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Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Chapter Contents Bio-macromolecules (proteins, nucleic acids, polysaccharides and lipids) in food cannot be utilized 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. DIGESTIVE SYSTEM In human beings, the digestive system consists of the alimentary canal and the associated glands (these glands help in digestion of food). Oral cavity Mouth Submaxillary and sublingual glands Liver Gall bladder Duodenum Transverse colon Ascending colon Ileum Caecum Vermiform appendix Anus Parotid gland Pharynx Oesophagus Stomach Pancreas Jejunum Descending colon Rectum Sigmoid colon Fig. : Human digestive system Chapter 3 Digestion and Absorption Digestive System Digestion of Food Absorption of Digested Products Disorders of Digestive System
80 Digestion and Absorption NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Alimentary Canal It starts with mouth and opens outside 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. Teeth Majority of mammals including human beings, have two sets of teeth during their life, i.e., one temporary set and one permanent set. A set of temporary milk or deciduous teeth is replaced by a set of permanent or adult teeth. Such type of dentition is called diphyodont. Each tooth is embedded in a socket of jaw bone. This type of teeth attachment is called thecodont. Incisor Canine Premolars Molars Socket of jaw 1 1 2 3 4 5 6 7 8 6 4 3 Fig. : Arrangement of different types of teeth in the jaws on one side and the sockets on the other side An adult human has 32 permanent teeth which are of four different types, namely incisors (I), canines (C), premolars (PM) and molars (M). Such type of dentition is called Heterodont dentition. The arrangement of teeth in each half of the upper and lower jaw in the order I, C, PM and M is represented by a dental formula. The dental formula of an adult human being is 2123 8 16 2 2 32 2123 8 16    . Out of these 32 teeth, premolars and last molar teeth appear only once i.e., are monophyodont and the remaining teeth are diphyodont The hard chewing surface covering the teeth is made up of enamel which helps in the mastication of food. Enamel is the hardest substance of the human body, secreted by ameloblasts (ectodermal in origin). Each tooth has three parts, namely crown, neck and root. Crown remains capped with hard enamel, formed principally of calcium phosphate; it forms the chewing surface of the tooth. Both crown and root are composed of dentine, secreted by odontoblasts (mesodermal in origin), inside which a central pulp cavity is present in which there is nerve and blood supply. Nerves and blood vessels in root canal Periodontal ligament Bony socket Pulp cavity Dentine Enamel Fig. : Vertical section of human tooth
NEET Digestion and Absorption 81 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Human teeth are: (a) Thecodont : Embedded in sockets and have well-developed roots (mammals and crocodiles), according to position. (b) Bunodont: Small, blunt and rounded cusps e.g., human, as per arrangement of enamel and dentine. Tongue The tongue is a voluntary, muscular and glandular structure which occupies the floor of the mouth. It is attached to the floor of the mouth by a fold called the frenulum of the tongue. An inverted V-shaped furrow termed the sulcus terminalis divides the upper surface of the tongue into anterior oral part and posterior pharyngeal part. Papillae present in tongue detect different tastes. The different areas of tongue are demarcated as follows : Tip – sweet Tips and sides – salt Sides – sour Base – bitter There are no taste buds for tasting chillies. The burning sensation is due to stimulation of pain receptors. Pharynx The oral cavity leads into pharynx. It is a common passage for food and air (breathing). Note : Tonsils : Lymphoid tissue of pharynx is called tonsils. It includes (i) Nasopharyngeal/pharyngeal tonsil/adenoids (ii) Palatine/faucial tonsils (iii) Lingual tonsils (iv) Tubal tonsils Oesophagus The pharynx leads into oesophagus. Glottis is the opening of the windpipe. A cartilaginous flap called epiglottis prevents the entry of food into the glottis 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-like structure called stomach. The opening of oesophagus into the stomach is regulated by a muscular (gastro-oesophageal) sphincter. Oesophagus is characterized by : Absence of digestive glands. Presence of mucus-secreting goblet cells. Presence of voluntary (in anterior 1/3rd) and involuntary muscle fibres (in posterior 2/3rd) Stomach It is 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 and a pyloric portion which opens into the first part of small intestine.
82 Digestion and Absorption NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Oesophagus Fundus Cardiac Pyloric Superior portion of duodenum Body Fig.: Anatomical regions of human stomach Cardiac sphincter is present at the opening of oesophagus into stomach and prevents the regurgitation of food into oesophagus. The pyloric part opens into duodenum (small intestine) and its opening is guarded by pyloric sphincter. Gastric Glands 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 (iii) Parietal or oxyntic cells which secrete HCl and intrinsic factor (factor essential for absorption of vitamin B12 in intestine). Small intestine: It is distinguished into three regions: (i) Duodenum (C-shaped) (ii) Jejunum (Long and coiled middle portion) (iii) Ileum (Highly coiled, longest portion) Ileum, the last part of small intestine leads into the large intestine. The wall of intestine has thin layers of longitudinal and circular muscles. Epithelial cells lining the villi have microvilli which further increase the absorptive area. Intestinal glands or Crypts of Leiberkuhn have epithelial cells (secrete mucus), Paneth cells (secrete digestive enzymes) and argentaffin cells (probably secrete hormones). In duodenum, Brunner’s glands are also present (located in submucosa) which secrete mucus. Diffused patches of lymphoid tissues are present through out the small intestine and are aggregated in ileum to form Peyer's patches. Large intestine: Large intestine consists of caecum, colon and rectum. (i) Caecum: It is a small blind sac which hosts some symbiotic micro-organisms. Vermiform appendix (a finger-like tubular projection) arises from the caecum and is considered vestigial in human beings. The caecum opens into the colon. (ii) Colon: It is divided into three parts, an ascending, a transverse and a descending part. The descending part of colon opens into rectum. Ascending colon is the smallest part and lacks mesentry. (iii) Rectum : It is the last part of alimentary canal. It is the storage site for faecal matter. It opens outside through anus.

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