A fall in glomerular blood flow/glomerular blood pressure/GFR can activate the JG cells to release renin which converts angiotensinogen in blood to angiotensin I and further to angiotensin ll. Angiotensin Il, being a powerful vasoconstrictor,increases the glomerular blood pressure and thereby GFR. Angiotensin Il also activates the adrenal cortex to release aldosterone. Aldosterone causes reabsorption of Na+ and water from the distal parts of the tubule. This also leads to an increase in blood pressure and GFR This complex mechanism is generally known as the Renin-Angiotensin mechanism.