An vital determinant of arterial oxygen stress and anxiety is the effectiveness of coupling of lung ventilation come lung perfusion. No all components of the lung are equally ventilated or perfused. The relationship in between ventilation and perfusion in a lung region is expressed as the ventilation-perfusion ratio (V/Q). The usually imbalance between ventilation and perfusion in normal people accounts because that the tiny alveolar-arterial oxygen gradient routinely measured v arterial blood gas testing.

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Equal ventilation and also perfusion

When breathing room air at an FIO2 that 0.21, one alveolus v one unit of ventilation and one unit of perfusion has actually a ventilation-perfusion ratio of one, a PAO2 of 100 mmHg, and also a PACO2 of 40 mmHg. 

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Figure 1. Equal ventilation-perfusion proportion (V/Q = 1.0) occurs once the breathing air has an FIO2 of 0.21, a partial pressure of oxygen (PAO2) the 100 mmHg, and also a partial pressure of carbon dioxide (PACO2) the 40 mmHg.

Perfused, not ventilated

In one extreme of ventilation-perfusion mismatch, an alveolus is perfused, however not ventilated; in various other words, it has a ventilation-perfusion ratio of zero. Because no waiting enters the alveolus as alveolar gas equilibrates with blended venous blood return to the lungs, the alveolar gas tensions are those of combined venous blood: PAO2 of 40 mmHg and PACO2 of 45 mmHg.

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Figure 2. A ventilation-perfusion proportion of zero (V/Q = 0.0) occurs once the alveolus is perfused however not ventilated. Since no wait enters the alveolus, the alveolar gas push is the very same as the combined venous blood returning to the lungs. 

 

Ventilated, no perfused

In one more extreme situation of ventilation-perfusion mismatch, the alveolus is ventilated, yet not perfused; in various other words, the ventilation-perfusion proportion is infinity. In the absence of blood circulation to the unit, the alveolar gas tensions space those of motivated air: PAO2 of around 150 mmHg and also PACO2 of practically 0 mmHg.

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Figure 3. A ventilation-perfusion ratio (V/Q) that infinity occurs as soon as the alveolus is ventilated however not perfused. Since there is an lack of blood flow to the unit, the alveolar gas anxiety is the very same as motivated air. 

 

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There actually is a spectrum that ventilation-perfusion relationships throughout the lung, developed by normal physiologic relationships that dictate regional perfusion and also ventilation

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Figure 4. The ventilation, perfusion, and the ventilation-perfusion ratio spectrums transparent the lungs, produced by regular physiology that dictate local perfusion and ventilation. 

In the upright lung, much more ventilation goes to the lung base than the lung apex. This arises because there are more alveoli in ~ the bigger bases. In addition, the basilar alveoli are much less stretched 보다 the apical ones and also can “give more” v inflation (i.e., lock are more compliant).

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Figure 5. Ventilation that the lung decreases as the rib number decreases to the apex lung. This arises due to the fact that there are much more alveoli in ~ the larger bases, and also basilar alveoli have larger inflation.

In the upright lung, an ext perfusion goes come the lung base 보다 the lung apex because there are an ext alveoli and pulmonary blood vessels in the bigger bases, and because gravitational results on pulmonary blood circulation favor perfusion come the bases.

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Figure 6. Blood flow in the lung decreases as the rib number decreases come the apex lung. This arises because there are much more alveoli and also pulmonary blood ship in the larger bases. 

Although the apical-basal gradients for ventilation and also perfusion space in the very same direction, the magnitudes of changes in every from apex come base are different. The steep of the perfusion curve is steeper than that because that ventilation. Together a result, the ventilation-perfusion ratio decreases indigenous apex come base.

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Figure 7. The steep of the ventilation-perfusion ratio decreases native apex to base. This occurs from the steep of the perfusion curve gift steeper than that that the ventilation slope. 

In disease states, ventilation-perfusion relationship throughout the lung room altered, developing abnormal gas exchange, specifically for oxygen. In particular, areas of the lung identified by ventilation-perfusion ratios of less than one contributes to hypoxemia and also widening the the alveolar-arterial oxygen gradient.

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