# 72 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.

Diffusion is responsible for the transfer of materials between capillaries and tissue fluid.

Sample questions 

Figure above shows a section through the heart 

i) Name the two blood vessels A and B                                         [2 marks]

ii) Which of blood vessels A, B, C or D carry oxygenated blood      [1 mark]

iii) Name valve E and state its function                                           [3 marks]

Student's answer 

i) A, vena cava (ü)       B, pulmonary vein (û)

ii) C (û)

iii) name: semilunar valve (ü)     function: to stop blood going backwards (ü)

Examiner's comments

Blood vessels B is the pulmonary artery. Arteries of the heart always carry blood from a ventricle. 

Part ii) needs two answers (blood vessels C and D) to gain the mark. 

D is the pulmonary vein, which carries oxygenated blood to the heart from the lung. 

C is the aorta, which carries oxygenated blood from the heart to the body.

In part iii) the name of the valve is correct, but there are two marks for its functions. This candidate has given only one statement: a second mark was available for stating that the valve prevents blood from going back into the left ventricle.