ABSTRACT
The distribution of ventilation and perfusion and, hence, the gas exchange function of the lung are governed by both gravitational and nongravitational factors. While the gravitational model provided a persuasive hypothesis to explain measurements made in the 1960s of the distribution of regional blood flow, recent observations using higher-resolution techniques have revealed factors that influence perfusion and ventilation independent of the gravitational vector. This chapter describes the nature and significance of these nongravitational factors.
