In addition to the changes due to the Physics of Dalton's Law, there are other physiological changes that take effect during shallow water blackout and free diving. The human body is capable of remarkable adaptations to the underwater environment. Even untrained divers will show a dramatic slowing of the heart when immersed. This is commonly referred to as the diving reflex. Immersion of the face in cold water causes the heart to slow automatically.
Chest compression can also slow the heart.
Untrained divers can experience up to a 40 percent drop in heart rate. Trained divers can produce an even lower heart rate some can slow to an incredible 20 beats per minute. Trained freedivers develop several other physiological adaptations that lead to deeper and longer dives. The spleen, acting as a blood reservoir, assists trained divers in increasing their performance. Apparently their spleen shrinks while diving, causing a release of extra blood cells.