I’m midway into my hour-long swim when it hits: I really have to pee. This always happens. It doesn’t help to curb my morning coffee or use the restroom beforehand. My bladder doesn’t care.

Why does this happen? “It’s a normal physiological response by the body to being immersed in water,” says Dr Stavros Kavouras, assistant dean, professor of nutrition and director of the Hydration Science Lab at Arizona State University. And it’s not just me: “It’s something that happens to all swimmers.”

Here’s what experts have to say about that inexplicable urge to pee when you’re surrounded by water.

What causes the urge to pee when you’re in the water?

The strong sudden need to urinate when submerged in water results from a process called immersion diuresis.

Diuresis is a medical term that refers to the increased production and excretion of urine by the kidneys. During this process, the body filters excess bodily fluid, water and waste products from the bloodstream and expels them through urination, according to Dr Scott Trappe, director of the Human Performance Laboratory at Ball State University.

It’s annoying, inconvenient and uncomfortable – but not usually dangerous, although sometimes it can lead to dehydration, he says.

Why does immersion diuresis occur?

Contrary to common misconception, you aren’t absorbing water from the pool through your skin while swimming.

When your body enters the water, “the relatively cooler water will cause the [blood] vessels in the skin to constrict to conserve core body temperature,” Trappe says. “This sends more fluid centrally.” (Most pools are heated to about 78-82F, according to the US Department of Energy; lakes and other bodies of water can be even cooler.)

After the blood moves to the chest, special cardiopulmonary receptors detect the volume increase, interpreting it as fluid overload. They signal the brain to halt production of antidiuretic hormone (ADH), a substance that tells the kidneys to retain urine, Kavouras says.

Almost simultaneously, your heart muscle cells also secrete a hormone called atrial natriuretic factor (ANF) in response to high blood volume. ANF widens blood vessels to reduce blood pressure, which then increases water excretion in an effort to restore fluid balance.

Put simply: your body thinks it has too much fluid. “The kidneys sense an increased fluid volume and balance this out by pulling some of the water from the blood – and you [then] have to pee,” says Trappe.

“Both of these factors make the kidneys produce more urine,” says Dr Michael Joyner, an anesthesiologist and researcher at the Mayo Clinic, who studies how the nervous system regulates blood pressure, heart rate and metabolism in response to stress. “This is a natural reflex that keeps the heart from getting overfilled with fluid.”

In addition to water pressure and water temperature, being prone in the water also plays a role, Kavouras says: “The blood more easily goes to your heart.”

Is that why the urge to pee happens far less often when, say, I go running? Yes, says Kavouras, but you also sweat less in the water, so you don’t lose as much fluid that way when swimming. Running causes more sweating, so the body doesn’t feel like it’s holding too much fluid.

So, is there any way to prevent immersion diuresis? “Not really,” Trappe says. “It’s all part of being a swimmer.”

So are people just peeing in the pool all the time?

When I get the urge, I am annoyed. I could quit my workout early or take a break and risk losing my pool lane while I’m gone. I could hold it, or give in to my inner toddler and pee in the pool – probably not a good idea if I want to keep swimming here. So I just hold on until my laps are done.

Other swimmers apparently have no such inhibitions. They just let go in the water. “Nobody talks about it, but everybody does it,” Trappe says.

Kavouras, a former competitive swimmer, agrees. “It’s long been part of swimming culture that swimmers pee in the pool,” he says. “But I’m not going to answer the question of whether or not I ever did it.”

Out of curiosity, I raised the issue with my son. Now in his 30s, he was a serious competitive swimmer starting at age five until he was in his 20s. I wanted to know if he had ever done it.

“Sure, all the time,” he says. “After a few laps I was always peeing on the swimmer behind me.”

The swimmer behind him could not be reached for comment.