High O2 levels are desirable in the respiratory system because O2 transport ultimately occurs by passive diffusion down an O2 gradient. Ventilatory and cardiovascular pumps are used to deliver O2 by bulk flow in air or blood to large surface areas, where diffusion of O2 can occur. The actual interface between the environment and the body's internal milieu occurs deep in the lungs at the pulmonary blood-gas barrier. Figure 3A illustrates the branching pattern of airways used to deliver gas to the terminal lung units called alveoli. The trachea divides into two smaller primary bronchi to each lung, and the primary bronchi divide into still smaller bronchi to the different lobes of the lung. The pulmonary blood vessels undergo a similar branching pattern so the alveoli are virtually covered with pulmonary capillaries (Fig. 3B).
The effect of this branching is to increase greatly the surface area available for gas exchange by diffusion. If the lungs were two simple spheres of about 3 L each, they would have a total internal surface area of only 0.1 m2. However, the lungs branch more than 20 times, so the lungs contain more than 300 million alveoli of only about 300 ^m in diameter. The surface-to-volume ratio increases as the diameter of a sphere decreases, so dividing the large volume of the lung into millions of smaller units results in an extremely large area available for gas exchange. The total surface area of the lung's blood-gas barrier is 50-100 m2, or about the area of a tennis court!
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