How does sodium ion increase blood pressure?
Sodium acts on the juxtaglomerular cells of the kidney, which are then activated to produce and secrete renin. Renin hydrolyses angiotensinogen into angiotensin I. Next angiotensin converting enzyme (ACE) converts angiotensin I to angiotensin II.

Angiotensin II is the product that causes the increase in blood pressure, because it is a potent vasoconstrictor. With peripheral vasocontriction the heart has more resistance to pump blood against, thus higher blood pressure. Additionally, angiotensin II stimulates secretion of aldosterone from the adrenal glands, which acts on the distal tubules and collecting ducts in the kidney to retain sodium and water, thereby increasing blood volume and thus blood pressure.


Image: RVC 2008

This pathway is one reason why ACE-inhibitors, such as benazepril, enalapril, lisinopril, or perindopril, are used to treat hypertension (high blood pressure). With less activity of ACE, there is less angiotensin II, and blood pressure goes down. Hypertensive patients are told to consume a lower sodium diet because that will lead to less activation of the JG cells in the kidney, which will eventually result in less angiotensin II, and thus lower blood pressure.