Constant and adequate pressure in the arterial system is required to drive blood into all of the organs. Abnormally low blood pressure results in inadequate perfusion of organs, while abnormally high blood pressure can cause heart disease, vascular disease and stroke. Therefore, it is essential that blood pressure be maintained within a narrow range of values that is consistent with the needs of the tissues.
Blood volume is directly related to blood pressure. If the blood volume is increased, then venous return of blood to the heart will increase. An increase in venous return will, by Starling’s Law, cause stroke volume to increase. As stroke volume goes up the cardiac output goes up and the blood pressure rises. Thus one way to control blood pressure over the long term is to control blood volume.
If the pressure in a vessel increases then the blood flow will increase. However, if the resistance in a vessel increases then the blood flow will decrease. Resistance in the blood vessels is effected by three parameters:
- Length of the vessel. The longer the vessel the greater the resistance.
- Viscosity of the blood. The greater the viscosity the greater the resistance.
- Radius of the vessel. The smaller the radius the greater the resistance.
There is no good correlation between pulse rate and blood pressure. Measuring pulse rate does not indicate high or low blood pressure. A rising heart rate does not cause blood pressure to increase at the same rate. The results of HR measurement are an indication of cardiovascular activity and oxygen consumption but it is not a substitute for measuring blood pressure. [1]
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