How Well Designed Is the Human Lung?

Abstract

Although it is often stated that the structure of the human lung is ideally suited to its gas exchange function, the lung is very vulnerable under some abnormal conditions. One example is the postoperative period in a patient with an otherwise normal lung where retained secretions can rapidly cause unventilated areas or atelectasis and substantial impairment of gas exchange. Some pulmonologists may be surprised to learn that evolution has provided a very different, and arguably superior, lung design in the bird. Here, the gas exchange and ventilatory functions of the lung are separated. Gas exchange occurs in relatively rigid parabronchi, which are more robust than the delicate alveoli in the human lung, and ventilation is performed by highly expandable air sacs. A comparison of these two completely divergent evolutionary paths throws light on some of the problems of the human lung. Suppose you were asked to design* a heat exchanger, such as the radiator of a car. The purpose of this is to enable heat from the engine to be eliminated to the outside air. One way would be to pump the hot coolant fluid from the engine through a grid of many small tubes and have air passed across these by means of a fan. In fact, this is the time-honored design for car radiators. If you suggested this, Henry Ford would have been proud of you. But you might come up with an alternative design in which the small tubes containing the coolant fluid were enclosed in a bellows that was alternately inflated and deflated so that the air in the bellows was heated and then expelled. In fact, you might even conceive of a design in which the small tubes themselves were part of the bellows. A critic might say that this alternative design was unnecessarily complicated and prone to problems, especially if the small tubes formed part of the moving bellows. But amazingly this is the design that evolution chose for the mammalian, and therefore human, lung. The lung is a gas exchanger, which is closely analogous to a heat exchanger. Although in the latter, heat is taken from the engine and eliminated into the surrounding air, the lung does this for carbon dioxide, and at the same time it takes up oxygen. The route taken by evolution for the mammalian lung is even more astonishing when we realize that a completely different path was pursued in designing the bird lung. Indeed, the bird has a lung rather like the classic car radiator. A comparison of these two quite divergent evolutionary paths helps us to understand some of the vulnerabilities of the human lung.