Identify the primary regulators of systemic vascular resistance.

• A. Only sympathetic nervous system
• B. Sympathetic nervous system, circulating catecholamines, local autoregulators (metabolites like CO2, H+, adenosine, NO), endothelins, and RAAS
• C. Only local autoregulators
• D. Endothelins alone

Answer: (B)

Systemic vascular resistance is controlled by a coordinated mix of neural, hormonal, and local signals that set the tone of arterioles throughout the body. The sympathetic nervous system provides rapid, global adjustment by activating alpha-adrenergic receptors to cause vasoconstriction in many vascular beds, with circulating catecholamines (epinephrine and norepinephrine) amplifying that effect. But local conditions finely tune this tone: autoregulatory signals such as metabolic byproducts (CO2, H+, adenosine) and endothelium-derived factors like nitric oxide promote vasodilation in tissues needing more blood flow, while endothelins act as potent vasoconstrictors when a stronger push is required. The renin-angiotensin-aldosterone system adds another layer, with angiotensin II causing vasoconstriction and aldosterone influencing blood volume to sustain changes in resistance. Because SVR reflects the balance of all these influences, the best answer recognizes that neural input, circulating hormones, local metabolic regulators, endothelins, and RAAS collectively regulate systemic vascular resistance.