Q. How does the blood reach the heart through the veins ?
A. The capillaries are the junction between the arteries and the veins. They enter small veins which join to form larger and larger veins. The pressure of blood in the veins is much lower than that in the arteries, because the only motive force for the flow of. blood in the veins is contraction of muscles, in which are embedded the veins. These contractions occur as a result of body movement, acting as a sort of peripheral pump to keep the blood moving towards the heart.
Q. What is the speed of circulation of blood ?
A. The circulation of blood is so rapid that the whole mass of blood (about 5 litres) passes through the body, heart and lungs in less than a minute. You can, therefore, imagine the magnitude of the work which the heart has to perform throughout our lives.
Q. Doctor, I have understood how the blood circulates, but I do not see why the heart has to circulate the blood ?
A. Oxygen, water and nutrients have to be collected and transported to every cell of the body. The blood is a carrier of these essential elements, without which the tissues cannot live. These elements are constantly being supplied to the tissues through the blood. That is the reason why heart has to circulate blood.