Immersion Diuresis

The urge to urinate is common in diving, even among divers who don’t usually need to urinate frequently. The explanation is rooted in dive physiology.

The phenomenon is known as immersion diuresis, and it occurs whenever the body is immersed in water. Immersion, along with cool water temperature, causes narrowing of the blood vessels in the extremities. This vasoconstriction occurs primarily in the skin and superficial tissues of the body as well as in the muscles of the arms and legs. The result: An increased volume of blood is sent to the central organs of the body (the heart, lungs and large internal blood vessels).

The hormone that controls the production of urine by the kidneys is called antidiuretic hormone (ADH). It controls when and how much urine your kidneys make. The increased blood volume to the major vessels is interpreted by your body as a fluid overload. This overload causes ADH production to stop, which in turn allows the kidneys to immediately produce urine to lower the centrally circulating blood volume; this is the body’s automatic response to preserve blood volume.

Once you exit the water, circulating blood volume returns to near normal — less the fluid taken to produce
urine, which is quickly replaced as the body draws fluid from body tissues such as muscles. You will probably leave the water with a full bladder. We are all subject to this phenomenon underwater, but if this situation causes problems such as urinary tract infections, see your doctor. Drinking caffeinated beverages may promote this phenomenon, as caffeine is a diuretic and interferes with the production of ADH.