Baroreceptors in the carotid sinus and aortic arch detect reduced stretch and trigger increased sympathetic outflow from the medulla.
The sympathetic nervous system is rapidly activated in response to decreased circulating blood volume and falling arterial pressure.
Vasoconstriction shunts blood away from nonessential organs such as the skin and gastrointestinal tract toward vital organs like the brain and heart. Additionally, sympathetic stimulation promotes venoconstriction, which helps increase venous return and temporarily supports cardiac output. These compensatory mechanisms aim to maintain blood pressure and tissue perfusion until volume can be restored.
This results in the release of catecholamines, primarily norepinephrine and epinephrine, which cause tachycardia, increased myocardial contractility, and peripheral vasoconstriction.