Which substance, stimulated by decreased arterial pressure, acts as a vasoconstrictor?

The correct substance that acts as a vasoconstrictor in response to decreased arterial pressure is angiotensin. When arterial pressure drops, the kidneys sense this change and release renin, an enzyme that converts angiotensinogen (a protein produced by the liver) into angiotensin I. Angiotensin I is then converted into angiotensin II, primarily by the action of angiotensin-converting enzyme (ACE) in the lungs.

Angiotensin II has potent vasoconstrictive properties; it narrows blood vessels, which helps to increase blood pressure. Additionally, it also stimulates the release of aldosterone from the adrenal glands, promoting sodium and water retention by the kidneys, further contributing to increased blood volume and blood pressure. This mechanism is crucial in maintaining cardiovascular stability under conditions of low arterial pressure, responding effectively to prevent hypotension.

While histamine is known mainly for its role in inflammatory responses and can cause vasodilation, epinephrine and norepinephrine are also vasoconstrictors but typically respond to different stimuli, such as stress or fight-or-flight responses, rather than solely to decreased arterial pressure. Thus, the specific response to low arterial pressure is primarily dominated by angiotensin.