Theory 02/Nephrology & Renal Physiology/Renal physiology
The Kidney Defends Oxygen
Autoregulation is taught as pressure control. The variable being defended may be oxygen.
Glomerular filtration rate is regulated to protect medullary oxygen tension rather than to hold perfusion constant, so the autoregulation curve shifts with oxygen demand and not only with pressure.
01The evidence
The medulla operates at low oxygen tension and is vulnerable to hypoxic injury, and filtration changes with sodium load even at constant pressure.
02The tension
Autoregulation is modelled as myogenic and tubuloglomerular feedback responding to pressure and salt. Both are real, and neither explains why filtration tracks metabolic demand at constant pressure.
03The mechanism
The medullary thick ascending limb consumes most of the kidney's oxygen while reabsorbing sodium. When demand rises or delivery falls, medullary oxygen tension drops, and the kidney adjusts filtration to restore it.
That makes oxygen the controlled variable, with pressure and sodium as inputs. The autoregulation curve is then not a fixed property but a contour of constant medullary oxygen, which shifts when transport demand shifts.
04What this is not
Those models defend pressure and salt. This defends oxygen, which predicts curve shifts with metabolic demand that pressure models cannot produce.
05Predictions
- P1Filtration should shift with transport demand at constant pressure.
- P2Medullary oxygen should predict filtration better than pressure.
- P3The autoregulation plateau should move when oxygen demand is altered.
06Refutation
An autoregulation curve invariant to metabolic demand would leave oxygen as a correlate.
07What it changes
It would change how renal perfusion is managed in critical care.