The blood transports substances around the body, e.g. oxygen.

The white blood cells help to fight disease.

(The blood transports food and wastes; it helps to maintain a constant body temperature.)

A is the aorta.

B is the left atrium.

A vein

X is a valve which prevents blood from flowing in the wrong direction.

As you exercise, your muscles are respiring and using oxygen. The oxygen has to get to them faster so the heart beat speeds up to pump more blood and therefore more oxygen.