When glomerular blood pressure or glomerular blood flow is decreased the JG cells (Cuff shaped or ring shaped cells present in the wall of afferent arteriole) are activated which secrete renin enzyme.
This renin converts angiotensinogen (made up of 453 amino acids) into angiotensin-I (made up of 10 amino acids).
This angiotensin–I is converted into angiotensin–II (made up of 8 amino acids) in presence of enzyme Angiotensin-Converting Enzyme (ACE), secreted from lungs.
The angiotensin–II stimulates the adrenal cortex for the secretion of the hormone aldosterone, which promotes the absorption of Na+ ions from nephric filtrate resultantly blood pressure is increased.
This complete mechanism is called as Renin-Angiotensin-Aldosterone System (RAAS).