Aldosterone: the salt-retaining hormone
Aldosterone, the main mineralocorticoid from the glomerulosa, has one central job: tell the kidney to hold onto sodium and, with it, water — while dumping potassium. More retained salt means more retained water means more blood volume and higher blood pressure. So aldosterone is, in effect, a sodium and blood-pressure dial.
What controls aldosterone is not ACTH but the renin-angiotensin-aldosterone system. When blood pressure or salt falls, the kidney releases renin, which sets off a cascade producing angiotensin II, and angiotensin II is the main trigger that tells the glomerulosa to release aldosterone. High blood potassium is the other strong trigger. This is a separate control loop from the cortisol axis — an important point clinically.
The medulla: instant adrenaline
While the cortex works in hours, the medulla works in seconds. When the brain senses acute danger, sympathetic nerves fire straight onto the medulla, which releases catecholamines — mainly epinephrine — into the blood. Heart rate and force jump, airways widen, the liver dumps glucose, pupils dilate, and blood is shunted to muscle. This is the fight-or-flight response in chemical form.
Together the medulla and the sympathetic nerves form the sympathoadrenal system: nerves give a fast, targeted jolt, and the medulla broadcasts a slightly slower, body-wide chemical echo that keeps the response going. The two arms of the stress response — fast catecholamines and slower cortisol — overlap and reinforce each other.
TWO TIMESCALES OF ADRENAL BLOOD-PRESSURE CONTROL
FAST (seconds): danger -> sympathetic nerve -> MEDULLA
-> epinephrine -> heart rate up, vessels constrict
-> BP up NOW
SLOW (minutes-hours): low BP/salt -> kidney RENIN
-> angiotensin II -> GLOMERULOSA
-> aldosterone -> kidney holds Na+ & water
-> blood volume up -> BP up over time