In human physiology and diving medicine, helium is added to the oxygen supply of deep sea divers because, compared with nitrogen, helium has which important property when breathed at high pressure under water?

Introduction / Context:
Deep sea divers breathe compressed gas mixtures at high pressures. At such depths, the behaviour of gases like nitrogen in the body changes and can lead to serious health problems, such as nitrogen narcosis and decompression sickness, also called the bends. To reduce these risks, helium is often added to the oxygen supply to form heliox or similar breathing mixtures. This question tests your understanding of why helium, rather than nitrogen, is preferred in deep diving breathing gases from a physiological and physical point of view.

Given Data / Assumptions:

• Divers breathe gases at pressures significantly higher than normal atmospheric pressure.
• Nitrogen is normally a major component of air, along with oxygen.
• At high pressure, nitrogen dissolves more in blood and body tissues.
• Helium is an inert noble gas with different physical properties from nitrogen.
• The question is asking for the key advantage of helium over nitrogen at high pressure.

Concept / Approach:
The solubility of a gas in blood and tissues increases with pressure according to principles similar to Henry law. When divers stay at depth breathing compressed air, more nitrogen dissolves into their blood and tissues. If they ascend too quickly, this dissolved nitrogen can come out of solution as bubbles, causing decompression sickness. Nitrogen can also cause narcotic effects at high partial pressures, leading to confusion and impaired judgement. Helium is much less soluble in blood and tissues at high pressure and is physiologically inert in this context. By replacing much of the nitrogen with helium, divers inhale a gas mixture that reduces nitrogen narcosis and lowers the risk of bubble formation during decompression, making deep diving safer when proper procedures are followed.

Step-by-Step Solution:
Step 1: Recall that ordinary air is about 78 percent nitrogen and 21 percent oxygen, and that nitrogen is normally harmless at surface pressure. Step 2: Understand that at high pressures under water, nitrogen dissolves into the diver's body fluids and tissues in larger amounts, and it can have narcotic effects on the nervous system. Step 3: When a diver ascends, the pressure decreases and dissolved nitrogen can form bubbles if it is released too rapidly, leading to decompression sickness. Step 4: Helium is an inert gas that is less soluble in blood and tissues at high pressure compared to nitrogen and does not have the same narcotic properties. Step 5: Therefore, by replacing nitrogen with helium in the breathing mixture, the amount of gas dissolved in the body at depth is reduced, and the risk of bubble formation and narcosis is lower. This matches the statement that helium is less soluble in blood than nitrogen at high pressure and helps to prevent decompression problems.

Verification / Alternative check:
Diving medicine texts and technical diving manuals explain the use of heliox (helium oxygen mixtures) and trimix (helium, nitrogen and oxygen mixtures) for deep dives. They highlight helium's low narcotic potential and lower solubility compared to nitrogen. They also discuss how helium based mixtures allow divers to function more clearly at depth and make decompression procedures more manageable. While helium has some disadvantages, such as increased heat loss and expense, its physiological behaviour under pressure makes it preferable to nitrogen for deep diving. These sources do not emphasise any special chemical reaction with oxygen or simple lightness as the main reasons for its use, confirming that reduced solubility and reduced narcosis are the key factors.

Why Other Options Are Wrong:
Option a, that helium is less poisonous than nitrogen, is misleading because nitrogen is not considered poisonous at normal pressures. The problem is not simple poisoning but pressure related effects and solubility.

Option b, that helium is lighter than nitrogen and therefore easier to carry, is not correct in this context. Gas cylinders are filled under pressure and the difference in weight between nitrogen and helium does not play a significant role in diving gas selection.

Option c, that helium mixes readily with oxygen and improves its combustion, is irrelevant, because there is no combustion in the diver's lungs. Helium is inert and does not burn; its role is not related to combustion.

Common Pitfalls:
Some learners think only in terms of simple ideas like being lighter or less poisonous without considering how gas solubility and pressure affect the body. Others mistakenly assume that any inert gas would be the same as nitrogen under pressure. Remember that the key differences are the narcotic effect and the degree of solubility in blood and tissues at high pressure. Linking this question to the concepts of nitrogen narcosis and decompression sickness helps to understand why helium is used in deep diving mixtures and to choose the correct option confidently.

Final Answer:
Helium is added because it is less soluble in blood than nitrogen at high pressure and therefore reduces decompression problems for deep sea divers.