# 71 Arteries, veins and capillaries - structure and functions
There are 3 main kinds of blood vessels – arteries, veins and capillaries.
- Arteries carry blood away from the heart. They divide again and again, and eventually form very tiny vessels called capillaries.
- The capillaries gradually join up with one another to form large vessels called veins.
- Veins carry blood towards the heart.
Plan of the main blood vessels in the human body
The comparison of blood vessels structure and functions
The transfer of materials between capillaries and tissue fluid
As blood enters capillaries from arterioles (small arteries), it slows down. This allows substances in the plasma, as well as O2 from red blood cells, to diffuse through the capillary wall into the surrounding tissues (the capillary wall is thin and permeable).
Liquid in the plasma also passes out. This forms tissue fluid, bathing the cells. Waste products from the cells, e.g. CO2, diffuse back through the capillary walls into the plasma. Some of the tissue fluid also passes back.
Liquid in the plasma also passes out. This forms tissue fluid, bathing the cells. Waste products from the cells, e.g. CO2, diffuse back through the capillary walls into the plasma. Some of the tissue fluid also passes back.
Diffusion is responsible for the transfer of materials between capillaries and tissue fluid.