Pulmonary Circulation and Systemic Circulation: Similar Problems, Different Solutions

Abstract

The number of sophisticated physical and mathematical systems that have been proposed to simulate the movement of oxygen from the environment to the cell probably equals (and may even exceed!) the number of investigators in this field. In terms of the problem with which we will deal in the next few pages, a much simpler model comes to mind. In this analogy, the integrated O₂ transport to tissue is compared to a tree, in which nutrients are absorbed by finely ramified roots and transported by the sap to a similarly subdivided set of branches through which they reach the delivery site. The two ends of the chain, dissimilar as they may look, share an important requirement, namely the need for proper functional distribution: clearly the roots should not be evenly spaced, like the spokes of a wheel, but rather must be more heavily concentrated in the area where more food or water is available; likewise the sap flow must go preferentially to parts that require more nourishment.