The capillaries in the alveolar walls are distended by the blood pressure inside them, but simultaneously, they are
Blood flow at different levels in the lung of an upright person at rest and during exercise. Note that when the person is at rest, the blood flow is very low at the top of the lungs; most of the flow is through the bottom of the lung.
Mechanics of blood flow in the three blood flow zones of the lung: zone 1, no flow—alveolar air pressure (Palv) is greater than arterial pressure; zone 2, intermittent flow—systolic arterial pressure rises higher than alveolar air pressure, but diastolic arterial pressure falls below alveolar air pressure; and zone 3, continuous flow—arterial pressure and pulmonary capillary pressure (Ppc) remain greater than alveolar air pressure at all times.
compressed by the alveolar air pressure on their out-sides. Therefore, any time the lung alveolar air pressure becomes greater than the capillary blood pressure, the capillaries close and there is no blood flow. Under different normal and pathological lung conditions, one may find any one of three possible zones of pulmonary blood flow, as follows:
Zone 1: No blood flow during all portions of the cardiac cycle because the local alveolar capillary pressure in that area of the lung never rises higher than the alveolar air pressure during any part of the cardiac cycle
Zone 2: Intermittent blood flow only during the pulmonary arterial pressure peaks because the systolic pressure is then greater than the alveolar air pressure, but the diastolic pressure is less than the alveolar air pressure Zone 3: Continuous blood flow because the alveolar capillary pressure remains greater than alveolar air pressure during the entire cardiac cycle Normally, the lungs have only zones 2 and 3 blood flow—zone 2 (intermittent flow) in the apices, and zone 3 (continuous flow) in all the lower areas. For example, when a person is in the upright position, the pulmonary arterial pressure at the lung apex is about 15 mm Hg less than the pressure at the level of the heart. Therefore, the apical systolic pressure is only 10 mm Hg (25 mm Hg at heart level minus 15 mm Hg hydrostatic pressure difference). This 10 mm Hg apical blood pressure is greater than the zero alveolar air pressure, so that blood flows through the pulmonary apical capillaries during cardiac systole. Conversely, during diastole, the 8 mm Hg diastolic pressure at the level of the heart is not sufficient to push the blood up the 15 mm Hg hydrostatic pressure gradient required to cause diastolic capillary flow. Therefore, blood flow through the apical part of the lung is intermittent, with flow during systole but cessation of flow during diastole; this is called zone 2 blood flow. Zone 2 blood flow begins in the normal lungs about 10 centimeters above the midlevel of the heart and extends from there to the top of the lungs.
In the lower regions of the lungs, from about 10 centimeters above the level of the heart all the way to the bottom of the lungs, the pulmonary arterial pressure during both systole and diastole remains greater than the zero alveolar air pressure. Therefore, there is continuous flow through the alveolar capillaries, or zone 3 blood flow. Also, when a person is lying down, no part of the lung is more than a few centimeters above the level of the heart. In this case, blood flow in a normal person is entirely zone 3 blood flow, including the lung apices.
Zone 1 Blood Flow Occurs Only Under Abnormal Conditions.
Zone 1 blood flow, which is blood flow at no time during the cardiac cycle, occurs when either the pulmonary systolic arterial pressure is too low or the alveolar pressure is too high to allow flow. For instance, if an upright person is breathing against a positive air pressure so that the intra-alveolar air pressure is at least 10 mm Hg greater than normal but the pulmonary systolic blood pressure is normal, one would expect zone 1 blood flow—no blood flow—in the lung apices. Another instance in which zone 1 blood flow occurs is in an upright person whose pulmonary systolic arterial pressure is exceedingly low, as might occur after severe blood loss.
Effect of Exercise on Blood Flow Through the Different Parts of the Lungs. Referring again to Figure 38-3, one sees that the blood flow in all parts of the lung increases during exercise. The increase in flow in the top of the lung may be 700 to 800 per cent, whereas the increase in the lower part of the lung may be no more than 200 to 300 per cent. The reason for these differences is that the pulmonary vascular pressures rise enough during exercise to convert the lung apices from a zone 2 pattern into a zone 3 pattern of flow.
Effect of Increased Cardiac Output on Pulmonary Blood Flow and Pulmonary Arterial Pressure During Heavy Exercise
During heavy exercise, blood flow through the lungs increases fourfold to sevenfold. This extra flow is accommodated in the lungs in three ways: (1) by increasing the number of open capillaries, sometimes as much as threefold; (2) by distending all the capillaries and increasing the rate of flow through each capillary more than twofold; and (3) by increasing the pulmonary arterial pressure. In the normal person, the first two changes decrease pulmonary vascular resistance so much that the pulmonary arterial pressure rises very little, even during maximum exercise; this effect is shown in Figure 38-5.
Cardiac output (L/min)
Cardiac output (L/min)
Effect on mean pulmonary arterial pressure caused by increasing the cardiac output during exercise.
The ability of the lungs to accommodate greatly increased blood flow during exercise without increasing the pulmonary arterial pressure conserves the energy of the right side of the heart. This ability also prevents a significant rise in pulmonary capillary pressure, thus also preventing the development of pulmonary edema.
Was this article helpful?
You are about to discover the "little-known" techniques, tricks and "mind tools" that will show you how to easily "program" your body and mind to produce an instant, rock-hard erection. Learn how to enjoy all of the control, confidence and satisfaction that comes from knowing you can always "rise to the challenge" ... and never have to deal with embarrassment, apologies, shyness or performance anxiety in the bedroom, ever again.