Partial pressure calculations
A partial pressure calculator shows how the total pressure of a gas mixture is distributed among its component gases. In physiology, this matters because oxygen, carbon dioxide, nitrogen, and other gases each contribute only a fraction of the total pressure, and that fraction determines the gas-specific partial pressure.
This topic is based on Dalton’s law of partial pressures. The key idea is that each gas behaves as if it alone occupied the space, contributing a pressure equal to its fractional concentration multiplied by the total pressure available to the dry gases.
Core definitions and formulas
The main relationship is:
\[
\begin{aligned}
P_{\text{gas}} &= F_{\text{gas}} \cdot P_{\text{total}}
\end{aligned}
\]
Here, \(P_{\text{gas}}\) is the partial pressure of the selected gas, \(F_{\text{gas}}\) is its fractional concentration, and \(P_{\text{total}}\) is the total pressure of the gas mixture.
If the gas fraction is unknown and the partial pressure is known, the equation can be rearranged:
\[
\begin{aligned}
F_{\text{gas}} &= \frac{P_{\text{gas}}}{P_{\text{total}}}
\end{aligned}
\]
For humidified inspired air, water vapor occupies part of the total pressure, so the effective dry-gas pressure becomes:
\[
\begin{aligned}
P_{\text{effective}} &= P_{\text{total}} - P_{H_2O}
\end{aligned}
\]
In that case, the selected gas partial pressure is calculated from the effective dry-gas pressure rather than the full total pressure.
How to interpret the result
A larger partial pressure means a larger pressure contribution from the selected gas. This usually occurs when the gas fraction is higher, when total pressure is higher, or both. A smaller partial pressure means the gas contributes less of the total mixture pressure.
The calculator outputs the partial pressure of the selected gas, the fractional concentration summary, the dry-versus-humidified comparison when enabled, and the step-by-step solution. In solve-for-fraction mode, it instead reports the gas fraction needed to produce the entered target partial pressure.
In physiology, the number matters because gas movement is driven by partial-pressure gradients, not by percentage alone. The same oxygen fraction can give a different oxygen partial pressure if total pressure changes, such as with altitude or humidification.
Common pitfalls
- Confusing gas percentage with gas partial pressure.
- Using a fraction as a whole number without converting it properly.
- Forgetting that humidified inspired gas reduces the dry-gas pressure available to oxygen and other gases.
- Assuming the same fraction always gives the same partial pressure at different altitudes.
Micro example: if oxygen fraction is 0.2093 and total pressure is 760 mmHg, then the oxygen partial pressure in dry atmospheric air is:
\[
\begin{aligned}
P_{O_2} &= 0.2093 \cdot 760 \\
&\approx 159\ \text{mmHg}
\end{aligned}
\]
This means oxygen contributes about 159 mmHg of the total dry atmospheric pressure.
This tool is best used for learning how gas fractions, total pressure, and humidification determine partial pressures in respiratory physiology. It is not a full alveolar gas or blood gas model by itself; the next step is to connect inspired partial pressures to alveolar gas equations, oxygen content, and ventilation/perfusion relationships.