Expansion
Reduced filtration, increased proximal reabsorption, and hormonal salt and water retention
Expansion
The sequence:
- Baroreceptors and reduced renal perfusion trigger sympathetic activation and renin release
- Angiotensin II constricts the efferent arteriole, defending GFR, and raises the filtration fraction
- The higher filtration fraction raises peritubular oncotic pressure, increasing proximal sodium and water reabsorption
- Aldosterone increases distal sodium reabsorption
- Antidiuretic hormone, released by the baroreflex even against osmotic signals, increases water reabsorption
- Prostaglandins dilate the afferent arteriole to preserve renal blood flow
The result: oliguria, urine osmolality above 500 mOsm/kg, urine sodium under 20 mmol/l, FENa under 1 per cent, and a urea to creatinine ratio above 100:1, since urea reabsorption rises with slow tubular flow.
Two therapeutic implications: this state is reversible with volume replacement if treated early, and blocking any of the compensations, with NSAIDs, ACE inhibitors or further diuretics, converts compensated hypovolaemia into established acute kidney injury.