The resistance in the pulmonary circulation is approximately 10 times lower than that in the systemic circulation. This unique system has two features that enable increased blood flow without an increase in pressure. Firstly, during exertion or exercise, closed pulmonary vessels are recruited. Secondly, the blood vessels in the pulmonary circulation are highly distensible, meaning their diameter can increase with only a minimal increase in pulmonary arterial pressure.

During end inspiration, the air-filled alveoli compress the alveolar capillaries, causing an increase in pulmonary vascular resistance (PVR). In comparison to the capillary beds in the systemic circulation, the capillary beds in the lungs contribute to approximately 40% of PVR. At the same time, the larger extra-alveolar vessels experience an increase in diameter due to radial traction and elastic recoil, resulting in a lower PVR at higher lung volume. During exhalation, the deflated alveoli offer less resistance to the alveolar capillaries, reducing PVR, while the higher pleural pressure increases PVR in the extra-alveolar vessels. As a result of these opposing effects, total PVR in the lungs is lowest at functional residual capacity (FRC).

Changes in vital capacity also affect total pulmonary vascular resistance. During lung inflation from residual volume (RV) to total lung capacity (TLC), resistance to blood flow through alveolar vessels increases, while resistance through extra-alveolar vessels decreases. This leads to a U-shaped curve of changes in total pulmonary vascular resistance, with the lowest point at FRC.

The distribution of blood flow in the lungs is significantly influenced by gravity due to the low-pressure/low-resistance nature of the pulmonary circulation. In a normal upright resting position, blood flow volume is highest at the base of the lungs and gradually decreases towards the apex. In contrast, in a supine position, blood flow is lowest in the uppermost (anterior) regions and highest in the lower (posterior) regions. During stress or exercise, the difference in blood flow between the apex and base of the lungs in upright individuals decreases, primarily due to an increase in arterial pressure.

In an upright position, blood leaving the pulmonary artery must move against gravity to reach the apex of the lung. For every 1-cm increase in elevation of a pulmonary artery segment above the heart, there is a corresponding decrease in hydrostatic pressure of 0.74 mm Hg. Consequently, a pulmonary artery segment located 10 cm above the heart experiences a pressure 7.4 mm Hg lower than a segment at heart level. Conversely, a pulmonary artery segment situated 5 cm below the heart has a 3.7 mm Hg increase in pulmonary arterial pressure. This gravitational effect on blood flow affects both arteries and veins equally, resulting in significant variations in arterial and venous pressure from the apex to the base of the lung. These variations have an impact on both blood flow and the ventilation/perfusion relationships in the lungs.

In addition to the gradients between pulmonary arterial pressure (Pa) and pulmonary venous pressure (Pv), differences in pulmonary alveolar pressure (PA) also influence blood flow within the lung. Traditionally, the lung has been divided into three functional zones.
This resistance is about 10 times less than that in the systemic circulation. The pulmonary circulation has two unique features that allow increased blood flow on demand without an increase in pressure: (1) With increased demand, as during exertion or exercise, pulmonary vessels that are normally closed are recruited; and (2) the blood vessels in the pulmonary circulation are highly distensible, and their diameter increases with only a minimal increase in pulmonary arterial pressure.
Fig. 23.5). At end inspiration, the air-filled alveoli compress the alveolar capillaries and increase PVR. In contrast to the capillary beds in the systemic circulation, the capillary beds in the lungs account for approximately 40% of PVR. The diameters of the larger extra-alveolar vessels increase at end inspiration because of radial traction and elastic recoil, and their PVR is lower at higher lung volume. During exhalation, the deflated alveoli apply the least resistance to the alveolar capillaries and their PVR is diminished, whereas the higher pleural pressure during exhalation increases the PVR of extra-alveolar vessels. As a result of these opposite effects of lung volume on PVR, total PVR in the lung is lowest at FRC.
• Fig. 23.5 Schematic Representation of the Effects of Changes in Vital Capacity on Total Pulmonary Vascular Resistance and the Contributions to the Total Afforded by Alveolar and Extra-Alveolar Vessels. During inflation from residual volume (RV) to total lung capacity (TLC), resistance to blood flow through alveolar vessels increases, whereas resistance through extra-alveolar vessels decreases. Thus changes in total pulmonary vascular resistance are plotted as a U-shaped curve during lung inflation, with the nadir at functional residual capacity (FRC).
Distribution of Pulmonary Blood Flow
Because the pulmonary circulation is a low-pressure/lowresistance system, it is influenced by gravity much more dramatically than is the systemic circulation. This gravitational effect contributes to an uneven distribution of blood flow in the lungs. In normal upright persons at rest, the volume of blood flow increases from the apex of the lung to the base of the lung, where it is greatest. Similarly, in a supine individual, blood flow is least in the uppermost (anterior) regions and greatest in the lower (posterior) regions. Under conditions of stress, such as exercise, the difference in blood flow in the apex and base of the lung in upright persons becomes less, mainly because of the increase in arterial pressure.
On leaving the pulmonary artery, blood must travel against gravity to the apex of the lung in upright people. For every 1-cm increase in location of a pulmonary artery segment above the heart, there is a corresponding decrease in hydrostatic pressure equal to 0.74 mm Hg. Thus the pressure in a pulmonary artery segment that is 10 cm above the heart is 7.4 mm Hg less than the pressure in a segment at the level of the heart. Conversely, a pulmonary artery segment 5 cm below the heart has a 3.7–mm Hg increase in pulmonary arterial pressure. This effect of gravity on blood flow affects arteries and veins equally and results in wide variations in arterial and venous pressure from the apex to • Fig. 23.6 Model to Explain the Uneven Distribution of Blood Flow in the Lung According to the Pressures Affecting the Capillaries.
PA, pulmonary alveolar pressure; Pa, pulmonary arterial pressure; Pv, pulmonary venous pressure. (From West JB, et al. J Appl Physiol. 1964;19:713.) the base of the lung. These variations influence both flow and ventilation/perfusion relationships.
In addition to the pulmonary arterial pressure (Pa) to pulmonary venous pressure (Pv) gradients, differences in pulmonary alveolar pressure (PA) also influence blood flow in the lung. Classically, the lung has been thought to be divided into three functional zones (