16. DIGESTION AND ABSORPTION
- Nutrition is the getting and utilization of energy rich nutrients (food) by an organism.
- Food consists of carbohydrates, proteins, fats (lipids), vitamins, minerals and water.
- Food provides energy for life activities, materials for growth, maintains body temperature and repairs tissues.
- The water plays an important role in metabolic processes and prevents dehydration of the body.
I. ALIMENTARY CANAL (GUT)
It includes the following parts:
· Mouth: To
receive the food (ingestion).
· Buccal cavity (oral or mouth cavity):
- Consists
of palate (roof), teeth & muscular tongue.
- Palate has anterior hard palate and posterior
soft
palate.
- Tongue is a muscular organ attached to the floor of oral cavity by the frenulum. Tongue has small projections called papillae. Some papillae bear taste buds.
- At the back, on either side of tongue tonsils present.
· Pharynx:
- Common passage for digestive and respiratory systems.
- When food materials pass through the pharynx, the cartilaginous epiglottis closes the glottis (opening of larynx) and prevents the entry of food into trachea.
· Oesophagus:
- Muscular
tube (30 cm) that conducts food into stomach.
- Posterior
part of the oesophagus has gastro-oesophageal sphincter (a
circular muscle). It controls the opening of oesophagus into stomach.
· Stomach:
- ‘J’ shaped structure for storage and digestion of food.
- 4 parts: a cardiac portion into which the oesophagus opens, a fundic region, body (main central region) and a pyloric portion (antrum).
- Pyloric stomach leads to small intestine by an opening called Pylorus, guarded by pyloric sphincter muscle.
- Inner wall of stomach bears rugae (longitudinal folds).
·
- Longest part of gut (7 m long and 2.5 cm diameter).
- Consists 3 parts, namely duodenum (C shaped first part), Jejunum (middle part) and Ileum (terminal part).
- Finger-like villi are seen at the mucosa. Each villus has a brush-bordered columnar epithelial layer provided with microvilli. Villus consists of a capillary network and a small lymph vessel (lacteal).
· Large intestine:
- 1.5 m long. Consists of caecum, colon and rectum.
- Caecum is well-developed in herbivores but very small in man. Arising from the caecum is a finger-like vestigial organ, the vermiform appendix.
- The colon consists of ascending colon, transverse colon, descending colon and Sigmoid colon.
- Pelvic colon leads to rectum that opens out by anus. Anus is guarded by anal sphincter (circular voluntary muscles).
- In some herbivores, the large intestine consists of several cellulose digesting bacteria.
TEETH
Human dentition is Thecodont, Heterodont & Diphyodont.
- Thecodont: It means teeth are placed in the jaw sockets.
- Heterodont: It means different kinds of teeth are present. They are incisors (I) for cutting, canines (C) for tearing, premolars (PM) & molars (M) for mastication. Premolars & molars are collectively called as cheek teeth which have cusps.
- Diphyodont: It means teeth appear twice in the lifetime. They are milk (deciduous) teeth and permanent teeth.
Milk teeth (20 in number) are erupted at 6-7 months of birth. They are replaced by permanent teeth (32 in number) at the age of 6-7.Last 4 molars (wisdom teeth) appear only at the age of 18.The hard chewing surface of teeth is made up of enamel.Dental formula: It explains the kinds and number of teeth.
Human dental formula (of permanent teeth):
I 2/2 C 1/1 PM 2/2 M 3/3 x 2 = 32
Dental formula of milk dentition:
I 2/2 C 1/1 PM 0/0 M 2/2 x 2 = 20
Histology of
human gut (Transverse section)
- Mucosa: Innermost, moist epithelial layer. Contains secretory and absorptive cells.
- Submucosa: Soft connective tissue layer just outside the mucosa. Nerves and blood vessels are present.
- Muscularis: Outer to submucosa. Smooth muscle layer (inner circular & outer longitudinal muscles).
- Serosa: Outermost fibrous layer.