There is a right shift of the HgbO2 curve. When the break point is reached , the chemoreceptors are stimulated by CO2, thus stimulation of respiration. Low O2 also stimulates respiration. On ascent the lungs re-expand reducing the favorable diffusion gradient for oxygen. Shallower depths cause this gradient to approach zero, the diver reaching a critical state of hypoxia.
Hypoxia causes unconsciousness, possibly before the diver reaches the surface.
Extreme weakness, trembling, unconsciousness in the water, amnesia of the event, drowning. 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.