One equation explains almost everything
Blood pressure (BP) is just cardiac output × total peripheral resistance. Cardiac output is how much blood the heart pumps per minute (heart rate × stroke volume); peripheral resistance is mostly how tight the small arteries are. If you want to lower BP, you can slow or weaken the heart, relax the arteries, or remove fluid so there is less blood to pump. Almost every antihypertensive does one of these three things.
Holding that equation steady is a job shared by four systems: the heart (rate and force), the vessels (constriction and dilation), the kidney (salt and water balance), and the nerves and hormones that coordinate them — chiefly the autonomic nervous system and the renin–angiotensin–aldosterone system.
The two control loops drugs love to target
The body has fast and slow controllers. The fast one is the sympathetic nervous system: when BP drops, it fires, releasing norepinephrine onto beta-adrenoceptors in the heart (speeding it up) and alpha-adrenoceptors in vessels (tightening them). A beta-blocker blunts the heart side of this loop.
The slow one is the RAAS. When the kidney senses low pressure or low salt, it releases renin, which triggers a cascade making angiotensin II — a powerful vessel-tightener — and aldosterone, which makes the kidney hold onto salt and water. Three big drug classes interrupt this loop, which is why it earns its own guide.
BP = Cardiac Output × Peripheral Resistance
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(heart rate × (artery tightness)
stroke volume)
Levers drugs pull:
heart -> beta-blockers, some Ca-channel blockers
vessel -> ACE inhibitors, ARBs, Ca-channel blockers, nitrates
kidney -> diuretics, aldosterone blockers (RAAS)Why we use combinations
The body fights back. Relax the arteries and the heart may speed up to compensate; remove fluid and the RAAS may switch on to refill it. So clinicians often combine a vasodilator with a low-dose diuretic or a RAAS blocker, hitting two levers at once with smaller doses and fewer side effects than pushing one drug hard.