Once food is digested into small soluble molecules, it must cross the wall of the gut into the blood. The small intestine does almost all of this, and most of the water is taken back there too. Its lining is folded into villi and covered with microvilli, which give it an enormous surface area. Villi, microvilli, capillaries and lacteals are Extended content.
The small intestine is the region where nutrients are absorbed. Most water is absorbed from the small intestine, but some is also absorbed from the colon. Extended: villi and microvilli greatly increase the internal surface area of the small intestine; each villus has a one-cell-thick epithelium, a network of capillaries that absorb glucose and amino acids, and a central lacteal that absorbs fatty acids and glycerol.
Fig. 1: a single villus; capillaries carry glucose and amino acids to the liver, and the lacteal takes fatty acids and glycerol into the lymph
Section 01
Where food and water are absorbed.
Start the sim on one villus. Coloured nutrient particles move into the villus and are carried away, and the ABSORBED counter keeps rising. Then switch to the intestine wall to see thousands of villi side by side.
Small intestine: the region where digested nutrients (glucose, amino acids, fatty acids and glycerol, vitamins and mineral ions) are absorbed into the blood.
Water: most of the water (from drinks, food and digestive juices) is absorbed in the small intestine; some more is absorbed from the colon.
What is left: undigested food, mostly fibre, with bacteria and some water, passes on as faeces to the rectum.
Steep gradient: blood flow carries absorbed nutrients away, keeping their concentration in the blood low so that absorption continues (by diffusion and active transport).
Examiner trap
Most water is absorbed in the small intestine, not the colon. The colon only absorbs some of the water that remains.
Section 02
Villi and microvilli.
Extended. Use the zoom chips in the sim: intestine wall, then one villus, then microvilli. The SURFACE AREA readout shows how each level of folding multiplies the area.
Villi: finger-like projections about 1 mm long covering the inside of the small intestine. The sim estimates they give about 10 times the area of a flat tube.
Microvilli: tiny projections on the surface of each epithelial cell, multiplying the surface area about 20 times more again.
Significance: a larger internal surface area means more nutrient molecules can be absorbed at the same time, so absorption is fast and almost complete before food leaves the small intestine.
Enzymes on microvilli: the membranes carry enzymes such as maltase, so digestion is completed right where absorption happens.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the one villus view, where do the fatty acid and glycerol particles go?
Quick check+10 XP
What does the sim say happens to glucose and amino acids absorbed into the capillaries?
Quick check+10 XP
According to the sim, how much do microvilli increase the surface area?
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Why does the sim stress that the villus wall is one cell thick?
Section 03
Inside a villus.
Extended. Return to the one villus view. It has been cut open to show the blood capillaries wrapped around a central lacteal.
Feature
How it helps absorption
Epithelium one cell thick
short distance for nutrients to cross
Microvilli on epithelial cells
further increase the surface area
Dense network of blood capillaries
absorb glucose and amino acids and carry them away (to the liver via the hepatic portal vein), keeping the concentration gradient steep
Lacteal (lymph vessel) in the centre
absorbs fatty acids and glycerol and carries them away in the lymph, which later drains into the blood
Examiner trap
Fatty acids and glycerol go into the lacteal, not the blood capillaries. Glucose and amino acids go into the capillaries. Keep the two routes separate.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
In which region of the alimentary canal are most nutrients absorbed?
Exam style+20 XP
Which statement about water absorption is correct?
Exam style+20 XP
Extended: what is the main significance of villi and microvilli?
Exam style+20 XP
Extended: which row correctly matches the structure in a villus with what it absorbs?
Skill unlocked
Absorption, mastered.
This skill is now recorded on your profile. Keep the chain going.