7.5 Absorption
- Syllabus
- 0610–2026–2027
- Topic
- 7.5
- Level
- —
Learning objectives
7.5.1Small intestine is the region where• State that the small intestine is the region where nutrients are absorbed7.5.2Most water is absorbed from the small• State that most water is absorbed from the small intestine but that some is also absorbed from the colon7.5.3Significance of villi and microvilli• Explain the significance of villi and microvilli in increasing the internal surface area of the small intestine7.5.4Structure of a villus• Describe the structure of a villus7.5.5Roles of capillaries and lacteals• Describe the roles of capillaries and lacteals in villi
Locate nutrient absorption in the small intestine
Absorption is the movement of small, soluble products of digestion from the alimentary canal into the blood or lymph.
The small intestine is the region of the alimentary canal where nutrients are absorbed. Its inner surface is specialised for moving digested nutrients out of the lumen and into transport systems in the villi.
digested nutrient in the lumen → crosses the small-intestinal epithelium → enters blood capillaries or a lacteal → transported around the body
Digestion makes molecules small and soluble; absorption is their later movement across the intestinal wall. The stomach is mainly a digestion site, not the main nutrient-absorption site.
Locate water absorption in the alimentary canal
Water is absorbed in more than one region of the alimentary canal, but the greatest amount is absorbed in the small intestine.
| Region | Relative role in water absorption |
|---|---|
| small intestine | absorbs most of the water |
| colon | absorbs some of the remaining water |
When a question asks where most water is absorbed, choose the small intestine. The colon still has an important role, but it does not absorb the largest total amount.
It is incorrect to say that all water, or even most water, is absorbed in the colon.
Explain how villi and microvilli increase absorption
The inner wall of the small intestine is folded into many villi. Each villus is covered by epithelial cells whose exposed membranes carry many microscopic projections called microvilli.
| Structure | Scale | Contribution |
|---|---|---|
| villi | projections of the intestinal lining | greatly increase the internal surface area |
| microvilli | projections on epithelial cell membranes | increase the surface area still further |
A larger internal surface provides more membrane through which nutrient molecules can move at the same time. This increases the rate and efficiency of absorption.
Villi and microvilli are not interchangeable names. Villi are multicellular projections of the lining; microvilli are much smaller projections on individual epithelial cells.
Describe the structure of a villus
A villus is a finger-like projection from the lining of the small intestine. It is built to place an absorptive surface close to transport vessels.
| Structural feature | Position or form |
|---|---|
| epithelium | thin outer layer, one cell thick |
| microvilli | on the exposed surface of epithelial cells |
| capillary network | many small blood vessels close to the epithelium |
| lacteal | lymph vessel running through the centre of the villus |
Many villi line the small intestine. Each combines a large surface area, a short distance across the epithelium and nearby routes that carry absorbed substances away.
The lacteal is the central lymph vessel; the branching vessels near the surface are blood capillaries. Do not reverse these labels.
Compare capillaries and lacteals in villi
A villus has two transport routes because different products of digestion enter different vessels.
| Vessel in the villus | Main absorbed products | Transport route |
|---|---|---|
| blood capillaries | glucose and amino acids | carried away in the blood |
| lacteal | fatty acids and glycerol | enters the lymphatic system before reaching the blood |
The capillary network carries water-soluble nutrients away from the villus. Continued blood flow helps maintain a concentration gradient between the intestinal lumen and the blood.
Glucose and amino acids enter blood capillaries; products of fat digestion enter the lacteal. A lacteal is not a blood capillary.