The proximal tubule reabsorbs a constant fraction of whatever is filtered
Expansion
The proximal tubule reabsorbs a constant fraction of the filtered load, about 65 to 70 per cent, rather than a constant amount. If GFR rises, absolute reabsorption rises proportionately.
Two mechanisms:
- Peritubular physical forces: a rise in GFR raises the filtration fraction, so peritubular oncotic pressure rises and hydrostatic pressure falls, both favouring reabsorption
- Luminal factors: more filtered glucose, amino acids and bicarbonate means more sodium is cotransported with them
The importance is one of scale. About 180 litres are filtered daily and less than 1 per cent excreted. A mere 1 per cent rise in GFR without compensation would increase the load to the distal nephron by nearly two litres a day, far beyond its reserve capacity, and would cause rapid volume depletion.
Glomerulotubular balance is therefore the first line of defence, buffering the distal nephron, while tubuloglomerular feedback provides a second line by stabilising GFR itself.