In diving, what primary physical principle explains why divers must ascend slowly?

The primary physical principle that explains why divers must ascend slowly is related to gas laws, particularly Henry's Law and the behavior of gases under pressure. As a diver descends, the pressure surrounding them increases, causing gases, especially nitrogen, to dissolve into the body’s tissues and blood due to the higher pressure.

When ascending, the pressure decreases, and the gases that were absorbed by the tissues become less soluble. If a diver ascends too quickly, these gases can form bubbles within the body, leading to decompression sickness, commonly known as "the bends." Slow ascension allows for a gradual decrease in pressure, giving the body time to safely eliminate the dissolved gases without forming harmful bubbles.

Understanding gas laws helps divers appreciate the risks associated with pressure changes and underscores the importance of controlled ascension to avoid potentially life-threatening conditions associated with rapid decompression.