A valuable characteristic of the vascular system is that all blood vessels are distensible. We have seen one example of this in Chapter 14: When the pressure in blood vessels is increased, this dilates the blood vessels and therefore decreases their resistance. The result is increased blood flow not only because of increased pressure but also because of decreased resistance, usually giving at least twice as much flow increase for each increase in pressure as one might expect.
Vascular distensibility also plays other important roles in circulatory function. For example, the distensible nature of the arteries allows them to accommodate the pulsatile output of the heart and to average out the pressure pulsations. This provides smooth, continuous flow of blood through the very small blood vessels of the tissues.
The most distensible by far of all the vessels are the veins. Even slight increases in venous pressure cause the veins to store 0.5 to 1.0 liter of extra blood. Therefore, the veins provide a reservoir function for storing large quantities of extra blood that can be called into use whenever required elsewhere in the circulation.
Units of Vascular Distensibility. Vascular distensibility normally is expressed as the fractional increase in volume for each millimeter of mercury rise in pressure, in accordance with the following formula:
Increase in volume
Increase in pressure x Original volume
That is, if 1 mm Hg causes a vessel that originally contained 10 millimeters of blood to increase its volume by 1 milliliter, the distensibility would be 0.1 per mm Hg, or 10 per cent per mm Hg.
Difference in Distensibility of the Arteries and the Veins. Anatomically, the walls of the arteries are far stronger than those of the veins. Consequently, the arteries, on average, are about eight times less distensible than the veins. That is, a given increase in pressure causes about eight times as much increase in blood in a vein as in an artery of comparable size.
In the pulmonary circulation, the pulmonary vein distensibilities are similar to those of the systemic circulation. But, the pulmonary arteries normally operate under pressures about one sixth of those in the systemic arterial system, and their distensibilities are correspondingly greater, about six times the dis-tensibility of systemic arteries.
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.