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Ventilation-perfusion Relationships

Ventilation/perfusion Ratio or V/Q. On average,alveolar ventilation is about 4 l/min and pulmonary capillary blood flow, or cardiac output, is about 5 l/min, making the overall ratio of ventilation to perfusion 4:5, or 0.8.This relationship is called the ventilation-perfusion ratio (V/Q ratio).The V/Q ratio varies throughout the lung. .

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Ventilation-perfusion Relationships

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    1. Ventilation-perfusion Relationships Cardiopulmonary A & P Chapter VIII

    2. Ventilation/perfusion Ratio or V/Q On average,alveolar ventilation is about 4 l/min and pulmonary capillary blood flow, or cardiac output, is about 5 l/min, making the overall ratio of ventilation to perfusion 4:5, or 0.8. This relationship is called the ventilation-perfusion ratio (V/Q ratio). The V/Q ratio varies throughout the lung.

    3. How the V/Q Ratio affects Alveolar Gas Ventilation and Perfusion (V/Q) has a profound effect on the partial pressures of O2 and CO2 in the alveolus. PA02 and PACO2 levels average about 100 mmhg and 40 mmhg respectively. Only in a small portion of the lungs are those numbers true. They are actually an average

    4. How the V/Q Ratio affects Alveolar Gas PA02 is determined by the amount of 02 entering the alveoli and its removal by capillary blood flow. If capillary blood flow is low PA02 will remain high. This occurs in the apices; Ventilation with very little perfusion results in a high PA02

    5. How the V/Q Ratio affects Alveolar Gas PAC02 is determined by: the amount of capillary blood perfusing an alveolus, allowing the CO2 to diffuse out of the capillary bed and into the alveolus and the amount of ventilation that alveolus receives. Areas of high perfusion and low ventilation have higher PAC02s. ie: The bases.

    6. Affects of Increased V/Q When the V/Q ratio increases, the PAO2 rises and the PACO2 falls. The PACO2 decreases because it washes out of the alveoli faster than it is replaced by venous blood. The PAO2 increases because it does not diffuse into the blood as fast as it enters the alveolus. This V/Q relationship is present in the upper segments of the upright lung. (zone I)

    7. Affects of Decreased V/Q When the V/Q ratio decreases, the PAO2 falls and the PACO2 rises. The PAO2 decreases because oxygen moves out of the alveolus and into pulmonary blood faster than it is replenished by ventilation. The PACO2 increases because it moves out of the blood and into the alveolus faster than it is washed out. This is seen in the lower lung segments.(zone III)

    8. How the V/Q ratio Affects end capillary gases The O2 and CO2 pressures in the end capillary blood mirror the PAO2 and PACO2 changes that occur in the lungs. Therefore, as the PAO2 decreases and the PACO2 increases from the top of the lung to the bottom, so does the PcO2 and PcCo2, respectively. The different PcO2 and PcCO2 levels mix together in the pulmonary veins, under normal circumstances, produce a PaO2 of 100 and a PaCO2 of 40 mm Hg.

    9. Respiratory Quotient Gas exchange between the systemic capillaries and the cells is called internal respiration. About 250 ml of O2 are consumed by the tissues during 1 minute. The cells produce about 200 ml of CO2. The ratio between the volume of O2 consumed and the volume of CO2 produced is called the respiratory quotient.

    10. Respiratory Exchange Ratio Gas exchange between the pulmonary capillaries and the alveoli is called external respiration. The quantity of O2 and CO2 exchanged during a period of 1 minute is called the respiratory exchange ration (RR). Under normal condition,s the RR equals the RQ.

    11. Respiratory Disorders and the V/Q In respiratory disorders, the V/Q ratio is always altered. Disorders that diminish pulmonary perfusion cause the affected lung areas to receive little or no blood flow in relation to ventilation, thereby, increasing V/Q ratio. This phenomenon leads to wasted or dead space ventilation.

    12. Respiratory Disorders and the V/Q In disorders that diminish pulmonary ventilation, the affected lung area receives little or no ventilation in relation to blood flow causing a decreased V/Q ratio. The pulmonary blood flow in this area of the lung to be physiologically ineffective in terms of gas exchange, and is said to be shunted blood.

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