6.5 Absorption and Assimilation of Digested Food

In the ileum, two processes occur which are digestion and absorption.

The process of digestion is completed in the ileum to produce simple sugars ( glucose, fructose and galactose), amino acids, glycerol and fatty acids.

The process of absorption also occurs in the ileum to absorb the products of digestion into the blood capillaries and to be used by the cells in the body.

The products of digestion are absorbed into the body by small finger-like projections called villi (singular: villus) in the walls of the small intestine. Each villus contains a network of blood capillaries and a lymphatic vessel called lacteal in the centre of the villus.

Absorption of digested food

  • Nutrient absorption involves both diffusion and active transport. Initially, glucose, amino acids, water-soluble vitamins and minerals diffuse into the epithelial cells and are absorbed into the capillaries. Subsequently, the transport of the remaining nutrients across the epithelial lining involves active transport during which energy is used.
  • In contrast, glycerol and fatty acids enter the epithelial cells where they recombine to form tiny droplets of lipids, which then move into the lacteals. Fat-soluble vitamins are also absorbed into the lacteals to be transported together with lipids.
  • The lacteals converge into larger vessels of the lymphatic system. The fluid carrying lipids and fat-soluble vitamins enters the lymphatic system which forms a network throughout the body. 
  • The contents are then drained into the right lymphatic duct and thoracic duct before being emptied into the bloodstream through the subclavian veins. Capillaries that drain water-soluble nutrients away from the villi converge into hepatic portal vein, which leads to the liver. From here, the nutrients are transported to all cells in the body.
  • Together with the small intestine, the colon reabsorbs almost 90% of the water and minerals. In the colon, water and minerals are reabsorbed into the cells lining the colon, and subsequently into the bloodstream, so that we do not constantly lose them.
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