The villus

Millions of villi line the small intestine, which is where most absorption of digested food into the blood takes place.

A villus is the point where the digested contents of the gut meet the blood and lymph. Questions on this structure usually test whether you can name each part and link it to one factor that makes absorption fast, and whether you know that glucose and amino acids are taken up by active transport as well as by diffusion.

Parts and functions

How structure suits function

The villus is adapted to absorb the products of digestion quickly and completely. Its shape and microvilli create surface area, its thin wall shortens the distance food must travel, its blood and lymph vessels carry absorbed food away, and its mitochondria supply energy for active transport.

Each adaptation links to one factor that speeds up absorption, and an examiner awards the mark for the link, not for the feature alone.

• Finger-like shape, with millions of villi, together they give the small intestine a very large surface area for absorption.
• Microvilli on each epithelial cell, increase the surface area still further, so more nutrients are absorbed per second.
• Epithelium one cell thick, the distance from the gut into the blood is short, so absorption is fast.
• Dense capillary network, carries away absorbed glucose and amino acids quickly, which keeps a steep concentration gradient.
• A lacteal at the centre, absorbs fatty acids and glycerol into the lymph.
• A rich supply of mitochondria in the epithelial cells, releases energy by respiration for the active transport of glucose and amino acids.

Related processes

The villus is the site of absorption, the stage that follows the completion of digestion in the small intestine. Bile and enzymes from the pancreas and the intestinal wall break food down to glucose, amino acids, fatty acids and glycerol, and these small, soluble molecules are then absorbed across the villus.

Glucose and amino acids move into the epithelial cells by diffusion when their concentration in the gut is higher, and by active transport when it is not, so absorption continues even against a concentration gradient. They pass into the blood capillaries, which carry them to the liver.

Fatty acids and glycerol enter the lacteal and travel through the lymphatic system before joining the blood. Once inside the body cells, the absorbed nutrients are used in respiration and to build new materials, a process called assimilation.

Common labelling errors

Worked question

Question. The diagram shows a single villus from the small intestine. (a) Name the structures that absorb (i) glucose and (ii) fatty acids.

(b) Give two ways in which the villus is adapted to absorb food quickly. (c) Explain why the epithelial cells of a villus contain a large number of mitochondria.

(d) Suggest why food moves along the small intestine slowly.

Model answer. (a)(i) the blood capillaries; (ii) the lacteal. (b) The villus wall, or epithelium, is one cell thick, giving a short diffusion distance; the villus and its microvilli give a large surface area.

(Alternative: a dense capillary network keeps a steep concentration gradient.) (c) Mitochondria release energy by respiration; this energy is used for active transport, which absorbs glucose and amino acids against their concentration gradient. (d) Slow movement keeps the food in contact with the absorbing surface for longer, so more of the digested nutrients are absorbed.

Source:SRC-DSKP-EN

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