I always thought the urge to urinate during a dive came from the pressure of the water on your body and your bladder. Instead, according to DAN* (Divers Alert Network), this has to do with 'immersion diuresis'. An expensive word I came accross during a scuba diver quiz. After I got the question wrong about I was curious about the real reason why you always need to urinate just when you start your dive.
In layman words the term 'immersion diuresis' comes down to the fact that your body is immersed in water, colder than air temperature. This sets in a chain reaction in your body, ending with you having a full bladder. I'm going to try to describe the proces** without using any scientific words.
The water temperature, colder than air, causes a narrowing of the blood vessels. This primarily occurs in the skin, superficial tissues of the body and the muscles of the arms and legs. This results in an increase of blood volume to the central organs such as the heart and lungs. 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 - the body's automatic response to preserve blood volume.ADH hormone is responsible for the production of urine in the kidney's. 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. Unfortunately, you probably will also leave the water with a full bladder.
*The largest association of recreational scuba divers in the world, DAN is supported by membership dues and donations. DAN's mission is to help divers in need of medical emergency assistance and to promote dive safety through research,education, products and diving services. The benefits of DAN membership include emergency medical evacuation assistance through DAN TravelAssist, a subscription to Alert Diver magazine and access to DAN's insurance services.