The blood pressure is the hydrostatic force that the blood exerts against the wall of a blood vessel and that propels blood around the body. It is determined partly by cardiac output and partly by the peripheral resistance. As already stated, it is greater in arteries than in veins and is highest in the arteries when the heart contracts (systole). This is called the systolic pressure. When the heart contracts blood enters the arteries faster than it can leave through the capillaries so the vessels stretch under the pressure. This bulging of the arteries is the pulse that can be felt at a number of sites in the body. During diastole,
the artery walls snap back, but the heart contracts again before enough blood has flowed into the capillary bed to relieve the pressure. This is the peripheral resistance. As a result, there is still a substantial blood pressure even during diastole, the diastolic pressure, and so blood flows into the arterioles more or less continuously.
Typically, at rest the systolic pressure is about 120 mmHg and the diastolic pressure approximately 70 mmHg. Blood pressure can be measured using a manual sphygmomanometer combined with a stethoscope (Figure 14.9 (A)) or with an automatic electronic one (Figure 14.9 (B)).Blood pressure is not constant because as demands on the circulatory system change, regulatory mechanisms ensure that adequate blood flow is maintained. Thus, during heavy exercise the arterioles in the working muscles dilate to allow increased delivery of oxygen to the muscles. This of course decreases the peripheral resistance, which, by itself, would cause a fall in blood pressure. However, the cardiac output is increased to counteract this effect.