Renin, also called angiotensinogenase, is an enzyme involved in the renin–angiotensin aldosterone system (RAAS), which regulates the body’s water balance and blood pressure level. The system regulates the extracellular volume in the blood plasma, lymph and interstitial fluid, as well as controlling constriction of the arteries and blood vessels.

Renin was first discovered by a physiology researchers Robert Tigerstedt and Per Bergman from the Karolinska Institute in Stockholm. They described and named the enzyme in 1898. The renin precursor protein is made up of 406 amino acids and possesses a pre- and a pro-segment made up of 20 and 46 amino acids, respectively. The mature form of the enzyme contains 340 amino acids and has a mass of 37 kDa.

This peptide hormone is secreted by the kidneys from specialized cells called granular cells found in the juxtaglomerular apparatus. The secretion of renin is stimulated by the following three factors:

• When a fall in arterial blood pressure is detected by pressure sensitive receptors (baroreceptors) in the arterial vessels.
• When a decrease in sodium chloride (salt) is detected in the kidney by the macula densa in the juxtaglomerular apparatus.
• When sympathetic nervous system activity is detected through beta1 adrenergic receptors.

There are two cellular pathways involved in the secretion of renin:

• Prorenin is secreted through a constitutive pathway
• Renin is secreted through a regulated pathway