The Uptake of Inert Gases in Diving

where Pabs is the absolute pressure and FtN2 is the fractional concentration of N2. If a diver, breathing 20% oxygen and 80% nitrogen, swims down to 30 MSW the inspired PN2 (PiN2) is calculated from equation 1 as: PiN2 = 4 bars x 0.8 = 3.2 bars. The lung alveolar PN2 (PAN2) will be slightly lower than this, but for the sake of simplicity we will assume that it also is 3.2 bars. The venous blood being brought to the lungs will have a PN2 (PvN2) of 0.8 bars as it will be in equilibrium with the tissues and N2 will diffuse from the alveoli into the blood. Equilibrium is quickly reached such that the arterial blood leaving the lung has a PN2 (PaN2) of 3.2 bars. Although some venous blood by-passes the lungs (a venous admixture or right to left shunt), again for simplicity it is assumed that the arterial blood entering the aorta from the left ventricle has a PN2 (PaN2) of 3.2 bars. It can be seen that ventilation is assumed to not be rate-limiting in the body's uptake of inert gases during diving. The blood that reaches the tissues has a PN2 of 3.2 bars, and nitrogen will diffuse into the tissues, which have an initial PN2 (PtN2) of 0.8 bar, until such time as the PtN2 also reaches 3.2 bars (equilibrium or saturation). The factors which determine how quickly the tissues reach equilibrium are: