Atmospheric pressure is the force exerted by the column of gas pressing down on the Earth’s surface. At standard sea level, the total pressure is defined as one atmosphere (atm), equivalent to 760 millimeters of mercury (mmHg) or 101.3 kilopascals (kPa). This total pressure results from the combination of all individual gases, including nitrogen, oxygen, and trace gases. The precise partial pressure of oxygen (\(P_{O_2}\)) at standard sea level is approximately 159 to 160 mmHg, or 21.2 kPa.
Understanding Partial Pressure
Air is a mixture composed mainly of nitrogen (about 78.1%) and oxygen (about 20.95%). Dalton’s Law of Partial Pressures governs how these individual gases contribute to the total pressure. This law states that the overall pressure of a gas mixture is the sum of the pressures that each gas would exert if it were the only gas present.
The partial pressure of a gas, such as oxygen, is determined by its fractional concentration within the total air volume. This means the pressure exerted by the oxygen molecules acts independently of the pressure exerted by the nitrogen molecules.
The proportion of oxygen molecules in the air directly dictates its share of the total atmospheric pressure. A change in the total atmospheric pressure, such as moving to a higher altitude, will proportionally change the partial pressure of every gas, including oxygen.
Determining the Oxygen Pressure at Sea Level
Determining the partial pressure of oxygen at sea level requires a straightforward calculation. Standard atmospheric pressure is 760 mmHg. The standardized fraction of oxygen in dry atmospheric air is approximately 20.95% (0.2095).
The partial pressure of oxygen (\(P_{O_2}\)) is found by multiplying the total atmospheric pressure by the fractional concentration of oxygen. The calculation is \(760 \text{ mmHg} \times 0.2095\), which equals approximately 159.22 mmHg. This value is commonly cited as 159 to 160 mmHg.
The remaining pressure is distributed among the other gases. For example, nitrogen makes up about 78.1% of the atmosphere, contributing a partial pressure of roughly 593 mmHg. Argon contributes about 0.93% of the total pressure, with other trace gases accounting for the remainder.
Why This Number Matters for Breathing
The atmospheric \(P_{O_2}\) of 160 mmHg is the initial driving force behind human respiration. Gas exchange relies on a pressure gradient, causing gases to move from higher pressure to lower pressure areas. For oxygen to enter the bloodstream, its partial pressure in the air must exceed the pressure inside the lungs and blood.
As air is inhaled, it enters the moist respiratory system and mixes with water vapor and carbon dioxide within the alveoli. Water vapor exerts its own pressure (about 47 mmHg at body temperature), which effectively dilutes all other gases in the lung air. This dilution causes the partial pressure of oxygen to drop significantly from the atmospheric 160 mmHg to an alveolar \(P_{O_2}\) of approximately 100 mmHg.
This pressure difference—from the alveolar \(P_{O_2}\) of 100 mmHg to the lower oxygen pressure in the returning blood—facilitates the movement of oxygen into the circulation. The sea level \(P_{O_2}\) of 160 mmHg provides a robust initial gradient. This ensures that sufficient oxygen pressure remains, even after the drop in the lungs, to drive the gas exchange necessary to sustain life.