Cambridge O Level Biology · Syllabus 5090 · Transport in Humans
Capillary
What is Capillary?
A capillary is the smallest blood vessel, connecting small arteries to small veins as it passes through a tissue. Its wall is one cell thick and its lumen is only about the width of one red blood cell, so the distance substances must diffuse between the blood and the body cells is extremely short. Capillaries branch to form a dense network, giving a very large total surface area for exchange, and blood flows through them slowly and at low pressure, allowing time for oxygen, glucose, carbon dioxide and other substances to be exchanged.
This definition is part of the Transport in Humans chapter in Cambridge O Level Biology.
Common mistakes with Capillary
- “Capillaries have no walls, so substances can just leak out.” Why wrong“Very thin” has been read as “absent”. WriteA capillary wall is one cell thick, which gives a very short diffusion distance while still holding the blood cells and large proteins in.
- “Red blood cells leave the capillary to form tissue fluid.” Why wrongThey are far too large to pass through the wall, which is why tissue fluid is not red. WriteTissue fluid is formed from plasma: water and small dissolved substances are forced out, while red blood cells and large plasma proteins remain in the capillary.
- “Tissue fluid contains the same large proteins as plasma.” Why wrongThe capillary wall holds them back. WriteLarge plasma proteins are too big to pass through the capillary wall, so they remain in the blood and tissue fluid contains very little of them.
Questions students ask about Capillary
Why are capillary walls only one cell thick?
So that the distance substances must diffuse between the blood and the body cells is as short as possible. A shorter diffusion distance means a faster rate of diffusion, which is what allows oxygen and glucose to reach cells, and carbon dioxide and wastes to leave them, quickly enough to keep up with respiration.
What is tissue fluid and how is it different from blood?
Tissue fluid is the fluid that bathes the body cells. It is formed when pressure forces water and small dissolved substances out of a capillary at its arterial end. Red blood cells and large plasma proteins are too large to pass through the capillary wall, so tissue fluid contains neither, which is why it differs in composition from blood and is not red.