All of the following are physiological mechanisms that regulate arterial blood pressure EXCEPT one. Which is the EXCEPTION?

Correct answer: C. Portal venous pressure

• A. This IS a regulator. Arterial pressure equals cardiac output multiplied by systemic vascular resistance; arteriolar tone is the principal determinant of resistance.
• B. This IS a regulator. Stroke volume × heart rate = cardiac output, one of the two terms in the pressure equation, and both are under moment-to-moment autonomic control.
• C. CORRECT — this is the EXCEPTION. PORTAL VENOUS PRESSURE is the pressure within the portal venous system draining the gut to the liver. It is a regional, low-pressure circuit; it does not participate in systemic arterial pressure regulation. Portal hypertension is a CONSEQUENCE of hepatic disease and produces varices, ascites and splenomegaly — not systemic hypertension.
• D. This IS a regulator. Renin initiates the renin–angiotensin–aldosterone system, producing angiotensin II (vasoconstriction) and aldosterone (sodium and water retention), which raises pressure over hours to days.

Note: The three timescales of blood-pressure control: SECONDS — the baroreceptor reflex from the carotid sinus and aortic arch; MINUTES TO HOURS — RAAS, and the sympathetic nervous system with ADH; DAYS — renal pressure–natriuresis governing blood volume.