At sea level, a small amount of nitrogen from the atmosphere is dissolved in the bloodstream. Scuba diving equipment provides a flowing mixture of oxygen and nitrogen to the lungs. The mixture of gas is subject to the pressure exerted by the surrounding water. As divers descend, the pressure of the surrounding water increases. As this pressure increases, more and more of the nitrogen becomes dissolved in the blood.

The surrounding water pressure decreases as a diver ascends to the surface. Scuba divers ascend slowly, so that the lungs have enough time to remove the extra nitrogen from the bloodstream as the pressure decreases. If a diver ascends too rapidly, the nitrogen does not remain dissolved in the blood. Instead, it turns back into a gas, causing bubbles to form in the blood stream. The bubbles interfere with blood flow, which reduces the supply of oxygen to all organs.