All of the following are physiological mechanisms involved in the regulation of systemic arterial blood pressure EXCEPT one. Which is the EXCEPTION?

Correct answer: D. Portal venous pressure

• A. RENIN release initiates the renin–angiotensin–aldosterone system: angiotensin II is a potent vasoconstrictor and aldosterone drives sodium and water retention. This is the dominant long-term regulator of blood pressure and the target of ACE inhibitors and ARBs.
• B. SYSTEMIC VASCULAR RESISTANCE is one of the two direct determinants of mean arterial pressure (MAP = CO × SVR). It is set principally by arteriolar tone under sympathetic and humoral control.
• C. STROKE VOLUME × HEART RATE = CARDIAC OUTPUT, the other direct determinant of mean arterial pressure, and the arm of the equation manipulated by beta blockers.
• D. CORRECT — this is the exception. PORTAL VENOUS PRESSURE is the pressure within the hepatic portal system, which drains the gut to the liver. It is a LOCAL splanchnic pressure and plays no part in regulating systemic arterial pressure. It becomes clinically important in liver cirrhosis, where portal HYPERTENSION produces varices, ascites and splenomegaly — but systemic arterial pressure in those patients is typically LOW, not high, because of splanchnic vasodilatation.

Note: Anchor everything on MAP = CARDIAC OUTPUT × SYSTEMIC VASCULAR RESISTANCE, with the RAAS and baroreceptor reflexes as the regulators. Any factor that cannot be mapped onto that equation is the exception.