During my swim, I suddenly realized I really needed to pee. It always happens, no matter how much I try to manage my coffee intake or use the restroom beforehand. My bladder doesn’t seem to care.
So, what’s going on here? “It’s a normal physiological reaction when you’re immersed in water,” explains Dr. Stavros Kavouras, a professor and director of the Hydration Science Lab at Arizona State University. And it’s not just me; “This is something that happens to all swimmers.”
Experts have some interesting insights about that unexpected urge to urinate when you’re surrounded by water.
What causes the urge to pee when you’re in the water?
The immediate need to urinate while submerged stems from something called immersion diuresis.
This term basically refers to how your body ramps up urine production thanks to your kidneys. When you’re in water, your body ends up filtering out excess fluid, which leads to more urination, according to Dr. Scott Trappe from Ball State University. It’s a hassle, quite uncomfortable really, but typically not dangerous—though it may occasionally risk dehydration.
Why does immersion diuresis occur?
When you enter the water, the cooler temperature causes your blood vessels to constrict, trying to conserve your core temperature. This shifts more fluid to your central body, as Trappe points out. (Pools are generally maintained at around 78-82°F, while lakes and rivers can be even chillier.)
Once the blood moves to the chest area, specialized receptors detect that increased volume, interpreting it as a fluid overload. They then signal your brain to reduce the production of antidiuretic hormone (ADH), which normally prompts the kidneys to hold onto urine, Kavouras explains.
Almost simultaneously, your heart cells release a hormone called atrial natriuretic factor (ANF) due to the high blood volume. This hormone widens blood vessels to lower blood pressure, thus prompting your body to excrete more water in order to restore fluid balance.
Essentially, your body mistakenly thinks it’s got too much fluid going on. “The kidneys recognize this increased volume and react by filtering out some of the excess water from the blood—prompting you to pee,” Trappe says.
“Both of these actions lead to increased urine production,” says Dr. Michael Joyner from the Mayo Clinic, who investigates how the nervous system reacts to stress. “This reflex helps prevent the heart from becoming overfilled with fluid.”
Besides the effects of water pressure and temperature, lying down in water also contributes to this response, Kavouras notes: “Blood flows more easily toward your heart.”
Is that the reason I feel this urge much less often while running? Yes, Kavouras confirms. But also, during swimming, you sweat less, so you’re not losing fluids that way. When you run, you sweat more, which makes it less likely your body thinks it’s retaining excess fluid.
So, can you avoid immersion diuresis? “Not really,” Trappe admits. “It’s just part of being a swimmer.”
So are people just peeing in the pool all the time?
Whenever I feel that urge, it’s definitely frustrating. I could either end my workout, risk losing my lane by taking a break, or hold it until I finish my laps. The thought of just giving in and peeing in the pool seems childish—definitely not a great idea if I want to keep swimming there. So I usually just hang in there.
Many other swimmers, though, seem unfazed. “Nobody talks about it, but it’s pretty common,” says Trappe.
Kavouras, a former competitive swimmer himself, backs this up. “It’s been an unspoken part of swimming culture for ages,” he adds, although he’s not revealing whether he ever participated.
Just for fun, I asked my son, now in his 30s and once an avid swimmer from age five into his twenties, if he ever did it. “Absolutely, all the time,” he replied. “After a few laps, I would always end up peeing on the swimmer behind me.”
The swimmer he mentioned didn’t have a chance to respond.
