Chapter 8, Problem 3RQ

Summary Introduction

Introduction:

The ventilation-perfusion $\frac{\text{V}}{\text{Q}}$ ratio represents the ratio of alveolar ventilation to pulmonary capillary blood flow. This ratio differs throughout the region of the lungs. It also mediates the pressure of oxygen (${\text{P}}_{{\text{A}}_{O_{{}_2}}}$) and the pressure of carbon dioxide (${\text{P}}_{{\text{A}}_{C{O}_{{}_2}}}$) inside the alveoli.

The pressure of carbon dioxide and oxygen at the capillary end is same as the alveolar end in lungs, that is, ${\text{P}}_{C_{O_{{}_2}}}$ and ${\text{P}}_{C_{C{O}_{{}_2}}}$ (the capillary end pressure of O₂ and CO₂ in blood, respectively) have the same pressure ${\text{P}}_{{\text{A}}_{C{O}_{{}_2}}}$ and ${\text{P}}_{{\text{A}}_{O_{{}_2}}}$, respectively.

Answer to Problem 3RQ

Correct answer:

1, 2, 3, and 4

Explanation of Solution

Justification/ Explanation for the correct answer:

Option (d) is given that when the $\frac{\text{V}}{\text{Q}}$ ratio decreases, the ${\text{P}}_{{\text{A}}_{O_{{}_2}}}$ level falls, ${\text{P}}_{C_{C{O}_{{}_2}}}$ increases, ${\text{P}}_{{\text{A}}_{C{O}_{{}_2}}}$ rises, and ${\text{P}}_{C_{O_{{}_2}}}$ decreases. The decrease in $\frac{\text{V}}{\text{Q}}$ ratio leads to multiple changes in O₂ and CO₂ pressure in the alveoli. As the $\frac{\text{V}}{\text{Q}}$ ratio decreases, the alveolar perfusion increases, leading to rapid movement of CO₂ from venous supply to alveoli, which occurs at a rate faster than its removal rate, and thus ${\text{P}}_{{\text{A}}_{C{O}_{{}_2}}}$ increases. This leads to a similar change at the capillary end and ${\text{P}}_{C_{C{O}_{{}_2}}}$ increases.

Similarly, as the ventilation rate of the alveoli decreases, the oxygen pressure inside the alveoli also decreases. Thus, the ${\text{P}}_{{\text{A}}_{O_{{}_2}}}$ level falls, which, in turn, leads to corresponding pressure change at the capillary end and decreases ${\text{P}}_{C_{O_{{}_2}}}$. Hence, option (d) is correct.

Explanation for the incorrect answer:

Option (a) is given that when the $\frac{\text{V}}{\text{Q}}$ ratio decreases, only the ${\text{P}}_{{\text{A}}_{O_{{}_2}}}$ falls. The supply of oxygen to the alveoli is mediated by ventilation. As the ventilation decreases and capillary blood flow increases, the oxygen replaced by the ventilation process is much slower than the rate with which it moves out of the alveoli. This leads to a fall in the ${\text{P}}_{{\text{A}}_{O_{{}_2}}}$ levels, but, in addition, ${\text{P}}_{{\text{A}}_{C{O}_{{}_2}}}$, ${\text{P}}_{C_{C{O}_{{}_2}}}$, and ${\text{P}}_{C_{O_{{}_2}}}$ levels are also changed. So, it is an incorrect option.

Option (b) is given that when the $\frac{\text{V}}{\text{Q}}$ ratio decreases, only ${\text{P}}_{{\text{A}}_{C{O}_{{}_2}}}$ rises. The flow of carbon dioxide (CO₂) inside the alveoli is mediated by the pulmonary capillary blood flow. The $\frac{\text{V}}{\text{Q}}$ ratio decreases when the blood flow rate is high and, as a result of high blood flow, the capillaries replace the CO₂ inside the alveoli faster than the rate it is removed from it. This leads to an increase in the ${\text{P}}_{{\text{A}}_{C{O}_{{}_2}}}$ levels, but, in addition, ${\text{P}}_{{\text{A}}_{O_{{}_2}}}$, ${\text{P}}_{C_{C{O}_{{}_2}}}$, and ${\text{P}}_{C_{O_{{}_2}}}$ levels are also altered with a decrease in the $\frac{\text{V}}{\text{Q}}$ ratio. So, it is an incorrect option.

Option (c) is given that when the $\frac{\text{V}}{\text{Q}}$ ratio decreases, the ${\text{P}}_{C_{C{O}_{{}_2}}}$ increases, ${\text{P}}_{{\text{A}}_{C{O}_{{}_2}}}$ rises, and ${\text{P}}_{C_{O_{{}_2}}}$ decreases. The pressure at the capillary end is similar to the main capillary branches present in the lungs. This change in pressure of carbon dioxide or oxygen corresponds to similar changes at the capillary end also, leading to an increase in ${\text{P}}_{C_{C{O}_{{}_2}}}$, the rise of ${\text{P}}_{{\text{A}}_{C{O}_{{}_2}}}$, and a decrease in ${\text{P}}_{C_{O_{{}_2}}}$ and, in addition, the ${\text{P}}_{{\text{A}}_{O_{{}_2}}}$ level is also changed. So, it is an incorrect option.

Hence, options (a), (b), and (c) are incorrect.

Conclusion

The ventilation-perfusion rate differs throughout the lungs. The decrease in the $\frac{\text{V}}{\text{Q}}$ ratio is due to a decrease in the ventilation rate, which leads to a decrease in the O₂ pressure in the alveoli and a similar decrease in pressure change is observed at the capillary end. Similarly, high capillary blood pressure leads to the venous supply of CO_2, which is in excess to its removal rate. It leads to an increase in the CO₂ pressure in the alveoli and a similar increase in pressure is observed at the capillary end also.