23262 biology amylase. About 30 per cent of starch is hydrolysed here by this enzyme(optimum pH ) into a disaccharide maltose. Lysozyme present insaliva acts as an antibacterial agent that prevents 8 The mucosa of stomach has gastric glands.