Why Do I Have to Run for the Toilet When I’m Swimming?© Felix Gmünder |
The short answer
Increased kidney activity can be triggered by the cold stimulus in the water, but what is far more significant for water sports enthusiasts is the increased return flow of blood and lymph from the legs to the heart, caused by the horizontal body position and/or, when the body is mostly vertical, by water pressure.The cold stimulus leads to activation of the parasympathetic nervous system
The parasympathetic nervous system (autonomic nervous system) in turn inhibits the release of antidiuretic hormone (ADH) by the pituitary gland. The main function of ADH is to regulate water balance. When there is a shortage of water, it is released and reduces urine output. A cold stimulus causes a reduction in ADH levels in the blood, which results in increased urine production.Lying flat or increased pressure on the legs
When swimming in a horizontal position, more blood and lymph flow from the legs to the heart than normal. In water aerobics, the water pressure on the legs – which is higher than that on the chest and arms – is the dominant factor. This also causes an increased return of blood and lymph to the heart. Both effects are familiar to astronauts when they experience weightlessness.The two atria contain stretch receptors which interpret the increased return as an increase in blood volume. Urine production is then stimulated via the parasympathetic-pituitary-ADH axis. Secondly, when under increased tension, the atrial muscle cells directly increase urine production in the kidneys by secreting atrial natriuretic peptide (ANP), which stimulates kidney function. The technical term for this physiological mechanism is the atrial stretch reflex or Gauer–Henry reflex.