One of the finest system of our body, to cope with drop in our blood pressure is through the renin angiotensin aldosterone system.
When blood pressure drops our kidneys can sense it hence perfusion of the kidney reduces too. Our kidneys in turn releases a hormone called renin.
Renin are produced from cells known as juxta glomerular cells. These specialized cells line the afferent arteriole, hence they can sense drop in blood pressure immediately.
Renin converts angiotensinogen (a product of the liver) into angiotensin I and the latter gets converted into angiotensin II by an enzyme called ACE (angiotensin converting enzyme).
Angiotensin II acts on adrenal gland’s (a cap like structure placed above both of the kidneys) cortex region’s outer portion i.e. the zona glomerulosa to secrete aldosterone.
Aldosterone is the particular product which is responsible reabsorption of sodium and water at distal part of nephrons in exchange of potassium (potassium is excreted). Therefore, finally sodium and water is retained in the body which is an attempt to restore the blood pressure.