Helium is used in deep-sea diving, not for respiration itself, but to replace nitrogen in the breathing gas mixture. This is done to prevent a serious condition called nitrogen narcosis.

Here's why:

* Nitrogen Narcosis: At increasing depths, the pressure of the surrounding water increases. This forces more nitrogen into the bloodstream. Nitrogen becomes a narcotic under pressure, leading to symptoms like confusion, euphoria, and impaired judgment - all highly dangerous in a diving situation.

* Helium as a Solution: Helium is much less soluble in blood than nitrogen, and it doesn't have the narcotic effect. Replacing nitrogen with helium in the breathing gas mixture prevents the build-up of nitrogen in the bloodstream, avoiding narcosis.

Important Note: Divers still need oxygen to breathe, so helium is mixed with oxygen, not used as a replacement.

Therefore, helium is used in deep-sea diving to prevent nitrogen narcosis, not as a direct replacement for respiration.