Blood Flow - Circulation
To understand how blood flows in vessels let us first understand the basic physics of flow in tubes and the special properties of blood as a fluid. The flow of a fluid may be smooth or regular in a straight tube so that each particle moves in a straight line. This is called laminar flow. Blood exhibits a similar kind of flow in the blood vessels.
Figure: Blood velocity is inversely proportional to cross-sectional area at any given point. The velocity is the highest in aorta and falls markedly in capillaries and again increases in vena cava.
Let us first consider the velocity of flow of blood in the vessels. At any point it is not related to the nearness to the heart but to the total cross-sectional area of that part. This cross-sectional area does not refer to that of a single artery or vein or capillary but to the sum of the cross-sections of all the arteries or capillaries of that area. You would have noted that the velocity of water in a river increases as the river narrows, similarly in circulation the maximum velocity occurs where total cross-sectional area is smallest. The arteries have the smallest cross-sectional area and the capillaries have the largest total cross-sectional area.