Hypertension Mechanisms: RAAS

Blood pressure is the product of how much blood the heart ejects and how tightly the arterial tree resists that flow. The renin-angiotensin-aldosterone system (RAAS) is the body's defence of kidney perfusion: when the kidney senses less flow or less sodium delivery, it releases renin, and the cascade that follows narrows arterioles and makes the kidney hold sodium and water. In a healthy person the signal switches off when perfusion recovers. In hypertension the defence is either switched on when it should not be, or it keeps running because the kidney is being underperfused by a narrowed artery, or aldosterone is being made without any renin signal at all. That last distinction is the one a prescriber uses. A drug that blocks the cascade works best when the cascade is driving the pressure, lowers glomerular filtration pressure and so can raise creatinine, and removes the signal that tells the kidney to excrete potassium, so it can raise potassium. Every monitoring and dose decision that follows rests on those three facts: pressure falls, creatinine may rise a little, potassium rises. The...