Absorption
Learning Objective 1: The small intestine is the primary region where nutrients are absorbed into the bloodstream. While digestion begins in the mouth and stomach, the chemical breakdown of food is completed in the small intestine, allowing for efficient absorption.
Learning Objective 2: Water absorption occurs in two main locations:
- Most water is absorbed from the chyme (partially digested food) as it passes through the small intestine.
- Some water is also absorbed from the remaining material in the colon (large intestine). This final absorption helps to concentrate the waste, turning liquid sludge into solid faeces.
A villus is a tiny, finger-like projection that lines the inner wall of the small intestine. Each villus is covered by a single layer of epithelial cells.
Inside each villus are two key transport systems:
- A network of capillaries (tiny blood vessels).
- A central lymphatic vessel called a lacteal. The lacteal is part of the lymphatic system, not the blood circulatory system.
1. Increased Surface Area:
The inner wall of the small intestine is folded into many villi. Furthermore, each individual epithelial cell covering the villus has microscopic projections called microvilli (forming the 'brush border'). These microvilli significantly increase the surface area available for absorption compared to a flat surface.
2. Thin Epithelium:
The epithelial layer lining the villus is only one cell thick. This creates a very short diffusion pathway, allowing nutrients to move quickly from the intestine into the blood or lymph.
| Adaptation for Absorption |
|---|
| Increases surface area for absorption. |
| Short diffusion distance for nutrients to enter the blood/lymph. |
| Maintains a concentration gradient by rapidly carrying away absorbed nutrients. |
| Absorb and transport fats/lipids which are too large for capillaries. |
Learning Objective 5: The two transport systems within the villus handle different types of nutrients based on their solubility and size.
Capillaries (Blood Vessels):
- Absorb glucose (from carbohydrates) and amino acids (from proteins).
- These substances are water-soluble and small enough to enter the blood plasma directly.
- The blood then carries these nutrients to the liver via the hepatic portal vein.
Lacteals (Lymphatic Vessels):
- Absorb fatty acids and glycerol (products of fat digestion).
- Fats are reassembled into triglycerides inside the epithelial cells. Because these molecules are large and hydrophobic (insoluble in water), they cannot enter capillaries easily.
- Instead, they enter the lacteal and are transported via the lymphatic system before eventually entering the bloodstream near the heart.
- Protein Digestion: Proteins are broken down into amino acids in the small intestine.
- Absorption: Amino acids diffuse across the thin epithelial cells of the villus.
- Transport: They enter the capillaries within the villus.
- Destination: They travel via the blood to the liver.
Contrast with Fat: Butter is digested into fatty acids and glycerol. These enter the lacteal because they are not water-soluble, bypassing the capillaries.
Mistake 1: Colon vs. Small Intestine for Water
- Incorrect: 'Most water is absorbed in the colon.'
- Correct: Most water is absorbed in the small intestine. The colon absorbs only the remaining/some water to solidify waste.
Mistake 2: Amino Acids and Lacteals
- Incorrect: 'Amino acids are absorbed into the lacteal.'
- Correct: Amino acids and glucose enter the capillaries. Only fatty acids and glycerol enter the lacteal.
Why examiners accept this: Examiners look for a direct link between structure and function. Simply listing features (e.g., 'has capillaries') is insufficient. You must explain why that feature helps absorption.
Correct Usage Example:
'The epithelium is one cell thick, which provides a short diffusion pathway for nutrients to enter the blood.'
'Villi have a rich blood supply, which maintains a steep concentration gradient by removing absorbed glucose quickly, allowing more glucose to diffuse in.'