Arterial blood pressure is generated by the left ventricle ejecting blood into the systemic vasculature, which acts as a resistance to cardiac output. With each ejection of blood during ventricular systole, the aortic blood volume increases, which stretches the wall of the aorta and increases aortic pressure. As the heart relaxes (ventricular diastole), blood flows from the aorta into distributing arteries that transport the blood to the various organs. Within the organs, the arterial vasculature undergoes extensive branching and the vessel diameters decrease. The smaller arteries and arterioles serve as the chief resistance vessels, and through changes in their diameter, regulate systemic vascular resistance and organ blood flow.
In the aorta and arterial vessels, the arterial pressure is pulsatile; however, by the time the blood reaches the capillaries, vascular dampening has removed the pulsatility. In hemodynamic terms, it is the mean arterial pressure (MAP) that drives organ blood flow. MAP can be described hemodynamically as the product of cardiac output (CO) and systemic vascular resistance (SVR), plus the central venous pressure (CVP).
Equation 1: MAP = (CO x SVR) + CVP
Based on this equation, increases in CO, SVR or CVP can lead to increases in MAP, with changes in CO and SVR having a quantitatively dominant effect in different hypertensive conditions.
While MAP is an important hemodynamic parameter that is required for calculating SVR, it is not measured in clinical practice unless a person's arterial pressure is being monitored with an indwelling catheter. The most common method for measuring arterial pressure is by use of a sphygmomanometer, which gives systolic and diastolic pressure values in mmHg. The systolic pressure is the peak arterial pressure that occurs during ventricular systole, whereas the diastolic pressure is the minimal aortic pressure just before the ventricle ejects blood into the aortic. At normal heart rates, MAP can be estimated from the systolic (Psys) and diastolic (Pdias) pressure values by the following equation:
Equation 2: MAP = 1/3 (Psys - Pdias) + Pdias
This equation is useful for estimating MAP from systolic and diastolic pressure values; however, this equation does not describe the hemodynamic variables that determine MAP. The hemodynamic variables that determine MAP are found in Equation 1.