Expansion
Renal blood flow falls as blood is diverted to muscle, with sodium and water conservation
Expansion
- Renal blood flow falls by up to 50 per cent in strenuous exercise, from sympathetic vasoconstriction, as flow is redirected to muscle
- GFR falls less, because efferent constriction partly preserves it
- Sodium and water are conserved, through renin-angiotensin-aldosterone and antidiuretic hormone activation, defending volume against sweat losses
- Urine output falls
Common benign findings after strenuous exercise:
- Proteinuria, from altered glomerular permeability and reduced tubular reabsorption
- Haematuria, from minor trauma and altered permeability
- Both resolve in 24 to 48 hours, so testing should be repeated after rest before investigating
Two important pathological states:
- Rhabdomyolysis with myoglobinuria, causing acute kidney injury through tubular obstruction, direct toxicity and vasoconstriction; suggested by very high creatine kinase and urine positive for blood on dipstick without red cells on microscopy
- Exercise-associated hyponatraemia, from drinking excess hypotonic fluid while antidiuretic hormone is inappropriately elevated by exercise, pain and nausea