Henry's Law is of central importance to scuba divers. It states that the amount of a gas that will dissolve into a liquid is directly proportional to the gas's partial pressure. Prior to making his first dive of the day, the partial pressure of each gas dissolved in a diver's bodily fluids (tissue pressure) is equal to the ambient partial pressure of each gas. At this state, the diver's body is saturated; that is, the rates of gas absorption and release are equal. When the diver submerges, his regulator's first stage balances the pressure of the his breathing mixture with the ambient water pressure. (Of course, this balancing act is absolutely necessary. Without it the water pressure would increasingly squeeze the diver's chest and air passages with increasing depth.) Thus a gas pressure gradient is established across the capillary membranes lining the alveoli of the diver's lungs, and the rate of gas absorption exceeds the rate of release across this boundary. The surplus nitrogen and oxygen (the primary constituents of air) readily dissolve into blood plasma and get whisked away by the circulatory system to be absorbed by the diver's various body tissues. If the diver were to stay at a constant depth long enough, his body would again become saturated.
While the body's tissues simply metabolize the excess oxygen, the excess nitrogen remains inert. If the diver slowly ascends to a shallower depth, the pressure gradient in the diver's body reverses, the tissues become slightly supersaturated, and the entire process runs smoothly in reverse: the excess nitrogen remains in solution while the circulatory system transports it back to the capillary-alveoli interface where it released into the lungs and exhaled normally.
The nitrogen elimination process occurs less smoothly if the diver's body contains a substantial amount of dissolved nitrogen (i.e. he has been at a great enough depth for a long enough period of time) and the diver ascends too quickly. If the pressure gradient is great enough upon ascent, microscopic bubbles of the gas phase will form on the surfaces of supersaturated tissues. These bubbles grow until they break free from the tissue's surface and are carried away by the circulatory system. Gas phase production within the body can afflict the diver with any of a host of ailments grouped under the term decompression sickness (a.k.a. "the bends").
Cases of decompression sickness range in severity from skin rashes and joint pain to paralysis and death. Symptoms depend on where in the "bent" diver's body the gas phase develops and where the bubbles end up after they break free and are circulated. Itchy rashes develop if bubbles form in skin capillaries. Joint pain is a particularly common symptom, and is thought to occur when bubbles form in connective tissues and in muscles surrounding the joint. Bubbles born in nervous tissue have the potential to paralyze the victim and cease neural transmission to the heart and lungs. If bubbles are of sufficient size and number, they may congest arteries leading to the spinal cord (causing paralysis) or brain (causing a stroke). If a large number of bubbles happen upon the lungs at once, they may overload the alveoli or block capillaries leading to the alveoli, restricting blood flow to the lungs which, aside from starving the victim of air, prevents blood intake by the heart. This restriction elevates the victim's heart rate, lowers his blood pressure, and can cause total heart failure.
