The ability of blood vessels to dilate
Normal vasoconstriction is expressed as a fraction of an increase in volume per mmHg increase in pressure.
A valuable feature of the vascular system is that all vessels can dilate. The natural elasticity of the arteries allows them to regulate the heartbeat and average the minimum blood pressure. This ability makes the blood flow always smooth and continuous in the capillaries of the arteries tissue.
Veins have the greatest dilatability of all blood vessels. Even a small increase in intravascular pressure can hold an extra 0.5 to 1 litre of blood. So, veins are like a storehouse of a large amount of blood that can be replenished when anywhere in the circulatory system needs it.
Unit of contraction of blood vessels. Normal vasoconstriction is expressed as a fraction of an increase in volume per mmHg increase in pressure, expressed by the following formula:
Vasodilation = (increase in volume)/(increase in pressure x initial volume)
According to the above formula, if 1mmHg is exerted on a blood vessel that initially holds 10 litres of blood to increase its volume by 1 litre, the potential for dilatation will be 0.1/1mmHg or 10%/1mmHg.
Veins are much more resilient than arteries, and their walls are thicker and stronger than corresponding veins. Accordingly, on average, veins are 8 times more elastic than arteries. Thus, for the same increase in pressure, the increased blood volume in the veins will increase more than 8 times compared with the same size arteries.
In the pulmonary circulation, the pulmonary veins dilate as in the systemic circulation. However, the normal pulmonary arteries operate under a pressure of about 1/6 of the pressure in the systemic artery and their ability to contract is respectively about 6 times better than the arterial capacity system.