Concept preview

What you will be able to do

1. 01Explain how villi and microvilli of the small intestine create a huge surface area for absorption.
2. 02Describe the absorption of glucose and amino acids into blood capillaries and of fats into lacteals.
3. 03State the role of the large intestine in absorbing water and vitamins.
4. 04Explain the role of the hepatic portal vein in carrying absorbed nutrients to the liver.

Absorption is the step where the small, soluble products of digestion — glucose, amino acids, fatty acids and glycerol — finally cross the gut wall and pass into the body's transport systems. Almost all of it happens in the small intestine, and the challenge is speed: food moves through in only a few hours, so the lining must grab nutrients extremely efficiently. Nature's solution is to maximise surface area. The rate at which molecules can be absorbed depends directly on how much surface is available, so evolution has folded and refolded the intestinal lining at three scales at once. The result is a lining with a total absorptive surface estimated at tens of square metres — far larger than a flat tube could ever offer, and the reason a person can extract almost every usable molecule from a meal before it moves on.