Expansion
Alpha-1 vasoconstricts, beta-1 stimulates the heart, beta-2 vasodilates
Expansion
| Receptor | Site | Effect |
|---|---|---|
| Alpha-1 | Arteriolar and venous smooth muscle | Vasoconstriction, raising systemic vascular resistance |
| Alpha-2 | Presynaptic nerve terminals, central | Reduced noradrenaline release, sympatholytic |
| Beta-1 | Heart (nodes and myocardium), kidney | Positive chronotropy and inotropy; renin release |
| Beta-2 | Skeletal muscle and coronary arterioles, bronchi | Vasodilatation, bronchodilatation |
One heart, two lungs keeps the numbering straight.
This table explains vasopressor choice: noradrenaline is predominantly alpha-1 and raises pressure with minimal tachycardia; adrenaline hits both, so it raises output and pressure but is arrhythmogenic; dobutamine is mainly beta-1 and increases output while often lowering resistance through beta-2.
It also explains why adrenaline at low dose can lower diastolic pressure, as beta-2 vasodilatation in muscle outweighs alpha-1 constriction elsewhere.