How We Match Flows

Vertebrates solve the ventilation-perfusion flow-matching problem by having the blood vessels of the lung and the airways as trees with their roots in the heart or in the upper airways extending down from the nose and mouth. Twigs of both of these trees intertwine and touch at the level of the alveolar-capillary units. Look at this match from the airway side. Air enters the trachea that soon divides into bronchi. The bronchial tubes branch and narrow their diameters over and over about twenty-three times until reaching their last division: the alveolar level. Each alveolus, with it skein of capillaries around it, behaves as a unit. However, shrinking the branching trees of airways, as with shrinking a simple pump-tube distributing system as we did in Chapter 1, would lead to blockage when the internal diameters of the airways grow too small so that flows diminish.

Look at the match-up now from the circulation's point of view. Blood leaving the right ventricle of the heart passes through pulmonary arteries and then analogously branching subdivisions of arteries all the way down to the pulmonary capillaries. The functional result of this design is that all points on the exchange surface of the lung are almost the same distance from the source of fresh air and blood: the main airways and the heart.

Tree distributions permit air flow and blood flow to remain almost entirely laminar and non-turbulent as they flow in very close proximity to each other on either sides of the thin alveolar capillary membrane. Lack of turbulence permits the work of pumping and the work of ventilating the alveoli to remain minimal.

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