RAAS: The Salt-and-Pressure Cascade

The trigger: a kidney that feels under-filled

The renin-angiotensin-aldosterone system (RAAS) is the body's main answer to falling volume. It begins in the kidney, which has its own little pressure-and-salt sensor. When blood pressure through the kidney drops, when the sympathetic nerves fire in a stress response, or when a sensor called the macula densa notices too little salt flowing past it, specialised kidney cells release an enzyme called renin into the blood. Renin is not a hormone that acts on tissues directly; it is the *starter* of a chain reaction. Think of it as striking the match.

Walking the cascade

1. The liver constantly puts an inactive protein, angiotensinogen, into the blood, just sitting there as raw material.
2. Renin snips angiotensinogen into angiotensin I, a still-inactive fragment. This step is the rate-limiting one — renin sets the pace of the whole system.
3. Angiotensin-converting enzyme (ACE), mostly in the lungs, trims angiotensin I into angiotensin II — the active player at last.
4. Angiotensin II does several things at once, all aimed at raising pressure and volume (see below).

Angiotensinogen  (liver, inactive raw material)
      |  renin   <-- the controlled, rate-limiting step
      v
Angiotensin I    (inactive)
      |  ACE     <-- mostly in the lung
      v
Angiotensin II   (ACTIVE) ---> squeezes blood vessels (raises pressure)
      |                   ---> triggers thirst + vasopressin
      |                   ---> tells kidney to keep sodium directly
      v
Adrenal cortex releases ALDOSTERONE
      |
      v
Kidney reabsorbs sodium (water follows) ---> volume rises

What angiotensin II and aldosterone actually do

Angiotensin II is the multitasker. It is a powerful vasoconstrictor — it tightens blood vessels to lift pressure immediately. It nudges the brain to make you thirsty and to release vasopressin. It tells the kidney directly to hold onto sodium. And, the slow but heavy hitter, it stimulates the adrenal cortex to release aldosterone. So the cascade gives both a fast effect (squeezing vessels in seconds) and a sustained effect (retaining salt over hours).

Aldosterone is a steroid hormone, so unlike fast-acting vasopressin it works by entering kidney cells, switching on genes, and building more sodium pumps and channels. The result: the kidney reabsorbs more sodium, and because water follows salt, volume rises. Aldosterone's role in RAAS also swaps sodium *in* for potassium *out*, which is why disorders of this system disturb potassium too. It takes hours to ramp up — a deliberate, sustained tightening of the salt budget rather than an emergency reflex.