Expansion
Two-thirds of sodium and water, and essentially all glucose, amino acids and bicarbonate
Expansion
Reabsorbed in the proximal convoluted tubule:
- Sodium and water: about 65 to 70 per cent, isosmotically
- Glucose: essentially 100 per cent, by sodium-glucose cotransport
- Amino acids: essentially 100 per cent
- Bicarbonate: about 80 to 90 per cent, via carbonic anhydrase
- Phosphate: about 80 per cent, inhibited by parathyroid hormone
- Potassium: about 65 per cent
- Urea: about 50 per cent, passively
Secreted: hydrogen ions, organic acids and bases, including many drugs and creatinine.
The driving force for all of it is the basolateral sodium-potassium ATPase, which keeps intracellular sodium low so that sodium entry from the lumen can be coupled to the transport of everything else.
Generalised failure of this segment gives Fanconi syndrome: glycosuria with normal blood glucose, aminoaciduria, phosphaturia with rickets or osteomalacia, and a proximal (type 2) renal tubular acidosis.