Subject: Are there gases that can be safely breathed, but will not pass into blood.

During SCUBA diving larger quanties of inactive gases, normally 
nitrogen, pass into the blood stream because of the pressure and volume  
differences created by the increased volume of gas breathed resulting from 
the increased ambient pressures. Sometimes helium and hydrogen are used to 
replace a portion of nitrogen because the gases equalize quicker than 
nitrogen.  Can another gas, possibly with a molecular size large enough to 
prevent passage through the alveoli wall into the blood stream, be used to 
replace a portion of the nitrogen to reduce the amount of nitrogen absorbed 
by the blood stream.  If such a gas exists an application would 
involve a computerized system that would control the partial pressure and 
gas volumes to maintain the same balance of nitrogen and oxygen at depth as 
found at the surface, thereby eliminating the need for decompression.

Re: Are there gases that can be safely breathed, but will not pass into blood.