Gas Density Helium is one-seventh the density of nitrogen. With greater depth and pressure, gases compress and become more dense. At this depth, nitrogen would be seven times denser than at the surface. With each breathing cycle, the lungs pull in the atmosphere. Then, they stop the gas flow and turn it around, pushing the breathing gases back out.
Denser gases take greater effort to pull in, stop, and then push back out in each cycle.
The work required to inhale and to exhale increases markedly as gas density increases. At some point, increased pulmonary work causes problems. The complete path for metabolic oxygen uptake and carbon dioxide removal includes lung function and heart function. At normal atmospheric gas densities, heart function limits a person's maximum aerobic capacity. As gas density increases, lungs work harder and harder.