Blood vessels

Blood is transported in arteries, veins and capillaries.

Blood is pumped from the heart in the arteries. It is returned to the heart in the veins.

The capillaries connect the two types of blood vessel and molecules are exchanged between the blood and the cells across their walls.

Arteries carry blood away from the heart

A cross-section of the blood vessels in the body
ArteriesVeins
Always carry blood away from the heartAlways carry 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 thin walls - have 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

Capillaries

Capillaries connect the smallest branches of arteries and veins

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.

Diagram of a capillary wall

Exchange of molecules

Into cells

  • oxygen diffuses through the capillary wall, into the tissue fluid, and the cells
  • glucose diffuses from the blood plasma, across the capillary walls to the tissue fluid, and then to the cells

Out of cells

  • carbon dioxide diffuses from the cells into the tissue fluid, then across the capillary walls into the blood plasma
  • 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

Looking at blood cells

You may be given some prepared slides of blood to examine with the microscope.

Many types of blood cell are 10 μm in diameter or less. You will need high magnification to examine them in detail.

Diagram showing red and white blood cells. There are more red than white

The slides will have been stained to show the cells, and cell features. This picture shows many red blood cells and three white blood cells.