Why does the heart need valves?

What is the liquid part of the blood called? What is the function of platelets in the blood?

How many types of blood vessels are there in the human body? Name them.

A dialysis machine contains long tubes coiled in a tank containing dialysing

(i) Of what substance are the tubes made ?

(ii) What does the dialysing solution contain?

(iii) Name the main waste which passes into the dialysing solution.

State the differences between artery, vein and capillary.