The kidneys control plasma volume by controlling how much Na and water are reabsorbed.
The juxtaglomerular apparatus is a structure formed by the distal convoluted tubule and the glomerular afferent arteriole which regulate blood pressure and filtration. The components of the apparatus are macula densa which detects Na concentration and juxtaglomerular cells which sense the blood pressure. When blood pressure and Na decrease, the juxtaglomerular cells then release an enzyme called renin. Renin converts angiotensinogen into angiotensin I, then angiotensin I is converted to angiotensin II which causes blood vessels to contract (elevate the blood pressure). Angiotensin II also stimulates the adrenal gland to secrete aldosterone which stimulates more Na and water reabsorption (also elevates the blood pressure).
