Expansion
Reabsorbed passively, with the amount depending on tubular flow rate
Expansion
- Freely filtered
- About 50 per cent reabsorbed passively in the proximal tubule
- Secreted into the loop, then reabsorbed again from the inner medullary collecting duct under the influence of antidiuretic hormone; this urea recycling contributes about half of the medullary osmotic gradient
- Net excretion: about 40 to 50 per cent of the filtered load
Because reabsorption is passive and flow dependent, urea is a poor marker of GFR but a useful marker of tubular flow:
- Slow flow (hypovolaemia, heart failure) means more time for reabsorption, so plasma urea rises disproportionately and the urea to creatinine ratio rises above 100:1 (or above 20:1 in mg/dl units)
- The same picture occurs with upper gastrointestinal bleeding, where digested blood provides a protein load, and with high protein intake, catabolism and steroids
- A low urea suggests liver failure, malnutrition or overhydration
This is why urea and creatinine must be interpreted together rather than either alone.