Three control systems on three time scales
Blood pressure is held within a workable range by overlapping systems that act at different speeds. The fastest is the baroreflex, a nervous-system loop that adjusts heart rate and vessel tone within seconds of a pressure change. Over minutes to hours, the renin-angiotensin-aldosterone system (RAAS) responds to low pressure or low kidney perfusion by producing angiotensin II, which constricts vessels, and aldosterone, which promotes salt and water retention.
Layered on top, hormones such as antidiuretic hormone and the natriuretic peptides fine-tune fluid balance.
The long-term anchor: pressure natriuresis
Over days, the kidneys exert the dominant long-term control through pressure natriuresis: when arterial pressure rises, the kidneys excrete more sodium and water, lowering blood volume and pressure; when pressure falls, they conserve them. Because this loop sets the long-run operating point, the kidneys are central to chronic blood pressure regulation.
The systems interact rather than acting alone, which is why blood pressure problems often involve more than one mechanism. This is general educational physiology, not medical advice; consult a qualified clinician for diagnosis or treatment.