Viewed in the past solely in the narrow setting of classical excretory and resorptive functions, the kidney's role in blood pressure and volume regulation has acquired added dimension. No longer seen as a mere target organ for extrarenal hormones in regulating water and electrolyte metabolism, the kidney is now recognized as a source of substances which play an essential part in hemodynamic homeostasis.
Most important in hemodynamic control is renin, a renal enzyme which activates a plasma protein to form angiotensin. A potent vasoconstrictor, angiotensin raises systemic blood pressure. It also expands blood volume by stimulating adrenocortical secretion of the sodium-retaining hormone, aldosterone. Both angiotensin and aldosterone inhibit the release of renin by direct or indirect negative feedback mechanisms. And though renin release is primarily controlled by the total delivered sodium load to the macula densa and possibly by renal arteriolar pressure changes, extrarenal hormonal and sympathetic nervous system influences