Blood Pressure - Blood Vessels

We are all familiar with the term high blood pressure. What exactly is blood pressure? By blood pressure we mean arterial blood pressure. During a heartbeat cycle the maximum pressure is referred to as systolic pressure and the minimum diastolic pressure. The difference between the two is the pulse pressure. Blood pressure is generally expressed in millimetres of mercury and as systolic/diastolic, that is 120/80 mm Hg.

Figure:  Change in blood pressure in different parts of the circulation. The major drop occurs in the arterioles

Blood is 12.9 times less dense than mercury so a blood pressure of 120 mm Hg is equal to 120 x 12.9 = 1550 mm or 155 cm of blood. In other words if the blood vessel was to be opened suddenly the blood would squirt out to a maximum height of 155 cm above the cut. Pressure generated by the heart decreases due to the flow of blood. Figure shows blood pressures in the circulatory system of man. The blood pressure falls as blood flows from the aorta to the vena cava. The greatest pressure drop takes place in the smallest arterioles which by changing their diameter can regulate the flow of blood to various body organs.