The blood vessels

Blood is transported in arteries, veins and capillaries.

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.

A cross-section of the blood vessels in the body
ArteriesVeins
Always carry blood away from the heartCarry blood to the heart
Carry oxygenated blood, except for the pulmonary arteryAlways carry deoxygenated blood, except for the pulmonary vein
Carry blood under high pressureCarry blood under low or negative pressure
Have thick muscular and elastic walls to pump and accommodate bloodHave thinner walls - less muscular tissue than arteries
A type of supporting tissue called connective tissue provides strengthHave less connective tissue than arteries
The channel in the blood vessel that carries blood - the lumen - is narrowHave a wide lumen
Arteries do not contain pocket valvesVeins contain pocket valves to stop the backflow of blood

Capillaries

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.

Blood travels from artery through the arterioles into the capillaries. From the capillaries it goes through the venules to the vein.

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.

Diagram of a capillary wall

Exchange of molecules

  • Oxygen diffuses through the capillary wall into the tissue fluid and the cells.
  • Carbon dioxide diffuses from the cells into the tissue fluid, then across the capillary walls into the blood plasma.
  • Glucose diffuses from the blood plasma across the capillary walls to the tissue fluid and then to the cells.
  • The waste product urea diffuses from the cells of the liver to the tissue fluid and then across the capillary walls into the blood plasma.
Diagram showing how oxygen diffuses through the capillary wall