12.5: Ventilation and Perfusion

Regional Distribution of Ventilation

During spontaneous breathing in an upright position, lung ventilation is unevenly distributed, with the upper lung regions being better ventilated than the lower (dependent) regions. This unequal distribution of ventilation results from the lung's weight, which causes compression of alveoli at the base and overexpansion of alveoli at the apex of the lung due to their relative positions. While alveolar pressure is relatively uniform throughout the lung, intrapleural pressure is more negative at the apex than at the base. The greater transpulmonary pressure, the difference between alveolar and intrapleural pressures, contributes to the overexpansion of apical alveoli. This negative intrapleural pressure creates higher distending pressures at the apex, leading to greater expansion of apical alveoli compared to those at the base. 

When breathing in an upright position, gravity affects the distribution of ventilation. Since alveoli at the base of the lungs are less expanded compared to those at the apex in an upright position, basal alveoli are more compliant and capable of expanding more during inspiration, thus receiving more ventilation compared to apical alveoli. This creates a gradient, where gas distribution favors the base of the lungs during quiet breathing. When lying supine, the vertical gradient of ventilation shifts, and gravity now affects the posterior and anterior regions differently. In this position, the posterior (dependent) regions of the lungs are compressed more than the anterior (non-dependent) regions, making the posterior lung regions more compliant and able to expand more. As a result, the posterior regions receive more ventilation than the anterior regions. In the lateral position, gravity once again affects ventilation distribution, with the dependent (lower) parts of the lung receiving more ventilation than the non-dependent (upper) regions of the lung. This shifting of ventilation distribution due to gravity and body position is critical in clinical settings, as it affects how patients are positioned, especially in cases of respiratory distress or ventilation support, to optimize gas exchange.⁴⁵

Regional Distribution of Pulmonary Blood Flow

In a standing individual, blood flow per unit of lung tissue tends to increase from the top (apex) to the bottom (base) of the lung due to the influence of gravity. This preference occurs because the pressure in the pulmonary arteries and arterioles is low, prompting blood to move more toward the lower segments due to gravitational effects. As the intravascular pressures in the lower lung regions are greater than those in the upper regions, the resistance in the pulmonary vessels diminishes because of their expansion or recruitment. However, this phenomenon is reversed when an individual is inverted or suspended upside down.

The regional distribution of pulmonary blood flow in a standing adult can be conveniently classified into three zones, as depicted in Figure 25.⁴⁶

Figure 25 | Organization of the Lung based on Ventilation and Perfusion |⁴⁷ This diagram illustrates the three zones of the lung, categorized based on the balance between ventilation (airflow) and perfusion (blood flow). Zone 1 (Apex): Located at the top of the lungs, Zone 1 is characterized by elevated ventilation (more air entering the alveoli) but reduced perfusion (less blood flow through the capillaries). This occurs because the alveolar pressure is greater than the pulmonary arterial pressure in this region, leading to potential capillary collapse. Zone 2 (Middle): In the middle region of the lungs, Zone 2 represents a more balanced state where ventilation is approximately equal to perfusion. The pulmonary arterial pressure is intermittently greater than the alveolar pressure, allowing for gas exchange. Zone 3 (Base): Situated at the bottom of the lungs, Zone 3 experiences reduced ventilation (less air reaching the alveoli) but elevated perfusion (increased blood flow). This is due to the hydrostatic pressure of blood being highest in this region, resulting in distended capillaries.

Zone 1

This is the upper part of the lung, where alveolar pressure is higher than the pressure in the pulmonary arteries. As a result, the capillaries in this zone can collapse, stopping blood flow. Normally, Zone 1 doesn’t exist, but it can appear when alveolar pressure is high (like with positive-pressure ventilation) or when blood pressure is low.

Zone 2

This area demonstrates the ideal situation where the pulmonary arterial pressure exceeds the alveolar pressure. The difference between these pressures determines blood flow in this zone. The alveolar pressure compresses the venules in this area.

Zone 3

Located at the base of the lung, both arterial and venous pressures exceed the alveolar pressure. The arteriovenous pressure difference controls blood flow in the capillaries. Despite appearances of collapsed alveoli, those in Zone 3 have greater compliance than those in Zone 1.

It is crucial to recognize that the demarcation between these zones is variable and can change based on physiological or pathological conditions. For instance, during exercise, increased cardiac output and pulmonary arterial pressure can recruit any existing Zone 1 to Zone 2, shifting the boundaries between Zone 2 and Zone 3. Changes in posture will also alter the relationship of these zones with respect to their anatomic locations in the lung. However, their associations with gravity, pulmonary flow, and alveolar pressure persist despite the change in posture. In addition to the gravitational pattern of pulmonary blood flow, local factors can influence the relationship between flow and ventilation. Hypoxic pulmonary vasoconstriction is an important local factor that contributes to these relationships.

Regional Differences in Ventilation/Perfusion Ratios

The gas tension in the alveoli of the lung is influenced not solely by ventilation or perfusion but by the ratio of ventilation to perfusion (VA/Q). This ratio is variable, spanning from zero to infinity. With a zero ventilation/perfusion ratio, there is perfusion but no ventilation. Conversely, if there is ventilation but no perfusion (creating a dead space effect), the expired gas resembles the inspired gas (Figure 26). Since alveolar pressure is greater than both arterial and venous pressures in zone 1, the alveolar pressure compresses the capillaries, causing them to collapse. This restricts blood flow, and, therefore, there is minimal or no flow in this zone. In zone 2, the arterial pressure exceeds the alveolar pressure, so blood can enter the capillaries. However, because alveolar pressure is still higher than venous pressure, blood flow depends on the difference between arterial and alveolar pressures (not arterial and venous pressures as in typical circulation). This leads to intermittent flow. Both arterial and venous pressures are higher than alveolar pressure in zone 3, and, therefore, there is continuous blood flow in this zone.

Figure 26 | V/Q Mismatch | Normal V/Q ratio 0.8. This is determined based on the volume of air moving into the lung (4 L) compared to the volume of blood the lung receives (5 L). V/Q = 0 when there is no ventilation. V/Q = ∞ when there is no perfusion (dead space). 

In the vertical gradient of the lung, ventilation is higher in the upper portion than at the base. In contrast, perfusion is greater at the base than at the top. In resting conditions, the apex of the lung serves as a reservoir for gas exchange due to its relatively poor blood perfusion. During exercise, the pulmonary flow significantly increases, resulting in a more uniform distribution of blood flow.⁴⁴