The kidneys receive about 20% of the heart's output, far more than their size suggests. This large flow is needed for filtering, not for the kidney's own oxygen needs.
The path of blood
The renal artery branches down to the afferent arteriole, which feeds the glomerulus. Blood leaves through the efferent arteriole and then supplies the tubules before draining to the renal vein.
Glomerular filtration rate
GFR is the volume of plasma filtered each minute, normally about 90 to 120 mL/min in young adults. It falls slowly with age.
Keeping GFR steady
The arterioles widen or narrow to hold GFR fairly constant across a range of blood pressures. When blood pressure falls too low, this control fails and filtration drops.
Measuring it
Creatinine, a waste from muscle, is filtered and not reabsorbed, so its blood level rises as GFR falls. Labs report an estimated GFR (eGFR) calculated from creatinine, age and sex.
| Stage | eGFR (mL/min/1.73 m²) | Meaning |
|---|---|---|
| G1 | 90 or more | Normal, if no other sign of kidney damage |
| G2 | 60 to 89 | Mildly decreased |
| G3 | 30 to 59 | Moderately decreased |
| G4 | 15 to 29 | Severely decreased |
| G5 | Under 15 | Kidney failure |
At the bedside
Check creatinine and eGFR before giving contrast, metformin or drugs cleared by the kidney, and adjust doses when eGFR is low. Low blood pressure and NSAIDs can both cut kidney blood flow.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).