Explain Regulation of Water Balance for Cells in the kidneys?

Cells in the kidneys respond to low blood pressure by releasing an enzyme called renin, which in turn causes the kidneys to reabsorb sodium through a series of events. Sodium reabsorption in turn is accompanied by water retention thus helping to restore blood volume and the blood pressure. Renin activates a protein called angiotensinogen in the blood to its active form, angiotensin. Being a powerful vasoconstrictor, angiotensin nasrows the blood vessel, thereby raising the blood pressure. Angiotensin also mediates the release of the hormone aldosterone, which causes the kidneys to retain more sodium and thus the water. Again, the effect is that when more water is needed, less is excreted.