Inspired air, drawn into the respiratory system, differs significantly from alveolar air, found within the tiny air sacs where gas exchange occurs. Understanding these differences is central to how our bodies efficiently take in oxygen and expel carbon dioxide.
The Air We Breathe In
Atmospheric, or inspired, air is a mixture of gases with relatively stable percentages. Nitrogen constitutes the largest portion, typically around 78% of the air volume. Oxygen follows, making up about 20.9% of the inspired air. Argon and other noble gases are present in trace amounts, approximately 0.9%. Carbon dioxide is a minor component, usually around 0.04%. The amount of water vapor in inspired air is variable, depending on environmental humidity, but is generally low compared to other gases.
Air Inside Your Lungs
Alveolar air contains a lower concentration of oxygen, typically around 13.7% to 14%. Conversely, the carbon dioxide concentration is significantly higher, ranging from about 5.2% to 6%. Nitrogen levels remain largely consistent with inspired air, at approximately 74.9% to 80%. Water vapor content in alveolar air is also considerably higher, as the air becomes fully saturated with water during its journey through the respiratory tract.
How Air Changes in the Lungs
The transformation of inspired air into alveolar air involves several physiological processes within the respiratory system. As air travels through the nasal passages, pharynx, trachea, and bronchi, it undergoes humidification. The moist lining of the airways adds water vapor to the inhaled air, ensuring it becomes nearly 100% saturated with water at body temperature (37°C) by the time it reaches the alveoli. This humidification increases the partial pressure of water vapor, effectively diluting the other gases present.
Gas exchange is a primary mechanism altering air composition in the lungs. In the alveoli, oxygen moves from the air sacs into the bloodstream, while carbon dioxide moves from the blood into the alveoli. This movement occurs due to differences in partial pressures, where each gas travels from an area of higher concentration to an area of lower concentration. For instance, oxygen’s partial pressure is higher in the alveoli (around 104 mm Hg) than in the pulmonary capillaries (around 40 mm Hg), driving its diffusion into the blood. Conversely, carbon dioxide’s partial pressure is higher in the blood (around 45 mm Hg) than in the alveoli (around 40 mm Hg), causing it to diffuse out of the blood and into the air sacs.
Another contributing factor is the mixing of fresh inspired air with the air already present in the lungs. Not all air is exhaled during a breath; a significant volume, known as residual volume (approximately 1.1 to 1.2 liters), always remains in the lungs, preventing them from collapsing. Additionally, air in the anatomical dead space (airways where no gas exchange occurs) also mixes with incoming fresh air. This mixing dilutes the oxygen content of the incoming air and increases its carbon dioxide content, as the residual air has already undergone gas exchange. This continuous mixing ensures a more stable alveolar gas composition, even with fluctuations in breathing patterns.
Why These Differences Matter
The composition of alveolar air enables efficient gas exchange and maintains bodily functions. The higher concentration of carbon dioxide and lower concentration of oxygen in alveolar air, compared to inspired air, creates the necessary partial pressure gradients that drive the diffusion of these gases. This ensures continuous and effective oxygen transfer into the bloodstream, supporting cellular respiration and energy production. Simultaneously, it facilitates the removal of carbon dioxide, a waste product of metabolism, from the blood into the alveoli for exhalation. This balance of gases in the alveoli regulates blood pH and supports physiological stability.