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Absorption of Digested Food

• Breakdown of bio-macromolecules during digestion basically occurs in duodenum region of the small intestine.
• Simple substances formed are absorbed in the jejunum and ileum regions of the small intestine.
• Undigested and unabsorbed substances are passed on to the large intestine. No digestive activity occurs in the large intestine.
• Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph.
• 90% of all absorption of nutrients occurs in small intestine; only 10% occurs in stomach and large intestine.
• It is carried out by passive, active or facilitated transport mechanisms.
• All dietary carbohydrates that are digested normally are absorbed, leaving only indigestible cellulose and fibers in the feces.
• Fructose and amino acids are absorbed by mucosal epithelium via facilitated diffusion / facilitated transport with the help of carrier ions Na+ .
• Small amounts of monosacharides like glucose, amino acids and some of electrolytes like chloride ions are generally absorbed by simple diffusion. The passage of these substances into the blood depends upon the concentration gradients.
• After entering mucosal epithelium these substances pass into the blood capillaries.

• Small short-chain fatty acids are absorbed by simple diffusion; while long-chain fatty acids (with more than 10–12 carbon atoms) and monoglycerides being insoluble cannot be absorbed as such.
• These long-chain fatty acids are first incorporated into small droplets called micelles with the help of bile. Micelle is a tiny sphere which has a diameter of 2–10 nm, which then move into the intestinal mucosa.
• Micelles are re-formed into very small protein coated fat globules called the Chylomicrons (80nm in dia.) which are transported into the lymph vessels (lacteals) in the villi. These lymph vessels ultimately release the absorbed substances into the blood stream.
• Chylomicrons consists of triglycerides, phospholipids, cholesterol and proteins.
• Chylomicrons are too large and bulky to enter the blood vessels directly, so they enter lacteals  which ultimately release the absorbed substances into the blood stream.
  
• Transport of water depends upon the osmotic gradient.
• Absorption of water, simple sugars, and alcohol takes place in stomach.
• Certain drugs get absorbed into mucosa of mouth and through lower side of tongue.
• Absorbed substances finally reach the tissues which utilize them for their activities. This process is called Assimilation.