The air surrounding the Earth is a dynamic mixture of gases, each contributing to the total atmospheric pressure. This pressure is the force exerted by the weight of the air column above a given point. To understand how oxygen sustains life, scientists isolate its specific contribution, known as the partial pressure of oxygen. Determining this value in atmospheric air is essential for understanding respiration.
Defining Atmospheric Pressure and Air Composition
Scientists define standard atmospheric pressure at mean sea level as one atmosphere (atm). This pressure equals 760 millimeters of mercury (\(\text{mmHg}\)), a unit often used in physiology. This measurement represents the total force exerted by all gas molecules in the air.
The composition of dry atmospheric air is remarkably consistent. Nitrogen accounts for approximately \(78.1\%\) of the volume, while oxygen is the second most abundant gas, making up about \(20.95\%\). The remaining fraction includes argon at about \(0.93\%\), with trace amounts of other gases.
The Calculation of Oxygen’s Partial Pressure
The specific pressure exerted by oxygen is calculated using Dalton’s Law of Partial Pressures. This law states that the total pressure of a gas mixture is the sum of the individual pressures of each component gas. To find the partial pressure of any gas, one must multiply its fractional concentration by the total atmospheric pressure.
Using the standard sea-level pressure of \(760 \text{ mmHg}\) and the oxygen concentration of \(20.9\%\), the calculation yields the partial pressure of oxygen (\(\text{P}_{\text{O}_2}\)). Multiplying \(760 \text{ mmHg}\) by \(0.209\) results in approximately \(159 \text{ mmHg}\). This value represents the maximum driving force for oxygen molecules to move into the lungs.
Oxygen Pressure in the Lungs and Bloodstream
The oxygen pressure changes significantly once atmospheric air is inhaled and reaches the lungs. The first major drop in \(\text{P}_{\text{O}_2}\) occurs because the inspired air is warmed and humidified by the upper airways. At body temperature, water vapor exerts a partial pressure of about \(47 \text{ mmHg}\), which dilutes the oxygen.
This humidification reduces the inspired \(\text{P}_{\text{O}_2}\) from \(159 \text{ mmHg}\) to approximately \(149 \text{ mmHg}\). A further reduction takes place within the alveoli, the tiny air sacs where gas exchange occurs. The continuous transfer of \(\text{CO}_2\) from the blood into the alveoli further lowers the available \(\text{P}_{\text{O}_2}\).
The resulting alveolar \(\text{P}_{\text{O}_2}\) is typically maintained around \(100\) to \(105 \text{ mmHg}\) at sea level. This pressure gradient between the alveolar air and the deoxygenated blood drives the diffusion of oxygen. Oxygen moves rapidly from the higher pressure alveoli into the blood until equilibrium is reached.
The oxygenated arterial blood leaving the lungs carries a partial pressure (\(\text{P}_{\text{a}\text{O}_2}\)) between \(95\) and \(100 \text{ mmHg}\). This gradient ensures oxygen diffuses from the blood into the tissues, which have a lower \(\text{P}_{\text{O}_2}\) due to cellular consumption.
The Impact of Altitude on Oxygen Availability
While the relative concentration of oxygen remains nearly \(20.9\%\) up to very high altitudes, the total atmospheric pressure decreases as elevation increases. This drop in total pressure is the primary reason why oxygen availability diminishes at higher elevations. A lower total pressure means the partial pressure of every gas in the mixture, including oxygen, is also lower.
For instance, at an altitude of approximately \(18,000 \text{ feet}\) (about \(5,500\) meters), the total atmospheric pressure is roughly halved compared to sea level. This means the \(\text{P}_{\text{O}_2}\) in the inspired air is also reduced by half, falling to about \(79 \text{ mmHg}\). This reduced driving pressure means fewer oxygen molecules are available to diffuse into the blood.
The physiological result of this drop is hypobaric hypoxia, a condition where the body struggles to maintain adequate oxygen delivery. The alveolar \(\text{P}_{\text{O}_2}\) falls dramatically, reducing the arterial \(\text{P}_{\text{a}\text{O}_2}\) and impairing tissue oxygenation. The reduced partial pressure of oxygen remains the limiting factor for human survival at extreme altitudes.