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Cardiopulmonary Anatomy & Physiolo...

7th Edition
Des Jardins + 1 other
ISBN: 9781337794909

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Chapter
Section
BuyFindarrow_forward

Cardiopulmonary Anatomy & Physiolo...

7th Edition
Des Jardins + 1 other
ISBN: 9781337794909
Textbook Problem

When the V/Q ratio decreases, the

1. PA o 2 falls

2. PC co 2 increases

3. PA co 2 decreases

4. PC o 2 decreases

A. 1 only

B. 3 only

C. 2, 3, and 4 only

D. 1, 2, 3, and 4

Summary Introduction

Introduction:

The ventilation-perfusion VQ 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 (PAO2) and the pressure of carbon dioxide (PACO2) inside the alveoli.

The pressure of carbon dioxide and oxygen at the capillary end is same as the alveolar end in lungs, that is, PCO2 and PCCO2 (the capillary end pressure of O2 and CO2 in blood, respectively) have the same pressure PACO2 and PAO2, respectively.

Explanation

Justification/ Explanation for the correct answer:

Option (d) is given that when the VQ ratio decreases, the PAO2 level falls, PCCO2 increases, PACO2 rises, and PCO2 decreases. The decrease in VQ ratio leads to multiple changes in O2 and CO2 pressure in the alveoli. As the VQ 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 PACO2 increases. This leads to a similar change at the capillary end and PCCO2 increases.

Similarly, as the ventilation rate of the alveoli decreases, the oxygen pressure inside the alveoli also decreases. Thus, the PAO2 level falls, which, in turn, leads to corresponding pressure change at the capillary end and decreases PCO2. Hence, option (d) is correct.

Explanation for the incorrect answer:

Option (a) is given that when the VQ ratio decreases, only the PAO2 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 PAO2 levels, but, in addition, PACO2, PCCO2, and PCO2 levels are also changed

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