The urinary system consists of the kidneys and bladder, along with the ureters and urethra. It has multiple functions, among them excreting toxins and metabolic by-products, maintaining the body's fluid and acid-base balance, regulating electrolyte levels and secreting several important hormones. Exercise affects the urinary system in multiple important ways.
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Blood Flow to the Kidneys
When you exercise, blood flow to your kidneys is diminished due to an increase in the activity of the sympathetic nervous system, the "fight or flight" component of the nervous system. This reduction in blood flow is necessary to maintain your blood pressure as blood vessels dilate in your working muscles. Because of the decrease in blood flow, the amount of fluid filtered by your kidneys also is reduced during moderate to intense exercise, resulting in decreased urine production.
Exercise, Sodium and Fluid Balance
You can lose significant amounts of fluid as well as some sodium in your sweat as you exercise. To maintain fluid balance, the kidneys conserve sodium and reabsorb water, contributing to the reduction in urine production. Although the amount of fluid conserved in this way during exercise is small compared to the amount you can sweat, the kidneys continue to conserve sodium for hours or even days after intense exercise to restore normal levels.
Hormonal Effects on the Kidneys
A major hormone involved in maintaining fluid balance during exercise is antidiuretic hormone, or ADH, which causes the kidneys to conserve sodium. ADH also results in more concentrated urine. The hormones aldosterone and angiotensin II are responsible for the kidney's restoration of normal electrolyte levels following exercise. Angiotensin II, a particularly powerful regulator of sodium balance, is produced from renin, which is a hormone secreted by the kidneys in response to sympathetic nervous system stimulation during exercise.
Other Effects of Exercise on the Kidneys
During exercise, the kidneys tend to filter out more protein, as well, producing elevated levels of protein in the urine. The kidneys are also partly responsible for maintaining acid-base balance. When you exercise intensely, you produce lactic acid, some of which the kidneys excrete. For this and other reasons, urine becomes more acidic during exercise. Following exercise, the kidneys help to metabolize the remaining lactic acid, converting it into glucose, or blood sugar.