Digestion and Nutrition
3. Absorption in the Small Intestine
Learning outcomes
- I can explain the process of nutrient absorption.
- I can describe the structure and function of villi and microvilli.
- I can explain why a large surface area is important for absorption.
- I can identify where carbohydrates, proteins, and lipids enter the body.
- I can describe how absorbed nutrients are transported to cells.
Introduction
After food has been digested into small molecules, the body must absorb these nutrients so they can be used by cells. Digestion alone is not enough—unless nutrients enter the bloodstream or lymphatic system, they cannot provide energy, support growth, or repair tissues.
Most nutrient absorption takes place in the small intestine, a long, highly specialised organ designed to maximise the movement of nutrients into the body. Its inner surface is covered with millions of tiny structures called villi and even smaller projections called microvilli, creating an enormous surface area for efficient absorption.
What Is Absorption?
Absorption is the process by which digested nutrients move from the small intestine into the bloodstream or lymphatic system.
Only small molecules produced during digestion can be absorbed.
Examples include:
- Glucose
- Amino acids
- Fatty acids
- Glycerol
- Vitamins
- Minerals
- Water
Once absorbed, these nutrients are transported to cells throughout the body.
Figure 1. Digested nutrients pass through the wall of the small intestine into the blood or lymph.
The Small Intestine
The small intestine is the main site of nutrient absorption.
It is about 6 metres long in adults and is specially adapted for efficient absorption.
Features include:
- Great length.
- Folded inner lining.
- Millions of villi.
- Microscopic microvilli.
- Rich blood supply.
These adaptations allow nutrients to be absorbed quickly and efficiently.
Villi
Villi are tiny finger-like projections that cover the inner lining of the small intestine.
Each villus contains:
- A network of blood capillaries.
- A lymph vessel called a lacteal.
Functions of villi:
- Greatly increase surface area.
- Absorb nutrients.
- Transfer nutrients into blood or lymph.
Without villi, nutrient absorption would be much less efficient.
Figure 2. Each villus contains blood capillaries and a lacteal for absorbing nutrients.
Microvilli
Each epithelial cell covering a villus has thousands of tiny projections called microvilli.
Microvilli:
- Increase the surface area even further.
- Form the brush border.
- Help maximise nutrient absorption.
Together, villi and microvilli create an enormous absorbing surface.
Scientists estimate that the total surface area of the small intestine is roughly 30–40 square metres in an adult—about the size of half a badminton court (estimates vary depending on how it is measured).
Why Is a Large Surface Area Important?
A larger surface area allows:
- More nutrients to be absorbed at the same time.
- Faster absorption.
- Greater efficiency.
The small intestine achieves this through:
- Its long length.
- Circular folds.
- Millions of villi.
- Billions of microvilli.
These features allow the body to absorb enough nutrients to meet its needs.
Figure 3. Villi and microvilli greatly increase the surface area available for absorption.
Where Different Nutrients Go
Different nutrients enter different transport systems.
Carbohydrates
Digested into:
- Glucose and other simple sugars.
These pass into the blood capillaries inside each villus.
Proteins
Digested into:
- Amino acids.
These also enter the blood capillaries.
Lipids (Fats)
Digested into:
- Fatty acids.
- Glycerol.
These enter the lacteals, which are part of the lymphatic system.
Eventually, they enter the bloodstream.
| Nutrient | Digested Into | Transport System |
|---|---|---|
| Carbohydrates. | Glucose | Blood capillaries |
| Proteins | Amino acids | Blood capillaries |
| Lipids | Fatty acids and glycerol | Lacteals (lymphatic system) |
Transporting Nutrients
Once absorbed:
Blood Capillaries
Carry:
- Glucose
- Amino acids
- Vitamins
- Minerals
- Water
The blood transports these nutrients first to the liver through the hepatic portal vein, where many are processed before being distributed to the rest of the body.
Lacteals
Carry:
- Fatty acids.
- Glycerol (mainly as part of larger fat particles called chylomicrons).
The lymphatic system eventually returns these fats to the bloodstream.
The circulatory system then delivers nutrients to cells for:
- Energy.
- Growth.
- Repair.
- Storage.
Figure 4. Different nutrients enter either blood capillaries or the lacteal inside each villus.
Adaptations of the Small Intestine
The small intestine is highly adapted for absorption.
| Adaptation | Benefit |
|---|---|
| Long length | More time for absorption |
| Circular folds | Increase surface area |
| Villi | Greatly increase surface area |
| Microvilli | Increase surface area even more |
| Thin epithelial wall. | Short diffusion distance |
| Rich blood supply | Quickly carries absorbed nutrients away |
| Lacteals | Absorb and transport fats |
These adaptations make the small intestine one of the most efficient absorbing organs in the body.
Why Absorption Is Essential
Without absorption:
- Cells would not receive glucose for respiration.
- Growth would stop.
- Tissue repair would not occur.
- Organs could not function properly.
Absorption allows the nutrients released during digestion to become available to every cell in the body.
Figure 5. After absorption, nutrients are transported throughout the body to support cellular functions.
Worked Example
Question
Complete the table.
| Nutrient | Absorbed Into |
|---|---|
| Glucose | ? |
| Amino acids. | ? |
| Fatty acids | ? |
Solution
| Nutrient | Absorbed Into |
|---|---|
| Glucose | Blood capillaries |
| Amino acids | Blood capillaries |
| Fatty acids | Lacteals (lymphatic system) |
Glucose and amino acids enter the bloodstream directly, while most fats first enter the lymphatic system.
Real-World Connection
Athletes often eat carbohydrate-rich foods before endurance events. During digestion, starch is broken down into glucose, which is absorbed through the villi into the bloodstream. The circulatory system transports this glucose to muscle cells, where it is used in cellular respiration to produce the energy needed for running, cycling, or swimming.
Did You Know?
The tiny microvilli on the cells lining the small intestine are so numerous that they form what scientists call the brush border. This surface not only increases the area available for absorption but also contains enzymes that complete the final stages of digestion before nutrients enter the body.
Key Terms
Absorption – The movement of digested nutrients from the small intestine into the blood or lymph.
Blood capillary – A tiny blood vessel where nutrients such as glucose and amino acids enter the bloodstream.
Brush border – The layer of microvilli covering the epithelial cells of the small intestine.
Lacteal – A lymph vessel inside each villus that absorbs most fats.
Lymphatic system – The body system that transports lymph and plays important roles in immunity and fat transport.
Microvilli – Microscopic projections on epithelial cells that greatly increase surface area for absorption.
Small intestine – The main site of digestion and nutrient absorption.
Villus (plural: villi) – A finger-like projection that increases the surface area of the small intestine.
Key Takeaways
- Absorption is the movement of digested nutrients from the small intestine into the blood or lymphatic system.
- The small intestine is specially adapted for absorption with its long length, folds, villi, and microvilli.
- A large surface area allows nutrients to be absorbed more quickly and efficiently.
- Glucose and amino acids enter the blood capillaries, while most fatty acids and glycerol enter the lacteals.
- The circulatory and lymphatic systems transport absorbed nutrients throughout the body.
- Efficient absorption ensures that every cell receives the nutrients needed for energy, growth, repair, and normal body function.