1.4.4 Absorption in the Small Intestine

Introduction

After food is digested in the small intestine, the small soluble molecules must pass into the blood so they can be carried to all parts of the body. This process is called absorption. The small intestine is specially adapted for absorption — its inner lining has tiny finger-like projections called villi that increase the surface area for absorbing nutrients. This topic explains how absorption occurs in the small intestine.

Definition

Absorption in the small intestine is the process by which digested food molecules (simple sugars, amino acids, glycerol, fatty acids, mineral salts, and vitamins) pass through the wall of the small intestine into the blood or lymph.

Main Idea

The internal lining of the small intestine is modified to make tiny finger-like projections called villi. These villi:

• Increase the surface area for absorption.
• Contain blood capillaries and a lacteal (lymph capillary).
• Allow simple sugars, amino acids, mineral salts, and vitamins to pass into the blood capillaries.
• Allow glycerol and fatty acids to be absorbed by the lacteal.

Absorption occurs by a combination of diffusion and active transport.

Core Concept

What is absorbed and how?

Detailed Explanation

1. Structure of Villi

The internal lining of the small intestine is modified to make tiny finger-like projections called villi (singular: villus).

What is a villus?

• A small, finger-like projection of the inner lining of the small intestine.
• Covered by simple columnar epithelium.
• Contains a blood capillary network and a lymph capillary called a lacteal.

How villi help in absorption:

• They increase the surface area for absorption.
• They have thin walls, so molecules can pass through easily.
• They have a rich blood supply, so absorbed nutrients are carried away quickly.

Simple explanation: Villi are like tiny fingers sticking out from the wall of the small intestine. They create a huge surface area, like a towel with many loops, to soak up nutrients.

2. Absorption of Simple Sugars and Amino Acids

Simple sugars (like glucose) and amino acids pass through the wall of the villi into the blood capillaries of villi.

How it happens:

• Simple sugars and amino acids are small, soluble molecules.
• They pass through the thin wall of the villus.
• They enter the blood capillaries inside the villus.

Method of absorption:

• Diffusion — molecules move from an area of higher concentration (in the intestine) to an area of lower concentration (in the blood).
• Active transport — some molecules are moved against the concentration gradient using energy (ATP).

Simple explanation: Simple sugars and amino acids are small enough to slip through the wall of the villus into the blood. Some need a little push (active transport), while others go on their own (diffusion).

3. Absorption of Mineral Salts and Vitamins

Mineral salts and vitamins also pass into the blood capillaries of villi.

How it happens:

• Mineral salts and vitamins are small, soluble molecules.
• They pass through the wall of the villus.
• They enter the blood capillaries.

Method of absorption:

- Diffusion and active transport.

Simple explanation: Vitamins and minerals are like tiny nutrients that slip into the blood through the villi.

4. Absorption of Glycerol and Fatty Acids

Glycerol and fatty acids are absorbed by the lacteal and ultimately will be poured into the blood.

How it happens:

• Glycerol and fatty acids are the products of fat digestion.
• They are absorbed into the lacteal (lymph capillary) inside the villus.
• The lacteal carries them to the lymphatic system.
• Eventually, they are poured into the blood.

Method of absorption:

- Diffusion.

Simple explanation: Fats are absorbed into a special vessel called the lacteal, not directly into the blood. The lacteal carries them to the blood later.

Why fats are absorbed differently:

• Fatty acids and glycerol are not water-soluble.
• They cannot dissolve in blood plasma directly.
• They are absorbed into the lacteal (which contains lymph) and then transported to the blood.

5. Pathway of Absorbed Food

The absorbed food in the blood capillaries of the villi passes into the blood stream.

What happens:

• Absorbed nutrients enter the blood capillaries of the villi.
• These capillaries join up to form veins.
• These veins unite to form one large vein called the hepatic portal vein.
• This vein carries all the blood from the intestine to the liver.
• The liver may store or alter any of the digestion products.

Simple explanation: After absorption, nutrients go into the blood. The blood from the intestine goes first to the liver through the hepatic portal vein. The liver processes the nutrients before they go to the rest of the body.

Why the liver is important:

• The liver stores excess glucose as glycogen.
• The liver converts amino acids and glycerol to glucose.
• The liver removes harmful substances.
• The liver releases nutrients into the general circulation as needed.

Intermediate Level

Absorption in the small intestine involves:

Structure of villi:

• Finger-like projections.
• Covered by simple columnar epithelium.
• Contain blood capillaries and a lacteal.

What is absorbed:

Method of absorption:

• Diffusion (passive movement from high to low concentration).
• Active transport (uses energy to move molecules against concentration gradient).

Pathway:

- Blood capillaries of villi → veins → hepatic portal vein → liver → general circulation.

Important Points

• - The internal lining of the small intestine is modified to make tiny finger-like projections called villi.
• - Villi increase the surface area for absorption.
• - Simple sugars and amino acids pass through the wall of the villi into the blood capillaries of villi.
• - Mineral salts and vitamins also pass into the blood capillaries of villi.
• - Absorption occurs by a combination of diffusion and active transport.
• - Glycerol and fatty acids are absorbed by the lacteal.
• - The absorbed food in the blood capillaries of the villi passes into the blood stream.
• - The hepatic portal vein carries all the blood from the intestine to the liver.
• - The liver may store or alter any of the digestion products.
• - Water and some vitamins are absorbed in the large intestine.

Remember

• - Villi = finger-like projections in small intestine
• - Villi increase surface area for absorption
• - Simple sugars + amino acids → blood capillaries
• - Mineral salts + vitamins → blood capillaries
• - Glycerol + fatty acids → lacteal
• - Absorption = diffusion + active transport
• - Hepatic portal vein = carries blood from intestine to liver
• - Liver = stores or alters digestion products

Common Mistakes

• 1. Thinking all nutrients are absorbed into the blood.
• - Glycerol and fatty acids are absorbed into the lacteal, not directly into the blood.
• 2. Forgetting the role of villi.
• - Villi increase the surface area for absorption. Without villi, absorption would be too slow.
• 3. Confusing diffusion and active transport.
• - Diffusion = passive movement from high to low concentration.
• - Active transport = uses energy to move molecules against concentration gradient.
• 4. Thinking absorption occurs in the stomach.
• - Most absorption occurs in the small intestine, not the stomach.
• 5. Mixing up the hepatic portal vein and hepatic vein.
• - Hepatic portal vein = carries blood from intestine to liver.
• - Hepatic vein = carries blood from liver to heart.
• 6. Believing the liver only stores nutrients.
• - The liver also alters nutrients, detoxifies harmful substances, and releases nutrients as needed.

Quick Revision

60-Second Revision

• Absorption = movement of digested food from small intestine into blood or lymph
• Villi = finger-like projections that increase surface area
• Simple sugars + amino acids → blood capillaries (diffusion + active transport)
• Mineral salts + vitamins → blood capillaries (diffusion + active transport)
• Glycerol + fatty acids → lacteal (diffusion)
• Hepatic portal vein = carries blood from intestine to liver
• Liver = stores or alters digestion products
• Water = absorbed in large intestine
• Warning: Do not confuse hepatic portal vein (intestine → liver) with hepatic vein (liver → heart).