Partial Pressure
பகுதி அழுத்தம்
Gas exchange is not magic; it is simple physics. Oxygen moves from the alveoli to the blood solely because of a pressure gradient. If you do not master partial pressure, you will fail the entire chapter on respiration.
The idea
Partial pressure is the pressure contributed by an individual gas in a mixture of gases. It is denoted as pO2 for oxygen and pCO2 for carbon dioxide. Gases always move from a region of higher partial pressure to a region of lower partial pressure. This gradient is the fundamental driver for gas exchange at the alveolar and tissue levels.
Why it matters
Gases move across membranes based on concentration gradients; partial pressure determines the direction and rate of gas diffusion.
NEET lens
NEET expects you to memorize the exact pO2 and pCO2 values for alveoli, oxygenated blood, deoxygenated blood, and tissues. Do not confuse the values of pO2 and pCO2 between oxygenated and deoxygenated blood. Questions often test your ability to predict the direction of gas diffusion based on these values. Memorize the table in the NCERT text; it is a high-yield area.
Clinical significance
High-altitude sickness is the direct result of low atmospheric partial pressure of oxygen. When pO2 drops, hemoglobin saturation decreases, leading to hypoxia. The body struggles to maintain cellular respiration, causing dizziness and fatigue.
Introduction
Partial pressure (Dalton's Law) is the pressure exerted by one component of a gas mixture — proportional to its mole fraction and total pressure. In physiology, pO₂ and pCO₂ drive gas exchange between alveoli, blood, and tissues.
Core concept
Dalton's Law: P_total = p₁ + p₂ + p₃... Atmospheric air: pO₂ = 159 mmHg (21% × 760 mmHg), pCO₂ = 0.3 mmHg (0.04%). Alveolar air: pO₂ = 104 mmHg (lower — diluted by water vapor, CO₂; O₂ consumed), pCO₂ = 40 mmHg. Oxygenated blood (pulmonary vein): pO₂ = 95 mmHg, pCO₂ = 40 mmHg. Tissues: pO₂ = 40 mmHg, pCO₂ = 45 mmHg. Gas always moves from HIGH to LOW partial pressure.
Go deeper
Why alveolar pO₂ < atmospheric? Three reasons: (1) Water vapor dilutes O₂ in warm humid alveoli. (2) CO₂ is added from blood → reduces O₂ fraction. (3) O₂ is continuously absorbed into blood → steady state pO₂ is 104, not 159 mmHg.
From Class 10
In a mixture of gases like air, each gas exerts its own pressure as if the others weren't there. This individual pressure is the partial pressure.