<prompt> In which of the following pa of the lung the ventilationperfusion ratio is maximum?Apical regionMiddle regionBaseAll of the above  <response>: The ratio of pulmonary ventilation to pulmonary blood flow for the whole lung at rest is about 0.8 4.2 Lmin ventilation divided by 5.5 Lmin blood flow. However, relatively marked differences occur in this ventilationperfusion ratio in various pas of the normal lung as a result of the effect of gravity. Ventilation, as well as perfusion in the upright position, declines in a linear fashion from the bases to the apices of the lungs. However, the ventilationperfusion ratios are high in the upper poions of the lungs. When widespread, nonuniformity of ventilation and perfusion in the lungs can cause CO2 retention and lowers systemic aerial PO2. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. 2012. Chapter 34. Introduction to Pulmonary Structure and Mechanics. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks Eds, Ganongs Review of Medical Physiology, 24e. 