DIGESTION AND ABSORPTION

POINTS TO REMEMBER :

 

Digestion : enzymatic conversion of complex food substances to simple absorbable forms in the alimentary canal.

DIGESTIVE SYSTEM :

Includes:

  • Alimentary canal
  • Digestive glands or associated glands.

Alimentary canal :

  • The alimentary canal begins with mouth and ends with anus.
  • Mouth leads to buccal cavity or oral cavity.
  • Oral cavity has teeth and muscular tongue.
  • Each tooth embedded in a socket of jaw bone: such attachment called thecodont.
  • Diphyodont : human has two sets of teeth in their life time:
    • Milk teeth or deciduous teeth
    • Permanent teeth.

  • Heterodont : teeth are of unequal shape and size.
    • Incisor (I)
    • Canine (C)
    • Premolar (PM)
    • Molar (M).

  • Dental formula : arrangement of teeth in each half of the upper jaw and lower jaw.

  • Dental formula of human adult is
  • The hard chewing surface of the teeth made up of enamel.
  • The tongue is a freely movable muscular organ attached to the floor of the oral cavity by the frenulum.
  • The upper surface of tongue has small projections called papillae, some of which bears taste buds.
  • The oral cavity leads into a short pharynx which serves as a common passage for food and air.
  • Oesophagus and the trachea open into the pharynx.
  • Opening of wind pipe or trachea called glottis, and that of oesophagus is called gullet.
  • The cartilaginous epiglottis prevents the entry of food into the glottis during swallowing.
  • Oesophagus connects pharynx with stomach.
  • Opening of oesophagus is regulated by gastro-oesophageal sphincter.
  • The stomach has three parts:
    • Cardiac: into which oesophagus opens.
    • Fundus: air filled portion of stomach.
    • Pyloric: portion opens into the small intestine.

  • Small intestine distinguished into three parts:
    • Duodenum: ā€˜Uā€™ shaped first part.
    • Jejunum: longer, coiled middle portion.
    • Ileum: highly coiled posterior part.

  • The opening of stomach into the duodenum is guarded by pyloric sphincter.
  • Large intestine consists of three parts:
    • Caecum
    • Colon
    • Rectum.

  • Caecum is a small blind sac which hosts some symbiotic micro-organisms.
  • Caecum has a finger-like blind tubular projection called vermiform appendix.
  • The Caecum opens into colon, which has three distinct part-
    • Ascending colon
    • Transverse colon
    • Descending colon

  • The descending colon opens into rectum which opens to out through anus.

Histolology of alimentary canal :

  • Alimentary canal from oesophagus to rectum has four layers.
    • Serosa.
    • Muscularis.
    • Sub mucosa.
    • Mucosa.

  • Serosa is the outermost layer and is made up of a thin mesothelium with some connective tissues.
  • Muscularis is formed by smooth muscles arranged outer longitudinal and inner circular layers.
  • Sub-mucosa is formed by loose connective tissues containing nerves, blood and lymph vessels.
  • Mucosa is the innermost layer made of endothelium.
  • Mucosa forms irregular folds (rugae) in the stomach and small finger like folding called villi in the small intestine.
  • The cells lining the villi produce numerous microscopic projections called microvilli giving a brush border appearance.
  • These modifications increase the surface area for absorption.
  • Villi are supplied with a network of capillaries and a central lymphatic vessel called lacteal.
  • Epithelial cells of mucosa contain secretory cells which secretes digestive enzymes.
  • Mucosa also forms glands in the stomach (gastric gland)
  • Mucosa forms crypts in between the bases of villi in the intestine called Crypts of Lieberkuhn.

 

CBSE Biology (Chapter Wise) Class XI ( By Mr. Hare Krushna Giri ) 
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