The smallest vessels

The idea

Arteries divide into ever smaller vessels; the smallest, capillaries, have walls one cell thick across which materials are exchanged between blood and cells, and they join to form veins.

Reaching every cell

A big vessel cannot touch every single cell of an organ.

How do you think the blood gets close to every cell?

• The vessels divide into smaller and smaller ones
• The cells come to the heart
• The blood leaves the vessels completely

From artery to vein

Open each tab.

Thin walls for exchange

An artery divides into smaller and smaller vessels inside an organ. The smallest are capillaries, and their walls are just one cell thick. This is where materials pass between the blood and the cells around it. The capillaries then join together and form veins that take the blood away.

Arteries divide into ever smaller vessels; the smallest, capillaries, have walls one cell thick across which materials are exchanged between blood and cells, and they join to form veins.

Notes

Capillaries, the smallest vessels, have walls one cell thick for exchange between blood and cells; arteries divide into them, and they join to form veins.

Check yourself

What are the smallest blood vessels called?

How many cells thick is the wall of a capillary?

Answer: 1

The book says the smallest vessels have walls which are one-cell thick.

Where does the exchange of materials between blood and cells take place?

What do the capillaries form when they join together?

• Capillaries — correct. Yes!
• Arteries. Arteries divide into smaller and smaller vessels; the smallest are the capillaries.
• Veins. Veins are formed when the capillaries join together.
• Inside the valves of the veins. Valves keep blood flowing one way in veins; exchange takes place across capillary walls.
• Across the thin wall of the capillaries — correct. Yes!
• Through the thick, elastic walls of the arteries. Exchange takes place across the thin walls of the capillaries.
• Arteries. An artery divides into smaller vessels; capillaries join together to form veins.
• Alveoli. Alveoli are balloon-like structures in the lungs.
• Veins — correct. Yes!