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The mechanism of absorption has been explained on the basis of several physico-chemical principles. Of them, diffusion is the most important. The concentration of the end products of digestion is much higher in the intestine than in the blood. This difference initiates diffusion currents from the lumen of the intestine into the bloodstream and thus helps in absorption. The active transport mechanism involves movement of materials from the intestine to the blood and lymph of villi against a concentration gradient and needs energy. Since the intestinal wall has the ability to transport many substances actively, they can be removed from the intestine even though they may be present in the blood in a higher concentration.
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