Blood pressure (BP) is defined as the force per unit area that contained blood exerts on a vessel wall. BP is expressed in millimeters of mercury (mmHg). The difference is blood pressure provides the force needed to keep the blood moving. Blood always moves from a place with high pressure to a place with a lower pressure. The pressure can be regulated by the arterial system.
Systemic blood pressure
When we speak of blood pressure we mean the systemic arterial blood pressure. This is the pressure in the large arteries near the heart. The pressure in the large systemic arteries depends on two factors: how much the arteries can be stretched and how big the volume of blood pumped in the arteries is. Because the heart pumps in pulses, the pressure in the arteries is also pulsatile. The pressure peak after a contraction of the left ventricle and the wall of the aorta is called the systolic pressure. In healthy adults this averages around 120 mmHg. During the contraction of the left ventricle, the aortic valve is open. But when the pressure in the aorta becomes higher than the pressure in the left ventricle, the valve closes. The recoil of the walls of the aorta generate a small pressure peak (see figure 1). When the pressure drops to its lowest point, it is called the diastolic pressure. In healthy adults this averages around 80 mmHg. The difference between the systolic and the diastolic pressure is called the pulse pressure. The pressure that propels the blood to the organs is called the mean arterial pressure (MAP).
MAP = diastolic pressure + 1/3 * pulse pressure
Because the diastole usually takes longer than the systole MAP is not simply in the middle of the two.
Venous blood pressure
Venous blood pressure is very low and reflects the effect the peripheral resistance has on the blood pressure. The venous pressure is usually too low to promote adequate venous return. There are three mechanisms that are important for venous return. These are:
- the respiratory pump
- the muscle pump
- the layer of smooth muscle cells around the veins that constrict under sympathetic control