Physiology · MU Sofia

39. Ventilation‐perfusion ratio. Matching of ventilation and perfusion.

Explains the ventilation-perfusion ratio as a measure of gas-exchange efficiency in the lungs, then how gravity makes this ratio differ between the apex and base. It sets out the resulting oxygen and carbon dioxide partial pressures in blood from each zone, and the consequences of mismatch, including dead space and shunt. A closing note covers how hypoxic vasoconstriction at altitude can help gas exchange, and the topic ends with recap questions.

Inside: 1 section · 3 labelled pictures · 15 flashcards · 5 min

Topic 39 of 104 in the MU Sofia syllabus for Physiology (Medicine, Year 2). The free AceMed note covers it in 1 section, about 681 words (5 min of reading), with 15 flashcards.

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  1. Ventilation‐perfusion ratio. Matching of ventilation and perfusion.

Ventilation‐perfusion ratio. Matching of ventilation and perfusion.

Ventilation-Perfusion Ratio (V̇/Q̇)

The ventilation-perfusion ratio (V̇/Q̇) represents the efficiency of gas exchange in the lungs. It is the ratio of alveolar ventilation (V̇A), the volume of air reaching the alveoli per minute, to pulmonary blood flow (Q̇), the volume of blood flowing through the pulmonary capillaries per minute.

  • Significance of V̇/Q̇: The V̇/Q̇ ratio is crucial for determining the amount of oxygen taken up by the blood and the amount of carbon dioxide removed from it.

Ideal V̇/Q̇ and Regional Variations

  • Optimal V̇/Q̇: Ideally, for optimal gas exchange, ventilation should perfectly match perfusion, with a V̇/Q̇ ratio of 1. However, the normal average V̇/Q̇ for the entire lung is around 0.8. This indicates that alveolar ventilation is typically 80% of pulmonary blood flow.
  • Gravity and V̇/Q̇: Gravity influences both ventilation and perfusion, leading to regional variations in V̇/Q̇ within the lungs. Perfusion is significantly affected by gravity, with higher blood flow at the base of the lungs than at the apex. Ventilation also varies, but to a lesser extent. Consequently, the V̇/Q̇ ratio is higher at the apex and lower at the base.
  • Apex: V̇/Q̇ is higher, around 3.6.
  • Mid-Lung: V̇/Q̇ approaches the optimal value of 0.8.
  • Base: V̇/Q̇ is lower but stable, around 0.6.

*Note: during high altitude hypoxia of the whole lung occurs. And we know that hypoxia causes vasoconstriction in the pulmonary blood vessels. But the hypoxic vasoconstriction can work in our favour when the increasing pulmonary pressure can recruit unused capillaries to increase the ↑surface area for gas diffusion.

  • V̇/Q̇ and Gas Partial Pressures: These regional V̇/Q̇ differences result in varying partial pressures of oxygen (PO2) and carbon dioxide (PCO2) in the blood leaving different lung zones.
  • Zone 1 (Apex): Highest V̇/Q̇ leads to the highest PaO2 (around 130 mmHg) and lowest PaCO2 (around 28 mmHg).
  • Zone 3 (Base): Lowest V̇/Q̇ results in the lowest PaO2 (around 89 mmHg) and highest PaCO2 (around 42 mmHg).

Matching of Ventilation and Perfusion

  • Importance of Matching: Precise matching of ventilation and perfusion is critical for efficient gas exchange. A mismatch can significantly impair this process.

Consequences of Mismatching (V̇/Q̇ Defects)

When ventilation and perfusion are not appropriately matched, it leads to V̇/Q̇ defects, causing abnormal gas exchange. These defects can range from ventilation of non-perfused areas (dead space) to perfusion of non-ventilated areas (shunt) and various imbalances in between.

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