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Cardiopulmonary Anatomy & Physiology
Cardiopulmonary Anatomy & Physiology
7th Edition
ISBN: 9781337794909
Author: Des Jardins, Terry.
Publisher: Cengage Learning,
Not helpful? See similar books
Cardiopulmonary Anatomy & Physiology
Cardiopulmonary Anatomy & Physiology
Ventilation-perfusion Relationships. 3RQ

The ventilation-perfusion V 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 ( P A O 2 ) and the pressure of carbon dioxide ( P 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, P C O 2 and P C C O 2 (the capillary end pressure of O 2 and CO 2 in blood, respectively) have the same pressure P A C O 2 and P A O 2 , respectively.

Question
Chapter 8, Problem 3RQ
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.

Expert Solution & Answer

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 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. So, it is an incorrect option.

Option (b) is given that when the VQ ratio decreases, only PACO2 rises. The flow of carbon dioxide (CO2) inside the alveoli is mediated by the pulmonary capillary blood flow. The VQ 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 PACO2 levels, but, in addition, PAO2, PCCO2, and PCO2 levels are also altered with a decrease in the VQ ratio. So, it is an incorrect option.

Option (c) is given that when the VQ ratio decreases, the PCCO2 increases, PACO2 rises, and PCO2 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 PCCO2, the rise of PACO2, and a decrease in PCO2 and, in addition, the PAO2 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 VQ 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 CO2, 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.

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Cardiopulmonary Anatomy & Physiology
Cardiopulmonary Anatomy & Physiology
7th Edition
ISBN: 9781337794909
Author: Des Jardins, Terry.
Publisher: Cengage Learning,
Not helpful? See similar books
Cardiopulmonary Anatomy & Physiology
Cardiopulmonary Anatomy & Physiology
Ventilation-perfusion Relationships. 3RQ
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