Expansion
The cortex receives most of the flow; the medulla is relatively hypoxic
Expansion
- Total renal blood flow: about 1.1 litres per minute, or 20 to 25 per cent of cardiac output
- Cortex: receives about 90 per cent, with a high oxygen tension
- Medulla: receives only about 5 to 10 per cent, with an oxygen tension of about 1.3 to 2.6 kPa (10 to 20 mmHg)
Low medullary flow is necessary, since a brisk flow would wash out the corticomedullary osmotic gradient. But it leaves the medulla operating close to hypoxia, and the thick ascending limb sits there while performing energy-intensive active transport.
Consequences:
- The outer medulla is the region most vulnerable to ischaemia, so acute tubular necrosis characteristically affects the thick ascending limb and the S3 segment of the proximal tubule
- NSAIDs, by blocking vasodilator prostaglandins that preferentially protect medullary flow, precipitate injury
- Contrast media cause vasoconstriction and increase medullary oxygen consumption
The kidney is therefore paradoxically vulnerable to hypoxia despite its enormous blood flow, because that flow serves filtration rather than oxygen delivery.