Evolution of the mammalian lung has taken a different track than that of the avian lung in that it forms a gas exchanger in the end part of a branched airway tree. This may appear to be a less efficient design than that of the bird lung, but it does have other advantages such as the facilitation of acid–base regulation of the internal milieu through the regulation of CO2 discharge.
An important question is whether the mammalian lung achieves functional efficiency and adequacy through the matching of the design of its structural apparatus, gas-exchange tissues, airways, and blood vessels, with the functional demands of the organism. In that sense, the lung is an essential part of the pathway for oxygen that leads to the sites at which energy is required, the mitochondria in the cells (Weibel 2000
). The largest demand for O2
occurs when the locomotor muscles are fully active in exercise, so we hypothesize that the lung's gas exchanger is quantitatively matched to the maximal O2
consumption (Weibel and Hoppeler 2000
), a value that varies greatly with both body size and physical capacity among different mammalian species, (Weibel 2000
). Studying the relationship between lung design and O2
needs among different mammalian species may offer some insights into the factors, genetic or epigenetic, that adapt the structural design in this vital organ to the variation in functional demand imposed by habitat and life style.
In the mammalian lung, the gas exchanger at the periphery of a closed-bag system is ventilated through a single port by to-and-fro air flow. On inspiration, the residual air near the gas exchange surface in the alveoli is thus replenished with O2
that is continuously transferred to the blood during gas exchange. The pulmonary gas exchanger is characterized by a very large surface of air-blood contact, nearly the size of a tennis court in humans (120 m2
), a very thin tissue barrier (~1 µm); and a large capillary blood volume (200 ml in humans), all of which determines the pulmonary diffusing capacity DLO2
; Weibel and Hoppeler 2000
Comparing different mammalian species, we find DLO2 about matched to the body's maximal O2 needs as induced by exercise, but not in a simple fashion. We find that in “athletic” species, such as dogs, horses, or antelopes, DLO2 is well matched to maximal O2 needs, whereas “normal” species—including humans—have about 50% excess diffusing capacity. When studying animals of different body size, we find DLO2 to be proportional to body mass (M), whereas maximal O2 consumption scales with M0.87. As a result, small mammals have a DLO2 that matches O2 needs, whereas larger animals have excess DLO2 that increases with body mass. Why is this? One possibility is that replenishing O2 in the alveoli may be more difficult in larger lungs.
The problem of supplying oxygen to all points on such a large surface was solved by the evolution of an airway tree with alveoli arranged around the last few generations, thus sequestering the alveolar surface in the pulmonary acini (Weibel et al. 2005
). Pulmonary arteries and veins branch in parallel with the airways and penetrate into the acinus for perfusion of the capillary networks associated with the alveoli. Because of the arrangement of alveoli along the last branches of the airway tree, alveoli are ventilated in series, whereas the units of the capillary network are perfused in parallel. In the acinus, replenishment of O2
at the alveolar surface occurs first by convective air flow, and then by diffusion of O2
in the air. This is combined with continuous absorption of O2
at the alveolar surface, beginning with the first alveolus on the airways, which removes O2
from the acinar air, a phenomenon called screening. The problem, therefore, is provision of enough oxygen for the final generation of the branching acinar tree where half the gas-exchange surface occurs. Adequate supply of oxygen depends on the size of the acinus and the dimensions (diameter and length) of the acinar airways, as well as on the permeability of the barrier. The size of the acini increases with body mass so that large species have larger acini with longer path length from the entrance to the end. Screening is therefore more pronounced in these large lungs, which may explain why they need a larger, seemingly excessive, diffusing capacity (Weibel 2000
; Weibel and Hoppeler 2000
; Weibel et al. 2005
In the preceding section, one comes to appreciate how a highly derived respiratory organ, namely the mammalian lung, can display functionally meaningful diversity. In terms of the metabolic spectrum of vertebrates in general, however, mammals are high-performance organisms. Where did lungs originate and how did the structural types that we see arise?