Urinary sodium excretion could be regulated through changes in glomerular filtration rate (GFR) or sodium reabsorption. However, marked changes in GFR may result in only small changes in urinary sodium excretion. This suggests that alterations in sodium excretion result from changes in net sodium reabsorption along the nephron. Several factors play a role in the regulation of sodium excretion, but their relative importance has not been defined perfectly. They include (1) the intrarenal distribution of glomerular filtrate, (2) hormonal effects (aldosterone), (3) peritubular physical factors (hydrostatic and oncotic pressure), and (4) the possible existence of a natriuretic material.
Edema-forming states result from the renal retention of salt and water. In these states, the rate of sodium and water excretion is less than the rate of entry of salt and water. Thus, the kidneys are responsible for salt and water retention.
Klahr S, Slatopolsky E. Renal Regulation of Sodium ExcretionFunction in Health and in Edema-Forming States. Arch Intern Med. 1973;131(6):780–791. doi:10.1001/archinte.1973.00320120020003