Peritubular capillaries and glomerulotubular balance
Proximal tubule · Oncotic ~32 mmHg GT balance
Low-pressure, high-oncotic-pressure capillary network fed by the efferent arteriole that surrounds the proximal and distal tubules and takes up everything the tubules reabsorb. Starling forces here couple proximal reabsorption to GFR (glomerulotubular balance).
Traced from the start
- Efferent arteriole
- Peritubular capillaries and glomerulotubular balance
Detail
- Starling forces
- Hydrostatic pressure only ~13 mmHg (two resistance vessels upstream) while plasma oncotic pressure is high (~32 mmHg) because ~20% of plasma was filtered off; net force strongly favours uptake
- Glomerulotubular balance
- The proximal tubule reabsorbs a near-constant fraction (~65%) of whatever is filtered, so a rise in GFR does not flood the distal nephron
- Mechanism
- Higher filtration fraction raises peritubular oncotic pressure and reabsorption; more filtered glucose and amino acids drive more Na+-coupled uptake
- Modulators
- Sympathetic tone and angiotensin II raise filtration fraction and hence proximal reabsorption; volume expansion lowers both
When it goes wrong
Volume depletion or heart failure with high angiotensin II (efferent constriction, high filtration fraction)
Avid proximal Na+, urea and urate reabsorption: urine Na+ under 20 mmol/L, fractional excretion of Na+ under 1%, urea rising out of proportion to creatinine, hyperuricaemia
Saline volume expansion
Peritubular oncotic pressure falls, proximal reabsorption drops and natriuresis follows
Practise this structure
1 question in the bank tagged Tubular Physiology.