The renin–angiotensin system (RAS) or the renin–angiotensin–aldosterone system (RAAS). Start reading this schematic from the left, where it says "Decrease in renal perfusion (juxtaglomerular apparatus)". Alternatively, the RAAS can also be activated by a low NaCl concentration in the macula densa or by sympathetic activation.
Legend info: Blue and red dashed arrows indicate stimulatory or inhibitory signals, which is also indicated by the +/-. In the tubule and collecting duct graphics, the grey dashed arrows indicate passive transport processes, contrary to the active transport processes which are indicated by the solid grey arrows. The other solid arrows either indicate a secretion from an organ (blue, with a starting spot) or a reaction (black). These 2 processes can be stimulated or inhibited by other factors.
This work is based on File:Renin-angiotensin-aldosterone system.png, which was created by User:A._Rad~commonswiki
Own work
Soupvector