The plasma protein concentration is important in the maintenance of plasma volume which, in turn, regulates reabsorption by the renal tubule through the renin-angiotensin aldosterone system and perhaps an additional natriuretic system. In addition, the plasma protein concentration in glomerular capillaries has direct effects on the formation of glomerular filtrate since the oncotic pressure of plasma opposes the hydrostatic pressure for filtration. Similarly, the plasma protein concentration in peritubule capillaries influences the uptake of reabsorbate from the renal tubules. Although there is a large body of evidence indicating a relationship between peritubule protein concentration and sodium reabsorption by the proximal tubule, the mechanisms involved and the importance of peritubule capillary protein concentration in the regulation of proximal sodium reabsorption remain unresolved. Finally, protein plays an important role in the concentrating ability of the kidney.