The ‘pee factor’ in diving: When the urge hits, should you?

Most experienced divers don't think twice about peeing underwater, but there are those -- especially new divers - who are hesitant. Should you hold your urine underwater? Is there a downside to holding it? Does holding or emptying your bladder affect thermal status?

Once underwater, the urge to urinate increases. During a dive, there is about a 60 percent increase in the work of breathing. Pressure outside the chest wall is positive and at the end of breath expiration, internal lung pressure is less negative. Negative pressure breathing causes divers to lose about 350 cc/hour from their circulating blood volume.

The cardiovascular system changes. More blood returns to the heart due to increased abdominal pressure and decreased pooling in peripheral veins. Cold inhibits the natural anti-diuretic hormone, so peripheral blood vessels constrict, driving fluid back into the core and stimulating urine discharge. Diving increases carbon dioxide in the blood, which also decreases a natural antidiuretic hormone, promoting fluid loss from blood.

There is no increased central blood volume and output from the heart increases up to 30 percent. The result? Urine flow increases 4-5 times during a dive.

Even with all those physiological changes, there should be no problem emptying the bladder while diving -- if the person is wearing a wet suit. The odor will wash out if care is taken after diving.

Holding the urine in could possibly be harmful. There have been cases of fainting when the stretch receptors located in the wall of the bladder are stimulated and a vagal nerve reaction -- a decrease in heart rate, blood pressure, and a feeling of light-headedness occurs. Fainting underwater is risky to say the least.

In addition, why ruin a perfect sport by the sense of urgency that occurs as well as the distraction from multitasking?

The problem is different when wearing a dry suit. Men have a "pee valve". Women have to wear some absorbent shorts or diapers (Depends, for example).

Is there a change in thermal status? Loss of heat from the urine might be counteracted by the temporary heating of the wet suit. If using a dry suit, it would likely be a wash. But, to my knowledge, the topic has not been studied.

So, my best answer is you need to go ahead and pee even if it is against your sensibilities.

-- Ernest Campbell, MD, the Scuba Doc