Digestion and Absorption
3. Absorption in the Small Intestine
Learning outcomes
- I can explain why absorption is necessary after digestion.
- I can describe how nutrients move from the digestive system into the bloodstream.
- I can identify the role of villi and microvilli in nutrient absorption.
- I can explain how the structure of the small intestine is adapted for absorption.
- I can trace the movement of absorbed nutrients through the body.
From Digestion to Absorption
Digestion breaks large food molecules into smaller molecules.
For example:
- Carbohydrates are broken down into simple sugars such as glucose.
- Proteins are broken down into amino acids.
- Fats are broken down into fatty acids and glycerol.
However, digestion alone is not enough.
These nutrients are still inside the digestive tract. To be useful to the body's cells, they must cross the wall of the digestive system and enter the body's transport systems.
This process is called absorption.
Absorption is the movement of digested nutrients from the digestive tract into the blood or lymph.
Most nutrient absorption occurs in the small intestine.
Why Is Absorption Necessary?
The digestive tract can be thought of as a tube running through the body.
Although food is inside your digestive system, its nutrients have not yet entered the body's internal tissues.
Digestion makes the molecules small enough to cross the intestinal wall.
Absorption then moves them across that wall.
Therefore:
Food → digestion → small soluble molecules → absorption → transport → use by cells
Without absorption, digested nutrients would simply continue through the digestive tract and eventually leave the body.
The Small Intestine
The small intestine is the main site of nutrient absorption.
It is particularly well suited to this function because it is:
- Several metres long.
- Highly folded.
- Covered with millions of villi.
- Covered with microscopic microvilli.
- Supplied with many blood capillaries.
- Connected to the lymphatic system.
Together, these features provide an enormous surface area for absorption.
Increasing Surface Area
The small intestine does not have a simple smooth inner surface.
Its absorptive area is increased at several levels.
Small intestine → folds → villi → microvilli
Each level provides additional surface area.
A greater surface area means that more nutrient molecules can cross the intestinal wall at the same time.
This makes absorption much more efficient.
Villi
The inner surface of the small intestine contains millions of tiny finger-like projections called villi.
A single projection is called a villus.
Each villus contains:
- A thin epithelial surface.
- A network of blood capillaries.
- A lymphatic vessel called a lacteal.
These structures allow different products of digestion to be transported away from the intestine.
Microvilli
The cells covering each villus have even smaller projections on their surfaces.
These are called microvilli.
Microvilli are microscopic extensions of the cell membrane.
Together they form a surface sometimes called the brush border.
Their main advantage is that they increase surface area even further.
Therefore:
Folds increase surface area.
Villi increase it further.
Microvilli increase it even further.
This creates an extremely large absorptive surface inside the small intestine.
How a Villus Is Adapted for Absorption
The structure of a villus is closely related to its function.
Large Surface Area
Millions of villi and microvilli provide an enormous surface area.
This allows many nutrient molecules to be absorbed simultaneously.
Thin Surface
The surface of a villus is only one cell thick.
This produces a very short distance between nutrients inside the intestine and the blood or lymph.
A short diffusion distance allows substances to cross more rapidly.
Good Blood Supply
Each villus contains a dense network of blood capillaries.
These capillaries rapidly carry absorbed substances away.
This helps maintain a concentration difference between the intestine and the blood, allowing continued absorption.
Substances entering the blood include:
- Glucose and other simple sugars
- Amino acids
- Many minerals
- Water-soluble vitamins
Lacteals
Each villus also contains a lacteal.
A lacteal is a small lymphatic vessel.
Many products of fat digestion are packaged within intestinal cells and enter the lacteals rather than moving directly into blood capillaries.
They are then transported through the lymphatic system before eventually entering the bloodstream.
Interactive View of Absorption
This visualization lets you trace products of carbohydrate, protein and fat digestion through a villus and compare whether they enter the blood capillaries or the lacteal.
How Nutrients Cross the Intestinal Wall
Different substances can cross the intestinal epithelium using different transport processes.
Important mechanisms include:
- Diffusion
- Facilitated diffusion
- Active transport
- Osmosis
The mechanism used depends on the substance and the conditions.
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.
Some substances can move across cell membranes by diffusion when an appropriate concentration gradient exists.
The thin surface of the villus makes this movement more efficient.
Facilitated Diffusion
Some molecules cannot easily pass directly through the lipid part of the cell membrane.
Instead, they move through specific membrane proteins.
This is called facilitated diffusion.
Like ordinary diffusion, facilitated diffusion moves substances down a concentration gradient and does not require cellular energy.
Active Transport
Sometimes intestinal cells need to absorb nutrients even when their concentration in the intestine is relatively low.
Active transport allows substances to move across membranes using energy and specific transport proteins.
This is particularly important for the efficient absorption of certain nutrients and ions.
The cells lining the small intestine contain many mitochondria, providing energy for active transport.
Osmosis
Water can move across partially permeable membranes by osmosis.
Osmosis is the net movement of water through a partially permeable membrane from a region of higher water potential to a region of lower water potential.
Water absorption occurs throughout the intestine, with substantial absorption taking place in the small intestine.
Absorption of Glucose
Carbohydrate digestion produces simple sugars, particularly glucose.
Glucose crosses the intestinal epithelium and enters the blood capillaries within the villi.
The simplified route is:
Small intestine → epithelial cells → blood capillaries → hepatic portal vein → liver
The liver helps regulate and process absorbed nutrients before blood carries them to the rest of the body.
What Happens to Glucose?
After passing through the liver, glucose can be transported in the blood to cells throughout the body.
Cells can use glucose in cellular respiration to release energy.
Some glucose can also be stored.
For example:
- The liver can convert glucose into glycogen.
- Muscles can store glycogen.
- Excess energy can ultimately contribute to fat storage.
Absorption therefore connects digestion directly with cellular metabolism.
Absorption of Amino Acids
Protein digestion produces amino acids.
Amino acids are absorbed through the intestinal epithelium into blood capillaries.
Their route is similar to that of glucose:
Small intestine → villus → blood capillaries → hepatic portal vein → liver → general circulation
Cells can then use amino acids to produce proteins required for:
- Growth
- Tissue repair
- Enzymes
- Some hormones
- Structural components
Absorption of Fats
Fat absorption follows a somewhat different pathway.
Digestion breaks fats into smaller products that are taken into intestinal epithelial cells.
Many long-chain fat products are then reassembled and packaged into particles before entering the lacteals.
They therefore enter the lymphatic system rather than immediately entering the blood capillaries.
The Route of Absorbed Fats
A simplified route is:
Small intestine → epithelial cells → lacteals → lymphatic vessels → bloodstream
Once they reach the blood, lipids can be transported to tissues throughout the body.
They may be:
- Used as an energy source.
- Used to build cell membranes.
- Used to make certain molecules.
- Stored in adipose tissue.
Blood Capillaries vs Lacteals
A useful distinction is:
Blood Capillaries
Primarily transport absorbed substances such as:
- Glucose
- Amino acids
- Many minerals
- Water-soluble vitamins
Lacteals
Primarily receive many products derived from the digestion and absorption of long-chain fats.
Both structures are therefore important components of each villus.
Maintaining a Concentration Gradient
Absorption is helped by the rich blood supply of the villi.
Suppose glucose enters the blood from the intestine.
If that glucose simply remained beside the villus, the concentration difference between the intestine and blood would decrease.
Instead, blood continuously flows through the capillaries and carries absorbed glucose away.
This helps maintain conditions favourable for continued absorption.
Therefore:
Good blood flow → nutrients removed quickly → concentration gradients maintained → efficient absorption
Why Villi Need Capillaries
Imagine a villus without a good blood supply.
Nutrients would cross into nearby tissues but would accumulate there.
As their concentration increased, further movement would become less efficient.
The dense capillary network prevents this by continually transporting absorbed substances away.
This demonstrates how the circulatory system and digestive system work together.
From the Small Intestine to the Liver
Blood carrying many absorbed nutrients does not immediately travel directly to every cell in the body.
Blood from much of the digestive tract first travels to the liver through the hepatic portal vein.
The liver can:
- Process absorbed nutrients.
- Store glucose as glycogen.
- Release glucose when needed.
- Process amino acids.
- Modify and store some nutrients.
- Remove or process certain potentially harmful substances.
Blood then leaves the liver and returns to the general circulation.
Tracing an Absorbed Nutrient
Consider a glucose molecule produced by digestion of starch.
Step 1: Digestion
Starch is chemically digested into smaller sugars.
Step 2: Absorption
Glucose crosses the epithelial surface of the small intestine.
Step 3: Capillary
The glucose enters a blood capillary inside a villus.
Step 4: Hepatic Portal Vein
Blood transports the glucose toward the liver.
Step 5: Liver
The liver processes the glucose. Some may be stored as glycogen.
Step 6: General Circulation
Glucose remaining in the blood can be transported around the body.
Step 7: Cells
Cells can take up glucose and use it during cellular respiration.
Therefore:
digestion → absorption → transport → assimilation/use
Worked Example: A Meal Containing Pasta and Chicken
Suppose a meal contains pasta and chicken.
The pasta contains large amounts of starch.
The chicken contains large amounts of protein.
Digestion
Starch is broken down into simple sugars such as glucose.
Proteins are broken down into amino acids.
Small Intestine
Glucose and amino acids cross the intestinal epithelium.
Villi
They enter blood capillaries within the villi.
Hepatic Portal Vein
The blood transports these nutrients to the liver.
Circulation
Nutrients can then be transported around the body.
Cells
Glucose may be used for respiration, while amino acids may be used to construct new proteins.
The nutrients originally contained in food have now become available to the body's cells.
Structure and Function in the Small Intestine
| Adaptation | How It Helps Absorption |
|---|---|
| Long small intestine | Provides a large area and time for absorption |
| Folded inner surface | Increases surface area |
| Villi | Greatly increase surface area |
| Microvilli | Increase surface area even further |
| Epithelium one cell thick | Creates a short transport distance |
| Dense capillary network | Rapidly carries absorbed nutrients away |
| Lacteals | Transport many absorbed lipid products |
| Transport proteins | Allow specific substances to cross cell membranes |
| Many mitochondria in epithelial cells | Provide energy for active transport |
The small intestine is therefore highly specialised for efficient absorption.
Absorption vs Assimilation
These two terms are easily confused.
Absorption is the movement of digested nutrients from the digestive tract into the blood or lymph.
Assimilation occurs when absorbed nutrients are taken up and used by cells and tissues.
For example:
Glucose crosses the intestinal wall into blood → absorption
A muscle cell uses glucose during respiration → assimilation/use
Absorption vs Digestion
Digestion and absorption are also different processes.
Digestion
Breaks large molecules into smaller molecules.
Absorption
Moves those smaller molecules across the intestinal wall.
For example:
Protein → amino acids is digestion.
Amino acids → through intestinal wall into blood is absorption.
Both processes are necessary for nutrients to become available to the body.
Common Mistakes
Saying Villi Digest Food
Villi are mainly specialised for absorption.
They are not simply structures for physically breaking food apart.
Confusing Villi and Microvilli
Villi are finger-like projections of the intestinal lining.
Microvilli are much smaller projections on the surfaces of individual epithelial cells.
Saying All Nutrients Enter Blood Capillaries Directly
Many absorbed products of long-chain fat digestion enter lacteals and travel through the lymphatic system before reaching the bloodstream.
Confusing Absorption and Digestion
Digestion breaks molecules down.
Absorption moves the products across the intestinal wall.
Thinking the Small Intestine Has a Smooth Surface
Its surface is highly folded and contains villi and microvilli.
Forgetting the Importance of Blood Flow
Blood does more than simply transport nutrients later. Continuous blood flow also helps maintain conditions for efficient absorption.
Saying Food Travels Through the Liver
Food does not pass through the liver.
Many absorbed nutrients are transported to the liver through the blood.
Check Your Understanding
1. Define absorption.
2. Why is absorption necessary after digestion?
3. Why must large food molecules be digested before they can be efficiently absorbed?
4. Where does most nutrient absorption occur?
5. What is a villus?
6. What are microvilli?
7. Explain why having both villi and microvilli is advantageous.
8. Give three adaptations of a villus for efficient absorption.
9. Why is the villus epithelium very thin?
10. Explain why villi contain many blood capillaries.
11. What is a lacteal?
12. Which products of digestion enter blood capillaries, and which commonly enter lacteals?
13. Trace the movement of glucose from the small intestine to the liver.
14. Explain how blood flow helps maintain efficient nutrient absorption.
15. Explain the difference between digestion, absorption and assimilation.
Key Terms
- Absorption – movement of digested nutrients from the digestive tract into the blood or lymph.
- Small intestine – the main region of the digestive system where nutrient absorption occurs.
- Villus – a finger-like projection of the small intestinal lining that increases surface area.
- Villi – plural of villus.
- Microvilli – microscopic projections on intestinal epithelial cells that further increase surface area.
- Epithelium – layer of cells covering a surface or lining an organ.
- Capillary – a very small blood vessel through which substances can be exchanged.
- Lacteal – a lymphatic vessel inside a villus involved in transporting absorbed fats.
- Diffusion – net movement of particles from higher concentration to lower concentration.
- Facilitated diffusion – passive movement across a membrane using transport proteins.
- Active transport – movement of substances across membranes using energy and transport proteins.
- Osmosis – net movement of water through a partially permeable membrane.
- Hepatic portal vein – blood vessel that carries nutrient-rich blood from much of the digestive system to the liver.
- Lymphatic system – vessel network involved in fluid balance, immunity and transport of absorbed lipids.
- Assimilation – uptake and use of absorbed nutrients by cells and tissues.
Key Takeaways
- Digestion produces small nutrient molecules, but these molecules must still be absorbed before the body can use them.
- Most nutrient absorption occurs in the small intestine.
- The small intestine has an enormous surface area because of its folds, villi and microvilli.
- Villi have a thin surface that creates a short transport distance.
- Each villus contains many blood capillaries and a lacteal.
- Glucose and amino acids enter blood capillaries.
- Many products of long-chain fat digestion enter the lymphatic system through lacteals.
- Continuous blood flow helps maintain conditions for efficient absorption.
- Blood containing many absorbed nutrients travels to the liver through the hepatic portal vein.
- The liver processes and regulates many absorbed nutrients before they enter the wider circulation.
- Absorbed nutrients are eventually transported to cells throughout the body.
- Digestion breaks molecules down, absorption moves them into the body's transport systems, and assimilation involves their uptake and use by cells.