Microcirculation of the Lung: Functional and Anatomic Aspects

Abstract

The fixed pulmonary vascular anatomy differs from the systemic anatomy in the arrangement and shape of the capillary segments, but an even more striking peculiarity of the lung is that it needs to adapt itself to three different pressures, which leads to considerable adaptations and changes in morphology. Because of the pressure changes required by respiratory mechanics, the morphology differs both at the level of the capillaries and at the level of the extraalveolar small arteries and veins as a function of the existing mechanical conditions, adapting them in a way that is best suited to fulfill their respective functions. The configuration of the pulmonary capillary network markedly differs from that in the systemic capillary bed. The gas exchange needs are different in the systemic and in the pulmonary capillaries. In the periphery, the capillaries are longitudinal and their number and density in the tissue reflect local needs; in the lung, their purpose is to be capable of picking up from the outside as much oxygen as possible to fulfill the most extreme conceivable needs for gas exchange (the “diffusing capacity”). This capacity is normally not reached and the capillaries tolerate recruitment, de-recruitment, and changes in configuration that support variable quantitative ­levels of gas exchange.