Q.Low concentration of oxygen in the blood and tissues of people living at high altitude is due to ____________.

Concept: Henry's Law — the solubility of a gas in a liquid is directly proportional to its partial pressure above the liquid.

At high altitude, atmospheric pressure drops significantly. Since air is roughly 21% oxygen, the partial pressure of $\mathrm{O_2}$ also decreases proportionally. According to Henry's Law, lower partial pressure of oxygen means less oxygen dissolves into the blood at the alveolar–capillary interface in the lungs. …

Why altitude affects oxygen availability

Henry's Law: the solubility of a gas in a liquid is proportional to its partial pressure. When you climb a mountain, total atmospheric pressure falls - and with it, the partial pressure of oxygen in the air you breathe. Even though oxygen still makes up about 21% of the atmosphere at any altitude, the absolute partial pressure of oxygen is lower, so less oxygen dissolves into blood plasma and binds to hemoglobin - hypoxia.

Step-by-step reasoning

1. Atmospheric pressure decreases with altitude (roughly 1 atm at sea level, ~0.33 atm at Everest's summit).
2. Partial pressure of oxygen falls proportionally: $p_{O_2}=0.21\times p_{atm}$.
3. Henry's Law governs oxygen dissolution in blood - lower $p_{O_2}$ means lower dissolved oxygen. …

Concept: Partial Pressure of Oxygen & Altitude Physiology

The correct answer is (ii) low atmospheric pressure.

Method: Henry's Law & Dalton's Law of Partial Pressures

Why this method?

Oxygen moves from air into blood by diffusion, which depends on the partial pressure of oxygen ($P_{O_2}$) in the atmosphere — not just its percentage.

Step-by-step reasoning

1. Dalton's Law Total atmospheric pressure ($P_{total}$) is the sum of partial pressures of all gases.

At any altitude, the fraction of oxygen in air remains ~21%.

2. Effect of altitude on $P_{total}$ At sea level: $P_{total} \approx 760 \, \text{mm Hg}$ At high altitude (e.g., 5000 m): $P_{total} \approx 400 \, \text{mm Hg}$ So,

3. Henry's Law The amount of gas dissolved in blood is proportional to its partial pressure. …

Here is the breakdown of the common mistakes students make on this question, along with the correct reasoning.

The Correct Answer

The correct answer is (ii) low atmospheric pressure.

The Core Concept: Partial Pressure of Oxygen

The key to this question is understanding Henry's Law and Partial Pressure.

• Henry's Law: The amount of gas dissolved in a liquid (like blood) is directly proportional to the partial pressure of that gas in the atmosphere above the liquid.
• Partial Pressure: The pressure exerted by a single gas (like oxygen) in a mixture of gases (like air).

At high altitude, the total atmospheric pressure drops. Since oxygen makes up a constant ~21% of the air, the partial pressure of oxygen also drops significantly. This lower partial pressure means less oxygen is forced into the blood and tissues.

Common Mistakes & How to Avoid Them

Mistake 1: Choosing "Low Temperature" (Option A)

Why students choose this:

Students often think, "It's cold on a mountain, so cold air must have less oxygen." They confuse the density of air with the concentration of oxygen.

The Reality:

• Temperature affects the density of air, but the percentage of oxygen in the air remains ~21%.
• More importantly, cold temperature actually increases the solubility of gases in liquids. If temperature were the only factor, cold blood would hold more oxygen, not less.
• The primary reason for low oxygen in the blood is the low partial pressure, not the cold.

How to Avoid:

• Focus on the "why": Ask yourself: "What is the driving force that pushes oxygen into the blood?" The answer is pressure (specifically, the pressure gradient). Temperature is a secondary factor that affects solubility, not the primary driving force.
• Remember the context: The question asks about high altitude. The defining characteristic of high altitude is low atmospheric pressure, not just low temperature.

Mistake 2: Choosing "Both Low Temperature and High Atmospheric Pressure" (Option D)

Why students choose this:

This is a trap for students who are guessing or who haven't read the options carefully. They might think "high altitude = cold" and then incorrectly assume pressure must also be high.

The Reality:

• Atmospheric pressure decreases with altitude. It is never "high" at high altitude. This option contains a direct factual error.
• The combination of "low temperature" and "high pressure" is actually the condition for maximum oxygen solubility, which is the opposite of what happens at high altitude.

How to Avoid:

• Know the basic relationship: Altitude $\uparrow$ → Atmospheric Pressure $\downarrow$.
• Eliminate contradictions: As soon as you see "high atmospheric pressure" in an option about high altitude, you can immediately discard it. It violates a fundamental fact.

Mistake 3: Confusing "Atmospheric Pressure" with "Partial Pressure" …