Pulmonary Blood Flow Distribution

Overview
  • Empirical studies have demonstrated that the blood flow through the resting lung is not uniform. Rather it appears that gravity exerts a strong influence on the distribution of pulmonary blood flow, resulting in higher levels of perfusion in the lower regions of the lung when an individual stands upright. Consequently, in an upright position, the lung apex displays the poorest perfusion while the lung base encounters the greatest blood flow. This blood flow gradient appears to be strongest at rest and decreases in magnitude during contexts of exercise.
Mechanism
  • Overview
    • For any column of liquid, the hydrostatic pressure of the liquid increases with depth. This progressively increasing pressure signifies the progressively added weight that liquid molecules exert on those below them. This principle is applicable to all bodies of liquids and explains why water pressure in the ocean progressively increases with oceanic depth. For all intensive purposes blood within the lung mimics a single body of liquid; consequently, blood pressures gradually increase as one travels down the lung's vertical axis. This vertical gradient of blood pressure can result in a nearly 20 mm Hg variation in pressure between the vasculature in the apex compared to that in the base of the lung
    • Given these large variations in pulmonary pressures, it is clear that the pressure gradient actuating blood flow through the vasculature will also vary widely, resulting in large qualitative differences in the blood flow distribution throughout the lung. In the systemic vasculature, the pressure gradients actuating blood flow are those between the arterial and venous pressures; however, in the pulmonary vasculature, the alveolar pressure is also a major factor governing blood flow. Given these unique considerations, physiologists have divided the lung into three basic zones which highlight major differences in how blood flow is actuated within the upright lung.
  • Zone 1
    • Zone 1 is defined as those areas of the lung where alveolar pressure exceeds that of the pulmonary arterial pressure. Consequently, the pulmonary arteries become compressed and shut, eliminating blood flow in this zone and thus creating a zone of dead space. Zone 1 likely does not exist in a healthy individual's lungs as pulmonary arterial pressures just exceed that of alveolar pressure even at the top of the lung apex. However, in scenarios of hypovolemia such as major hemorrhage, reductions in pulmonary arterial pressure may result in the generation of a Zone 1 in the apical lung.
  • Zone 2
    • Zone 2 is defined as those areas of the lung where the pulmonary arterial pressure exceeds that of alveolar pressure; however, alveolar pressures exceed those of pulmonary venous pressures. When the alveolar pressure is larger than the pressure in the pulmonary veins, the relevant pressure gradient determining the rate of blood flow is that between the arterial and alveolar pressures, and not between the arterial and venous pressures. This arises because the alveoli begin to compress the pulmonary capillaries and thus the alveolar pressure becomes a more limiting factor to blood flow than the pressure within the pulmonary veins. Zone 2 usually includes the top and middle portions of the lung in a healthy upright individual at rest. Because pulmonary pressures increase during exercise, the lower border of Zone 2 may begin to rise during contexts of exercise.
  • Zone 3
    • Zone 3 is defines as those areas of the lung where the alveolar pressure falls below that of the pulmonary venous pressure. Consequently, the blood pressure gradient determining blood flow is the traditional one between the pulmonary arterial pressure and the pulmonary venous pressure. Zone 3 is located in the bottom sections of the lung and represents the areas which receive the greatest rates of pulmonary blood flow.


Zones of Pulmonary Blood Flow
Three functional zones of pulmonary blood flow can be defined based on the relevant pressure gradient actuating flow of blood. In Zone 1 where no blood flows the alveolar pressure (PA) is greater than both the arterial (Pa) and venous pressures (Pv). In Zone 2 where moderate blood flow occurs the relevant pressure gradient is between the arterial pressure and the alveolar pressure. In Zone 3 where the greatest blood flow occurs the relevant pressure gradient is between the arterial and venous pressures.