14 Human Nutrition - part 05 - Absorption, assimilation and egestion:

 

14 Human Nutrition - part 05 - Absorption, assimilation and egestion:


Absorption, assimilation and egestion:

  • The passage of end products of digestion through the mucosal lining of alimentary canal into blood and lymph is called absorption.
  • Absorption takes place by various ways like
  1. simple diffusion 
  2. osmosis
  3. facilitated transport and 
  4. active transport. 
  • About 90% of absorption takes place in the small intestine and the rest in mouth, stomach and large intestine.


Mouth: 

  • Absorption takes place through mucosa of mouth and lower side of tongue into the blood capillaries. 
  • e.g.Some drugs like certain painkillers.


Stomach: 

  • Gastric mucosa is impermeable to most substances hence nutrients reach unabsorbed till small intestine.
  • Little water, electrolytes, alcohol and drugs like aspirin get absorbed in stomach.


Small Intestine:

  • Glucose, fructose, galactose, amino acids, minerals and water soluble vitamins are absorbed in blood capillaries in villi.
  • Lipids and fat soluble vitamins (A, D, E, K) are absorbed in lacteals.


Large intestine:

  • Absorption of water, electrolytes like sodium and chloride, drugs and some vitamins takes place.


  • Absorption of part of glucose, amino acids and some electrolytes like chloride ions are absorbed by simple diffusion depending on concentration gradient.
  • Some amino acids as well as substances like fructose are absorbed by facilitated transport. 
  • In this method, carrier ions like Na+ bring about absorption. 
  • Some ions are absorbed against concentration gradient. It requires energy. This type of absorption of mineral like sodium is called active transport. 
  • Water is absorbed along the concentration gradient.
  • Monoglycerides and fatty acids can not be absorbed in blood. These dissolve in the centre of spherical aggregates formed by bile salts called micelles.
  • Micelles enter into intestinal villi. Here, they are reformed into chylomicrons. 
  • Chylomicrons are small protein coated fat globules. 
  • They are transported into lymph vessels called lacteals. From here, they are transported to blood stream.


Assimilation : 

  • The absorbed food material finally reaches the tissue and becomes a part of protoplasm. This is called as assimilation


Egestion : 

  • Undigested waste is converted to faeces in colon and reaches rectum. 
  • Faeces contain water, inorganic salts, sloughed of mucosal cells, bacteria and undigested food.
  • Distension of rectum stimulates pressure sensitive receptors that initiate a neural reflex for defecation or egestion. 
  • It is a voluntary process that takes place through anal opening guarded by sphincter muscles.

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