Blood is pumped from the heart into the arteries. It is returned to the heart by the veins.
The capillaries connect the two types of blood vessel (arteries and veins). Molecules are exchanged between blood and cells across the capillary walls.
|Always carry blood away from the heart||Carry blood to the heart|
|Carry oxygenated blood, except for the pulmonary artery||Always carry deoxygenated blood, except for the pulmonary vein|
|Carry blood under high pressure||Carry blood under low or negative pressure|
|Have thick muscular and elastic walls to pump and accommodate blood||Have thinner walls - less muscular tissue than arteries|
|A type of supporting tissue called connective tissue provides strength||Have less connective tissue than arteries|
|The channel in the blood vessel that carries blood - the lumen - is narrow||Have a wide lumen|
|Arteries do not contain pocket valves||Veins contain pocket valves to stop the backflow of blood|
Arteries branch to become smaller arteries called arterioles, these arterioles branch to become capillaries. The capillaries connect the smallest branches of the arteries (arterioles) and the smallest branches of veins (venules).
Capillaries form extensive networks throughout the body so that all cells are near a capillary and the concentration gradients can be maintained for the delivery of substances and the removal of waste around cells.
The walls of capillaries are just one cell thick. Capillaries therefore allow the exchange of molecules between the blood and the body's cells - molecules can diffuse across their walls. This exchange of molecules is not possible across the walls of other types of blood vessel. Capillaries form extensive networks so that every cell is near a capillary carrying blood.