Which of the following is NOT a physiological mechanism that regulates systemic blood pressure?

Systemic blood pressure is regulated by multiple physiological mechanisms: the baroreceptor reflex (rapid neural regulation), the renin-angiotensin-aldosterone system (hormonal, longer-term), antidiuretic hormone/vasopressin (fluid retention and vasoconstriction), the sympathetic nervous system (heart rate and vascular tone), and autoregulation of vascular tone. An increase in portal venous pressure (portal hypertension) affects hepatic blood flow and is a consequence of liver disease, not a regulator of systemic blood pressure. Portal hypertension is a local phenomenon in the portal circulation, not a systemic blood pressure regulatory mechanism.