1.6 Structure of Villus

Introduction

The small intestine is the main site of absorption in the digestive system. To absorb nutrients efficiently, its inner lining is specially adapted. It has millions of tiny finger-like projections called villi. These villi greatly increase the surface area for absorption. This topic explains the structure of a villus and how its parts work together to absorb digested food.

Definition

Villi (singular: villus) are small, finger-like projections found in the inner lining of the small intestine. Each villus is covered by simple columnar epithelium and contains a blood capillary network and a lymph capillary called a lacteal.

Main Idea

Villi are found in the small intestine. Each villus:

• Is covered by simple columnar epithelium.
• Contains a blood capillary network.
• Contains a lymph capillary called a lacteal.
• Has goblet cells that secrete mucus.
• Increases the surface area for absorption.

The structure of a villus is specially adapted to absorb digested food efficiently.

Core Concept

Parts of a Villus and Their Functions

Detailed Explanation

1. What is a Villus?

Villi (plural: villi) are found in the small intestine. Each villus is covered by simple columnar epithelium. It contains a blood capillary network and a lymph capillary called a lacteal.

Simple explanation: A villus is like a tiny finger sticking out from the wall of the small intestine. It is packed with blood vessels and a special vessel for fats.

How many villi are there?

• There are millions of villi in the small intestine.
• They cover the entire inner surface of the small intestine.
• They give the inner lining a velvety appearance.

2. Simple Columnar Epithelium

Each villus is covered by simple columnar epithelium.

What is simple columnar epithelium?

• A single layer of tall, column-like cells.
• The cells are tightly packed together.
• The outer surface of these cells has microvilli (tiny finger-like projections).

Function:

• Provides a thin barrier for absorption.
• The single layer of cells allows nutrients to pass through easily.
• Microvilli further increase the surface area for absorption.

Simple explanation: The epithelium is like a thin skin covering the villus. It is only one cell thick, so nutrients can pass through quickly.

3. Blood Capillary Network

Each villus contains a blood capillary network.

What is a blood capillary network?

• A network of tiny blood vessels.
• The capillaries are very thin-walled (one cell thick).
• They are connected to an arteriole (brings blood) and a venule (takes blood away).

Function:

• Simple sugars (like glucose) and amino acids pass through the wall of the villi into the blood capillaries.
• Mineral salts and vitamins also pass into the blood capillaries.
• Absorption occurs by a combination of diffusion and active transport.

Simple explanation: The blood capillaries are like tiny pipes that collect nutrients. They absorb sugars, amino acids, minerals, and vitamins from the digested food.

4. Lacteal

Each villus contains a lymph capillary called a lacteal.

What is a lacteal?

• A lymph capillary located in the center of the villus.
• It is part of the lymphatic system.

Function:

• Glycerol and fatty acids are absorbed by the lacteal.
• The lacteal carries them to the lymphatic system.
• Ultimately, they are poured into the blood.

Simple explanation: The lacteal is a special vessel that absorbs fats. Fats cannot go directly into the blood, so they enter the lacteal first.

Why fats are absorbed by the lacteal:

• 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. Goblet Cells

A villus also has goblet cells.

What are goblet cells?

• Mucus-secreting cells.
• They are shaped like a goblet (cup).
• They are found among the columnar epithelial cells.

Function:

• The primary function of goblet cells is to secrete mucin and create a protective mucus layer.
• Mucus lubricates the intestine and protects the epithelium.

Simple explanation: Goblet cells are like tiny cups that release mucus. The mucus protects the villi and helps food slide along.

6. Intestinal Crypts

Crypt means small cavity. Interstitial crypts are small tubular glands formed in the small intestine in between the bases of the villi.

What are intestinal crypts?

• Small tubular glands located between the bases of villi.
• They contain stem cells.

Function:

• Crypt cells of the small intestine provide stem cells for renewal of the intestinal epithelium.
• They replace the epithelial cells that are worn out or damaged.

Simple explanation: Crypts are like factories that produce new cells to replace old ones. This keeps the villi healthy and functional.

7. Arteriole and Venule

Each villus has an arteriole and a venule.

Arteriole:

• A small artery that brings blood to the villus.
• It branches into the blood capillary network.

Venule:

• A small vein that takes blood away from the villus.
• It collects blood from the blood capillary network.

Simple explanation: The arteriole is like an inlet pipe that brings blood in. The venule is like an outlet pipe that takes blood out.

8. Lymph Vessel

Each villus has a lymph vessel.

What is a lymph vessel?

• A vessel that carries lymph away from the lacteal.
• It is part of the lymphatic system.

Function:

• Takes lymph (containing absorbed fats) away from the villus.
• Lymph eventually drains into the blood.

Simple explanation: The lymph vessel is like a drainage pipe that carries fats away from the villus.

Intermediate Level

The structure of a villus is adapted for absorption:

How villi increase surface area:

• Villi are finger-like projections.
• Microvilli are tiny projections on the surface of epithelial cells.
• Together, they increase the surface area by about 600 times.

Important Points

• - Villi are found in the small intestine.
• - Each villus is covered by simple columnar epithelium.
• - It contains a blood capillary network and a lymph capillary called a lacteal.
• - Crypt means small cavity.
• - Interstitial crypts are small tubular glands formed in the small intestine in between the bases of the villi.
• - Crypt cells of the small intestine provide stem cells for renewal of the intestinal epithelium.
• - The villi increase surface area for absorption.
• - A villus also has goblet cells.
• - The primary function of goblet cells is to secrete mucin and create a protective mucus layer.
• - Simple sugars and amino acids pass into the blood capillaries of villi.
• - Mineral salts and vitamins also pass into the blood capillaries of villi.
• - Glycerol and fatty acids are absorbed by the lacteal.

Remember

• - Villus = finger-like projection in small intestine
• - Simple columnar epithelium = covering
• - Blood capillary network = absorbs sugars, amino acids, minerals, vitamins
• - Lacteal = absorbs glycerol and fatty acids
• - Goblet cells = secrete mucus
• - Intestinal crypts = stem cells for renewal
• - Arteriole = brings blood
• - Venule = takes blood away
• - Lymph vessel = takes lymph away
• - Villi increase surface area for absorption

Common Mistakes

• 1. Thinking villi are found in the stomach.
• - Villi are found in the small intestine, not the stomach.
• 2. Confusing villi and microvilli.
• - Villi = finger-like projections of the intestine.
• - Microvilli = tiny projections on the surface of epithelial cells.
• 3. Thinking all nutrients are absorbed into the blood.
• - Glycerol and fatty acids are absorbed into the lacteal, not directly into the blood.
• 4. Forgetting the role of goblet cells.
• - Goblet cells secrete mucus, which protects and lubricates the intestine.
• 5. Mixing up the lacteal and blood capillaries.
• - Lacteal = absorbs fats.
• - Blood capillaries = absorb sugars, amino acids, minerals, vitamins.
• 6. Believing villi are only for increasing surface area.
• - Villi also contain blood vessels and lacteals for absorption.
• 7. Thinking crypts are not important.
• - Crypts contain stem cells that renew the intestinal epithelium.

Quick Revision

60-Second Revision

• Villus = finger-like projection in small intestine
• Simple columnar epithelium = covering
• Blood capillary network = absorbs sugars, amino acids, minerals, vitamins
• Lacteal = absorbs glycerol and fatty acids
• Goblet cells = secrete mucus
• Intestinal crypts = stem cells for renewal
• Arteriole = brings blood
• Venule = takes blood away
• Lymph vessel = takes lymph away
• Villi increase surface area for absorption
• Warning: Do not confuse villi (finger-like projections) with microvilli (tiny projections on epithelial cells).