1. Overview
Absorption is the critical process where the small, soluble molecules produced during digestion move from the lumen of the digestive system into the circulatory system. This process ensures that nutrients like glucose, amino acids, and fats reach the cells of the body to be used for energy, growth, and repair.
Key Definitions
- Absorption: The movement of small food molecules and ions through the wall of the intestine into the blood.
- Villi: Tiny, finger-like projections found on the inner wall of the small intestine that increase surface area.
- Microvilli: Even smaller projections on the cell membranes of the epithelial cells lining the villi.
- Epithelium: The thin layer of cells (one cell thick) forming the outer layer of a villus.
- Lacteal: A central vessel in the villus that absorbs fatty acids and glycerol into the lymphatic system.
Core Content
In the human digestive system, the majority of absorption occurs after chemical digestion has broken down large, insoluble polymers into small, soluble monomers.
- The Small Intestine: This is the primary region where nutrients are absorbed into the blood. By the time food reaches the end of the small intestine (the ileum), most digested food molecules, as well as mineral ions and vitamins, have been absorbed.
- Absorption of Water:
- The small intestine absorbs the largest volume of water (approximately 5–8 liters per day).
- The colon (part of the large intestine) absorbs most of the remaining water, helping to solidify the waste material into faeces.
A cross-section of the small intestine showing the folded inner lining (mucosa) covered in finger-like villi to illustrate how the internal surface is maximized.
Extended Content (Extended curriculum only)
To maximize the efficiency of absorption, the small intestine is highly adapted to increase the rate of diffusion and active transport.
The Significance of Surface Area
The rate of absorption is directly proportional to the surface area available. The small intestine achieves a massive internal surface area through three levels of folding:
- Inner folding of the intestinal wall.
- Villi: Millions of finger-like projections.
- Microvilli: Microscopic folds on the surface of individual epithelial cells.
Structure of a Villus
Each villus is a functional unit of absorption with specific adaptations:
- One-cell thick wall (Epithelium): Provides a very short distance for molecules to travel, speeding up the rate of diffusion.
- Large Surface Area: Provided by the shape of the villus and the presence of microvilli on the epithelial cells.
- Network of Blood Capillaries: A rich blood supply that absorbs glucose and amino acids. The constant flow of blood maintains a steep concentration gradient between the lumen and the blood.
- Lacteal: A tube in the center of the villus that specifically absorbs fats (fatty acids and glycerol) and transports them into the lymphatic system.
A detailed diagram of a single villus. Label the single layer of epithelial cells on the outside, the network of red/blue blood capillaries inside, and the central yellow/clear lacteal vessel.
Structure to Function Summary
| Structure | Function |
|---|---|
| Microvilli | Further increases surface area for maximum absorption. |
| Capillaries | Transports glucose and amino acids away to the liver; maintains concentration gradient. |
| Lacteal | Transports fatty acids and glycerol away from the intestine. |
| Mitochondria | Epithelial cells contain many mitochondria to provide energy (ATP) for active transport of nutrients. |
Key Equations
While there are no specific chemical equations for absorption, you must be able to calculate magnification for diagrams of villi:
- Magnification = Image size / Actual size
- $M = I / A$
- I: The measurement of the diagram (usually in mm).
- A: The real size of the villus (usually in $\mu m$).
- Note: 1 mm = 1000 $\mu m$.
Common Mistakes to Avoid
- ❌ Wrong: Thinking that all water is absorbed in the colon.
- ✓ Right: Most water is absorbed in the small intestine; the colon absorbs the remainder.
- ❌ Wrong: Stating that the lacteal absorbs glucose.
- ✓ Right: The lacteal only absorbs lipids (fats); glucose and amino acids go directly into the blood capillaries.
- ❌ Wrong: Confusing "Villi" with "Villi" in the lungs.
- ✓ Right: Villi are in the small intestine; Alveoli are the structures in the lungs.
Exam Tips
- Command Word "Describe and Explain": If asked to "describe and explain how the small intestine is adapted for absorption," you must name the structure (e.g., villi) and then state the benefit (e.g., increases surface area for faster diffusion).
- Identify the Lacteal: In diagrams, the lacteal is the large central vessel. Capillaries are the smaller, branching network surrounding it.
- Contextual Questions: Exams often use "Celiac disease" as a context. This disease flattens villi. You would be expected to explain that this leads to poor growth/weight loss because there is less surface area for nutrient absorption.
- Typical Values: A human villus is roughly 0.5 to 1.6 mm long. If your calculation for "Actual size" results in 10cm, you have likely made a unit conversion error.