Absorption of digested food in animals
Absorption of digested food in animals is the process whereby the products of digestion and the digested foodstuff form the lumen of the gut is transferred to the blood or the lymph across the epithelial cell membrane.
Site of absorption
No absorption of food or end products of digestion take place in the mouth and oesophagus area. In the monogastric animal, absorption in the stomach also very limited.
However certain drugs can be absorbed from the pharynx and to a limited extent from the oesophagus.
The small intestine is the chief site of absorption in the carnivores and omnivores. Villi is the the chief site of absorption in small intestine
The large intestine is the chief organ of absorption in all herbivores (eg. horse) and to a limited extent in the carnivores and man where it is restricted to the initial colon.
In ruminants, the digestion and absorption of contents is of special importance in the anterior part of the digestive tract.
The large intestine has specific absorptive sites with respect to water and electrolytes.
In herbivores, especially in equines, the large intestine absorbs volatile fatty acids and ammonia.
Routes of absorption
Small intestine has extremely well developed lymphatic and blood system which function as a route of absorption of digestive products in animals-
1. Absorption by Lymph
In the core of the villus lymph capillary originates as lacteal near the tip of the villus and enters into a plexus of lymph vessels lying just on the inner side of the muscular coat. Branches of these plexus then enter into the submucosa and form a loose plexus of large lymphatics, finally pass into mesentry.
The lymph capillaries drain their content into large lymph vessels, which intern empty into the mesentric vessels. These mesentric vessels are then connected with mesentric lymph nodes. The contents of the mesentric vessels empty into the cisterna chyli which is continued forward as thoracic duct, finally empties into the venous system anterior to heart.
Glycerides, long chain fatty acids, cholesterol and the immuno globulins during the first 24 hours of life are absorbed by the lymphatic system. The rate of lymph flow increases after a meal.
2. Absorption by Blood
Each villus contains several small arteries, which enter the base of the villus and form a dense capillary network immediately under its epithelium. Near the tip of the villus, one or two veins arise from a capillary network and run downward.
The venules and veins, drain into the portal vein. The portal vein enters into the liver where its blood is mixed with that of hepatic artery. The hepatic vein conveys the blood from the liver to the posterior vena cava.
Amino acids, monosaccharides, free glycerol, water, inorganic salts and short chain fatty acids are absorbed through blood route. After a meal, rapid flow of blood causes increased absorption rate, but this increase is less than that of lymph.
Mechanism of absorption
The possible mechanisms of absorption are broadly classified into three groups-
- Non-carrier mediated transport (passive diffusion)
- It depends on the electro-chemical gradient, occurs through channel pathways of ions.
- Non-carrier mediated process aids in the absorption of short chain fatty acids, inorganic salts and lipid soluble compounds.
- Carrier mediated transport – includes
- Facilitated transport from higher to lower concentration
- Exchange diffusion Na+/H+ exchanger: transports H+ out and Na+ into the cell
- Active transport against concentration gradient with expenditure of energy
- The carrier-mediated process may help water-soluble materials to pass the lipid layer of the cell membrane, whereas glucose and amino acids are absorbed by active transport.
- Pinocytosis transport of intact luminal materials in the form of vacuoles into the mucosal cells. This is important for absorption of intact proteins and intact triglycerides.
