Absorption in the ileum

Where it happens

Absorption takes place across the wall of the ileum, the final section of the small intestine. Its inner surface is folded into thousands of finger-like villi, and each villus is covered with even smaller microvilli, which together give it a very large surface area.

Each villus contains a network of blood capillaries and a central lymph vessel called the lacteal. The epithelium covering the villus is a single layer of cells, so the absorbed molecules cross only one cell before reaching a capillary or the lacteal.

Absorption of glucose and amino acids begins in the duodenum and jejunum, but the ileum is where most of the remaining products of digestion are taken up before the undigested material passes into the large intestine.

Inputs and outputs

• Input: glucose and other monosaccharides from the digestion of carbohydrates.
• Input: amino acids from the digestion of proteins.
• Input: fatty acids and glycerol from the digestion of lipids by lipase.
• Input: vitamins, mineral ions and water, which also cross the ileum wall.
• Output to the blood capillaries: glucose, amino acids, water-soluble vitamins and mineral ions, carried to the hepatic portal vein.
• Output to the lacteal: fatty acids and glycerol, reassembled into lipids and carried as lymph before joining the blood.

The steps

1. Chemical digestion is completed in the ileum, leaving small soluble molecules, glucose, amino acids, fatty acids and glycerol, in the lumen close to the villi.
2. Glucose and amino acids move across the epithelium of the villus into the blood capillaries inside it, by diffusion while the concentration in the lumen is higher than in the blood.
3. When the concentration in the lumen falls below that in the blood, the epithelial cells use active transport, powered by ATP from their abundant mitochondria, to keep taking up glucose and amino acids.
4. Fatty acids and glycerol diffuse into the epithelium cells and are reassembled into lipids before entering the lacteal, the lymph vessel at the centre of the villus.
5. Blood carrying glucose and amino acids flows away from the villus in the capillary network, towards the hepatic portal vein leading to the liver.
6. Lymph carrying fat products flows away from the villus through the lymphatic system, eventually joining the bloodstream near the heart.
7. Continuous blood flow through the capillaries keeps the concentration of absorbed substances low inside the villus, maintaining the concentration gradient for diffusion.

Why it matters and how it is controlled

The villi and microvilli of the ileum are adapted to absorb digested food efficiently: a large surface area allows more molecules to cross at once, a thin, one-cell-thick epithelium keeps the diffusion distance short, and a rich network of capillaries and a lacteal carry absorbed substances away quickly, keeping the concentration gradient steep.

The rate of absorption is set by three things that the exam expects you to name: the steepness of the concentration gradient, which the blood flow maintains; the supply of ATP for active transport, which depends on the mitochondria in the epithelial cells; and the surface area available, which the villi and microvilli maximise. Peristalsis of the ileum wall also keeps the contents moving and mixing so that fresh digested food is brought against the villi.

Once glucose and amino acids reach the liver through the hepatic portal vein, the liver regulates what enters the general circulation: excess glucose is stored as glycogen and excess amino acids are deaminated. This link between absorption and the liver's role in homeostasis is a common bridge question.

How it is examined

You may be asked to list the adaptations of the ileum for absorption, to label a diagram of a villus, to explain why glucose absorption sometimes needs active transport as well as diffusion, or to explain why fatty acids and glycerol enter the lacteal rather than the blood capillary.

Structured questions often present a diagram of a single villus with the epithelium, capillary network and lacteal numbered. A typical follow-up asks you to explain how one labelled feature increases the rate of absorption, which needs a feature plus its effect, for example, one-cell-thick epithelium therefore short diffusion distance.

Essay questions may ask you to describe how the ileum is adapted for absorption, which is best answered as a list of paired feature-and-function points rather than a single long paragraph.

Common misconceptions

Worked exam-style question

Question. Diagram X shows a longitudinal section of a villus from the ileum. Structure P is the epithelium, structure Q is the blood capillary network and structure R is the lacteal.

(a) State one function of R. (b) Explain two ways in which structure P is adapted to absorb glucose efficiently.

(c) A student claims that glucose is absorbed by diffusion only. Explain why this claim is incomplete.

Model answer. (a) R, the lacteal, transports fatty acids and glycerol as lymph away from the villus. (b) P is one cell thick, so the diffusion distance is short; its surface bears microvilli, which increase the surface area for absorption.

The epithelial cells also contain abundant mitochondria to supply ATP for active transport. (c) Diffusion moves glucose only down its concentration gradient.

When the concentration of glucose in the lumen becomes lower than in the blood, the cells use active transport, which needs energy from respiration, to continue absorbing glucose against the gradient.

Source:SRC-DSKP-EN

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