<prompt> Ventilation-perfusion ratio is maximum atApex of lungBase of lungPosterior lobe of lungMiddle of the lung  <response>: Apical alveoli have low ventilation and perfusion per unit volume of the lung, whereas the basal alveoli have high ventilation and perfusion.Ventilation-perfusion ratio and the PaO2 is maximum at the apex. PCO2 is maximum at the base. 