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How does renin affect blood vessels and blood pressure?

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Human kidneys — renal system Diagram showing the location of the kidneys in the abdominal cavity and their attachment to major arteries and veins.

Renin does not directly affect blood vessels; rather, it initiates a physiological cascade known as the renin-angiotensin system that regulates blood pressure.21

When the kidneys secrete renin into the blood, it catalyzes the conversion of angiotensinogen into angiotensin I. An enzyme called angiotensin-converting enzyme (ACE) then converts this into angiotensin II.21

Angiotensin II is the active agent that increases blood pressure through several mechanisms:
* Direct Constriction: It acts directly on small arteries (arterioles), causing them to constrict.21
* Hormonal Influence: It stimulates the adrenal glands to release aldosterone, which promotes salt and water reabsorption, and catecholamines.21
* Nervous System Interaction: It inhibits the reuptake of the hormone norepinephrine into nerve terminals, which further promotes vessel constriction.21

By increasing systemic vascular resistance and fluid retention, these processes effectively raise blood pressure.21