Small intestine structure and function

The small intestine is where digestion is completed and where most digested nutrients are absorbed into the blood and lymph. It has two main regions, the duodenum and the ileum.

It is the longest part of the alimentary canal, a coiled tube about six to seven metres long in an adult. Its great length and its heavily folded inner lining keep food in contact with digestive enzymes and with the absorbing surface for a long time, and this is the point in the gut where questions usually test whether you can link each feature of the wall to the job of absorbing food.

Parts and functions

How structure suits function

The small intestine is adapted to complete digestion and then absorb the products efficiently. Its length, its lining of villi and microvilli, its thin epithelium and its rich blood and lymph supply all serve absorption, while its muscular wall keeps food moving and mixed with enzymes.

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

• Very long and coiled, food stays in contact with enzymes and with the absorbing surface for a long time.
• Millions of villi, each covered in microvilli, the internal surface area for absorption is enormous, so more nutrients are absorbed per second.
• Epithelium one cell thick, the distance from the gut contents into the blood is short, so absorption is fast.
• Dense capillary network in each villus, absorbed glucose and amino acids are carried away quickly, which keeps a steep concentration gradient.
• A lacteal in each villus, fatty acids and glycerol are absorbed into the lymph rather than the blood.
• Moist, thin lining, dissolved nutrients cross the membrane readily by diffusion and by active transport.

Related processes

Digestion is completed in the duodenum. Bile from the liver emulsifies fats into small droplets, increasing their surface area for lipase, and neutralises the acidic chyme from the stomach so that the enzymes of the small intestine can work in slightly alkaline conditions.

Pancreatic juice and intestinal juice supply amylase, protease and lipase, which break carbohydrates down to glucose, proteins to amino acids, and fats to fatty acids and glycerol.

Absorption then takes place across the villi. Glucose and amino acids move into the epithelial cells partly by diffusion and partly by active transport, which uses energy to absorb them even against a concentration gradient, and pass into the blood capillaries.

Fatty acids and glycerol enter the lacteal. Peristalsis, a wave of muscle contraction along the gut wall, keeps the food mixed with enzymes and pushes it along, and the absorbed nutrients are carried to the liver and then to body cells, where they are used in a process called assimilation.

Common labelling errors

Worked question

Question. Diagram Y shows a single villus from the ileum. (a) Name the part labelled P, a network of small vessels, and the part labelled Q, a single central vessel.

(b) Explain how two features of the villus adapt it for the absorption of digested food. (c) Glucose continues to be absorbed from the ileum even when its concentration in the blood is higher than in the gut.

Name the process responsible and state why it is needed. (d) Explain why a person whose ileum is damaged may lose weight even when they eat enough food.

Model answer. (a) P is the blood capillary network; Q is the lacteal. (b) The villus wall, or epithelium, is one cell thick, so the diffusion distance is short and nutrients cross quickly.

The villus and its microvilli give a large surface area, so more nutrients are absorbed at once. (Alternative: a dense capillary network maintains a steep concentration gradient.) (c) Active transport; it uses energy from respiration to absorb glucose against the concentration gradient, so absorption is complete instead of stopping when the concentrations become equal.

(d) Damaged villi reduce the surface area for absorption, so fewer digested nutrients are absorbed into the blood; undigested food is lost in the faeces, so the body is short of glucose and amino acids and draws on its own stores, causing weight loss.

Source:SRC-DSKP-EN

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