PULMONARY GAS EXCHANGE

This chapter describes how the lungs perform their principal task — the uptake of O₂ from the air into the blood, and the simultaneous movement of CO₂ from the blood to the air. The unique structure of the lungs as 300 million separate alveoli (300 μm diameter gas-filled spaces whose walls are essentially made of capillary networks) facilitates this exchange through a linked series of transport functions that employ both convective and diffusive movements of gas. The alveoli lie at the distal ends of a complex branching network of hollow airways much as grapes connect to a stalk. The alveolar wall capillaries are fed by a similar, branching pulmonary arterial tree, and are drained by corresponding pulmonary veins. Ventilation (a convective process) brings O₂ from the air to the alveoli during inspiration, while during expiration the gas in the alveoli is moved back to the outside air by flow reversal through the same airways to eliminate CO₂. The two gases exchange between blood and air in the alveoli by diffusion. This avoids energy expenditure and accounts for the very large number of very small exchange units, a strategy that greatly increases surface area without also requiring a large lung volume. The third process is blood flow, again convective, that moves the blood out of the alveolar capillaries and back to the left heart for distribution to the tissues. These three transport functions are well-understood and can be modeled mathematically with remarkable accuracy using simple mass conservation principles. The lung is the only organ whose major function can be described and thus understood adequately in terms of such simple transport equations, as this chapter shows. More complete descriptions can be found in the references,^1–6 which are intended for further reading for those interested in additional details.