The Perfusion Pressure for the Systemic Circulation Is Much Greater Than the Perfusion Pressure for the Pulmonary Circulation
WTien the left ventricle contracts and ejects blood into the aorta, the aorta becomes distended with blood, and aortic pressure rises to a peak value called systolic pressure (typically 120 mm Hg).
Between ejections, blood continues to flow out of the aorta into the downstream arteries. This outflow of blood from the aorta causes aortic pressure to decrease. The minimal value of aortic pressure, just before the next cardiac ejection, is called diastolic pressure (typically 80 mm Hg). The mean aortic pressure (average value of the pulsatile pressure in the aorta) is about 98 mm Hg. The mean aortic pressure represents a potential energy for driving blood through the systemic circulation. As blood flows through the systemic blood vessels, this pressure energy is dissipated through friction. The potential energy (blood pressure) remaining by the time the blood reaches the venae cavae is only 3 mm Hg. Therefore the perfusion pressure for the systemic circuit is typically 98-3 mm Hg, or 95 mm Hg.The pressures in the pulmonary artery are typically 20 mm Hg (systolic) and 8 mm Hg (diastolic); the typical mean value is 13 mm Hg. Pulmonary venous pressure is typically 5 mm Hg. Under these conditions the perfusion pressure for blood flow through the lungs is 8 mm Hg (Leo 13 - 5 mm Hg).
The same volume of blood (the cardiac output) flows each minute through the systemic circulation and through the lungs; however, the perfusion pressure for the systemic circuit is much greater than the perfusion pressure for the lungs. The reason for this difference in perfusion pressure is that the systemic vessels offer more friction against blood flow (have a higher resistance) than do the pulmonary vessels. Therefore the systemic circulation is referred to as the high-pressure, high-resistance side of the circulation. The pulmonary circuit is called the low-pressure, low-resistance side.
By convention, blood pressures are always measured with reference to atmospheric pressure. Thus an aortic pressure of 98 mm Hg means that the blood pressure in the aorta is 98 mm Hg higher than the atmospheric pressure outside the body. Also, by convention, blood pressure is measured at heart level. This is why, in human medicine, blood pressure cuffs are typically applied over the brachial artery (in the upper arm); the brachial artery is at the same level as the heart. If blood pressure is measured in an artery or vein at a level different from heart level, an arithmetic correction should be made so that the pressure is reported as if it had been measured at heart level. This correction is necessary because gravity pulls downward on blood and therefore affects the actual pressure of blood within vessels. Gravity increases the actual pressure in vessels lying below heart level and decreases the actual pressure in vessels above heart level. The gravitational effect is significant in an animal the size of a dog and substantial in an animal the size of a horse. The correction factor for the effect of gravity is 1 mm Hg for each 1.36 cm above or below heart level.
Table 18-1
Distribution of Blood Volume in the Cardiovascular System of a Normal Dog
| Distribution | Percent |
| Between central and systemic circulations | |
| Central circulation | 25 |
| Systemic vessels | 75 |
| TOTAL | 100 |
| Within the systemic circulation | |
| Arteries and arterioles | 15 |
| Capillaries | 5 |
| Venules and veins | 80 |
| TOTAL | 100 |