Lung ventilation-perfusion ratio

Ventilation-perfusion ratio (VA/Q) A comparison of the proportion of lung tissue being ventilated by inhaled air to the rate of oxygenation of pulmonary blood. 

Normal ventilation- perfusion ratio. The function of the lung is to maintain P02 and PC02 within the normal range. This is accomplished by matching 1 ml mixed venous blood with 1 mL fresh air (V/Q = 1). Normally, there is less ventilation (V) than perfusion (Q), and the V/Q ratio is 0.8. 

The V/Q term describes the imbalance between ventilation (V) and perfusion (Q) in different areas of the lung. Given that alveolar ventilation is 4.5 l.min and pulmonary arterial blood flow is 5.0 l.min 1, the overall V/Q ratio is 0.9. Both ventilation and perfusion increase from top to bottom of the lung, but perfusion by much more than ventilation. 

Although the reversibility of airways obstruction is an important characteristic of asthma, those asthmatics with chronic symptoms do not reverse completely. The chronic airways obstruction is probably related to mucus plugging of bronchi and hypertrophy of the bronchial smooth muscle. These changes are not reversible by bronchodilators, and it is not clear that they can be reversed with any specific therapy. Both types of obstruction contribute to an imbalance of the ventilation to perfusion ratio in the lung and can result in hypoxemia and hypocarbia. The decreased carbon dioxide content of the blood is the result of the asthmatic s hyperventilation and will persist until the respiratory muscles fatigue and hypoventilation becomes prominent. Asthmatics can die quite quickly when this occurs. 

TABLE 7.3 Ventilation-to-Perfusion Ratios from the Top to Bottom of the Lung of Normal Man in the Sitting Position... 

The overall effect of nonuniform ventilation and perfusion is that both decrease as one progresses vertically upward in the upright lung. But perfusion decreases more rapidly so that the dimensionless ratio of ventilation to perfusion, VJQ, decreases upward, and can vary fin>m approximately 0.5 at the lung s bottom to 3 or more at the lung s top. Extremes of this ratio are ventilated regions with no blood flow, called dead space, where VJQ — , and perfused regions with no ventilation, called... 

Bronchiolar smooth muscle is sensitive to changes in carbon dioxide levels. Excess carbon dioxide causes bronchodilation and reduced carbon dioxide causes bronchoconstriction. Pulmonary vascular smooth muscle is sensitive to changes in oxygen levels excess oxygen causes vasodilation and insufficient oxygen (hypoxia) causes vasoconstriction. The changes in bronchiolar and vascular smooth muscle tone alter the amount of ventilation and perfusion in a lung unit to return the V/Q ratio to one. 

In a lung unit with high blood flow and low ventilation (airway obstruction), the level of carbon dioxide is increased and the level of oxygen is decreased. The excess carbon dioxide causes bronchodilation and an increase in ventilation. The reduced oxygen causes vasoconstriction and a decrease in perfusion. In this way, the V/Q ratio is brought closer to one and gas exchange is improved. 

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