Question

Which gas has higher solubility in blood?

• A. Oxygen
• B. Nitrogen
• C. Carbon dioxide
• D. Hydrogen

Hint:

High solubility of \(CO_2\) is the reason why it can be transported in significant amounts in the dissolved state in plasma compared to oxygen.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Solubility is the physical property of a gas to dissolve in a liquid (blood or plasma) at a constant temperature and pressure. It determines the rate of diffusion across the respiratory membrane.

Step 2: Detailed Explanation:
The solubility of carbon dioxide (\(CO_2\)) is much higher than that of oxygen (\(O_2\)).
In physiological conditions, \(CO_2\) is approximately \(20\) to \(25\) times more soluble in blood and plasma than oxygen.
Because of this high solubility, \(CO_2\) can diffuse across the alveolar-capillary membrane much more efficiently than oxygen, even though the partial pressure gradient for \(CO_2\) (\(5\) mmHg) is much smaller than for \(O_2\) (\(64\) mmHg).
Nitrogen and Hydrogen have very low solubility in blood at normal physiological pressures.

Step 3: Final Answer:
Among the options, Carbon dioxide has the highest solubility in blood.

Answer 2

This question can be resolved by looking at how each gas actually behaves once it reaches the blood, rather than by simply ranking solubility values, examining the chemical fate of each gas in plasma one at a time.

1. Oxygen: Oxygen is a small nonpolar diatomic molecule with weak interaction with water molecules, so its physical solubility in plasma is low. Most oxygen transport happens because it binds reversibly to hemoglobin inside red blood cells, not because it dissolves well in the liquid plasma itself, so oxygen cannot be the most soluble gas.
2. Nitrogen: Nitrogen is chemically inert and nonpolar, so it neither reacts with plasma components nor forms any hydrated species. It stays dissolved only in the small amount predicted by Henry's law at normal pressure, which is why nitrogen bubbling out of solution during rapid pressure changes, as in decompression sickness, is a well known problem, confirming nitrogen has low, not high, solubility.
3. Carbon dioxide: Carbon dioxide is unusual because a large fraction of it does not stay as a simple dissolved gas molecule. Once inside plasma and red blood cells it combines with water, helped along by the enzyme carbonic anhydrase, to form carbonic acid, which then dissociates into bicarbonate and hydrogen ions. This reaction continuously removes free dissolved carbon dioxide and pulls more gas into solution, so the effective solubility of carbon dioxide in blood ends up far higher than a simple nonpolar gas like oxygen or nitrogen, even though its partial pressure gradient across the alveolar membrane is small.
4. Hydrogen: Hydrogen is an extremely small nonpolar molecule with essentially no chemical affinity for plasma proteins or water, so it behaves like nitrogen, dissolving only sparingly and playing no meaningful role in normal blood gas physiology.

Comparing all four gases this way, only carbon dioxide undergoes a chemical transformation in blood that keeps drawing more of it into solution, which is what gives it its unusually high solubility compared to oxygen, nitrogen and hydrogen.

So the correct answer is Carbon dioxide.