Updated

Updated · The Guardian · Jun 16

Experts Link Swimmers' Mid-Lap Urge to Pee to 78-82F Water and Immersion Diuresis

Updated

Updated · The Guardian · Jun 16

Experts Link Swimmers' Mid-Lap Urge to Pee to 78-82F Water and Immersion Diuresis

Q: What does the urge to pee during a cold plunge reveal about your heart's hidden emergency response system?

The urge to pee during a cold plunge shows that your heart has a built-in overflow protection system that rapidly tries to reduce what the body perceives as excess central blood volume. When cold water hits the body, blood vessels in the skin and limbs constrict to preserve heat. Water pressure and the horizontal position in water also push more blood back toward the chest. That increases filling of the heart and central circulation. Sensors in the heart and nearby vessels read this as a fluid overload risk, even if you have not actually gained water. That triggers a hormonal response. The brain reduces antidiuretic hormone, which normally tells the kidneys to conserve water. At the same time, stretched heart muscle cells release atrial natriuretic peptide or factor, which promotes sodium and water excretion and helps relax blood vessels. The kidneys then make more urine, producing the sudden urge to urinate. Experts including Dr. Stavros Kavouras, Dr. Scott Trappe, and Mayo Clinic physiologist Dr. Michael Joyner describe this as a normal protective reflex designed to keep the heart from becoming overfilled. In practical terms, the bladder signal is evidence of a fast heart-kidney communication system responding to cold stress and blood redistribution. It is usually harmless, but it can contribute to dehydration because cold also blunts thirst. That is why steady hydration before and after immersion matters, even when you are surrounded by water.

3 articles · Updated · The Guardian · Jun 16

Immersion diuresis drives the sudden urge to urinate during swims, experts said, calling it a normal response that affects essentially all swimmers and is usually harmless, though it can sometimes contribute to dehydration.
Cool water—many pools run at 78-82F—constricts skin blood vessels and shifts more blood to the chest, where receptors read the change as fluid overload and trigger the kidneys to make more urine.
Two hormone changes help cause that response: the brain reduces antidiuretic hormone, which normally helps retain urine, while the heart releases atrial natriuretic factor to lower pressure and increase water excretion.
Swimming also differs from running because swimmers sweat less and lie prone in the water, both of which make the body more likely to sense excess fluid rather than fluid loss.
Experts said there is little way to prevent the reflex, which they described as a long-standing part of swimming culture—along with the taboo reality that many swimmers simply urinate in the pool.