From the 1975 programme notes:
The heart was at one time thought to be the very seat of life and personality, and indeed we still speak of black hearted villains and soft hearted aunts. This was so presumably because the most obvious immediate sign of death was cessation of the heart beat. We now know that the heart is just a blood pump, or more correctly a pair of pumps, which are responsible for keeping the major transport system of the body in continuous motion. This knowledge has not decreased the importance of the heart because at normal body temperature we cannot survive a cardiac arrest of more than a few minutes.
The blood is the distribution system which supplies every cell in the body with nutrients and oxygen; the refuse collection agency which removes carbon dioxide and waste products from the cells; a branch of the post office which carries chemical messages from one organ or tissue to another, and the central heating or cooling system which ensures that heat is taken away from hard working parts of the body and supplied to other parts which only work correctly if kept at a constant temperature.
The demands of the body can vary considerably between say sleep and running as hard as you can. These can be met by increasing the speed and stroke volume of the pump, and also by adjusting the diameter of the blood vessels so that the blood can be directed to where it is needed most. Because one does not normally want to puncture the system one has to use indirect methods in order to find out what goes on inside it; for instance at what pressure is the heart delivering the blood, how fast is it flowing in different large arteries or veins, or even how much oxygen is being carried? One of the more difficult measurements is to find out how much blood the heart is actually pumping and how efficiently it does it. We shall try and find out all these things without spilling a drop!