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Alveolar Air Composition

Overview
  • The composition of alveolar air is determined by the balance between two basic processes. The first involves the refreshing of air in the alveolar space with that of fresh external air due to alveolar ventilation. The second involves exchange of gases with blood perfusing the pulmonary capillaries, and includes carbon dioxide diffusing out of the blood and into the alveolar space in conjunction with oxygen diffusing out of the alveolar space and into the blood. Thus the composition of alveolar air is ultimately determined by the balance between the ventilation and perfusion of the alveolus. Below we engage in a qualitative discussion of how ventilation and perfusion modify the composition of alveolar air and subsequently endeavor to provide an intuitive physical analogy that we hope readers will find helpful.
Contribution of Alveolar Ventilation
  • Inspiration results in oxygen-rich external air entering the alveolus whereas expiration pushes carbon dioxide-rich air out of the alveolus. This process is known as alveolar ventilation; however, in reality, ventilation of the alveoli does not occur simply by bulk entry and exit of air into and out of the alveolar space. Rather, the mammalian lung is structured in such a way that bulk airflow ends at the terminal bronchioles; consequently, upon inspiration fresh external air moves only up to the point of the terminal bronchiole and upon expiration, only air within the terminal bronchiole and larger bronchi is actually expelled from the lung. This arises from the fact that the terminal bronchioles rapidly branch into numerous alveoli, resulting in a near-immediate rise in the cross-sectional area of the airway.
  • A general rule of airflow is that the speed at which air travels is inversely proportional to the cross-sectional area of the airway; therefore, if the airway cross-sectional area suddenly expands, the speed of airflow suddenly drops, and in the case of the terminal bronchiole-alveoli transition point, approaches zero. Given the fact that bulk flow of air ends at the terminal bronchiole, one may ask how air within the alveoli is actually refreshed. Although coordinated unidirectional motion of a population of inspired air molecules, representing bulk airflow, may end at the terminal bronchiole, individual external air molecules can still enter the alveoli through random thermodynamic diffusion. Similarly, carbon dioxide molecules within the alveoli can diffuse into the terminal bronchioles and be expelled by bulk flow outward from that point. These diffusing processes ultimately form the basis of how ventilation can modify the composition of alveolar air.
  • When ventilation is increased, the rate at which fresh oxygen rich external air is delivered to the terminal bronchioles rises and the rate at which carbon dioxide-rich air within the alveolus is expelled from the terminal bronchioles also rises. Therefore, increased levels of alveolar ventilation modify the composition of alveolar air such that the oxygen tension rises and the carbon dioxide tension declines whereas lower levels of alveolar ventilation reduce the oxygen tension and increase the carbon dioxide tension within the alveolus.
Air Exchange Within the Alveolus
Bulk flow of air during inspiration ends at the border of the respiratory bronchiole and the alveolus due to the enormous expansion of surface area at this transition point. Consequently, exchange of gas between the alveolus and the respiratory bronchiole occurs purely by random diffusion. The same is true during expiration.

Contribution of Perfusion
  • The blood entering the pulmonary capillaries is rich in carbon dioxide and relatively poor in oxygen because it is derived from venous blood that has traveled through the peripheral tissues. As blood perfuses the pulmonary capillaries, carbon dioxide diffuses out of the capillaries and into the alveoli whereas oxygen diffuses from the alveoli and into the capillaries. Consequently, pulmonary perfusion plays an important role in determining the composition of alveolar air as it controls the rate at which carbon dioxide is added to the alveolar space and the rate at which oxygen is carried away.
Physical Determinants of Alveolar Air Composition
The composition of alveolar air is determined by the balance between two basic processes: Alveolar Ventilation and Perfusion. Alveolar Ventilation refreshes the alveolar air, increasing the oxygen tension while reducing the carbon dioxide tension. Conversely, perfusion of alveoli brings carbon dioxide transported from the periphery and carries away oxygen. The balance between alveolar ventilation and perfusion ultimately determine the partial pressures of oxygen and carbon dioxide within the alveolus and in turn the arterial blood.

Physical Analogy
  • Let us imagine a door-less igloo with a fire burning inside of it which consumes oxygen and releases carbon dioxide. Because of igloo does not possess a door, the only way that the air can be refreshed inside the structure is through a central rooftop chimney at the outlet of which has been a placed a pump. This pump gently gently pushes fresh external air into chimney for a few seconds and then pumps air out of the chimney for several seconds in a cyclical pattern. Importantly, the capacity of the pump is quite limited and with each cycle it can only suck out the volume of air within the chimney, not the entire igloo, and replace it with fresh external air. Once again, this is analogous to the lung's ventilation capacity which can push and pull air only to the level of the terminal bronchioles. Therefore, the air within the igloo can only be refreshed by diffusion of oxygen-rich air from the chimney when the pump pushes external air inward, followed by diffusion of carbon dioxide-rich air into the chimney during the period that the pump expels air from the chimney.
  • It should be clear that the composition of the air within the igloo is essentially dependent on two processes. The first is the rate at which the pump atop the chimney cycles, analogous to the rate of alveolar ventilation. If the pump cycles quickly, then the rate at which fresh external air is introduced into the chimney during the inward phase of pump's cycle rises as well as the rate at which carbon dioxide-rich air within the chimney is expelled during the outward phase of the pump's cycle. Consequently, at high rates of pump cycling, the composition of the igloo's air will become more similar to the fresh external air.
  • The second process that determines the composition of the igloo's air is the size of the fire, which consumes oxygen and produces carbon dioxide. If the fire expands, more carbon dioxide is produced and more oxygen is consumed; consequently, the composition of the air within the igloo will become more carbon dioxide-rich and oxygen-poor. The size of the fire is analagous to the perfusion rate of the alveoli, which introduces carbon dioxide into the alveolus and carries away oxygen. The igloo example is instructive because major changes to the pump's action or size of the fire will lead to predictable changes to the composition of igloo air. If the pump breaks then the igloo will become full of concentrated carbon dioxide, whereas if the fire is extinguished and the pump continues to operate, the composition of air within the igloo will eventually approach that of external air. The reader may want to keep the igloo analogy in mind when considering the rest of our discussion of topics within ventilation-perfusion Relationships.