Cardiac output is the amount of blood the heart pumps each minute, about 4 to 8 liters at rest in an adult. It equals heart rate times stroke volume, the blood ejected with each beat.
Preload
Preload is how much the ventricle is stretched by blood at the end of filling. Within limits, more stretch gives a stronger beat, so fluids raise output in a dry patient, while bleeding and dehydration lower it.
Afterload
Afterload is the resistance the ventricle pushes against to eject blood, set mostly by arterial pressure and vessel tone. High blood pressure and a narrow aortic valve raise afterload and make the heart work harder.
Contractility
Contractility is the strength of the heart muscle itself at any given stretch. Adrenaline and some drugs raise it; heart attack, heart failure and acidosis lower it.
Heart rate
A faster rate raises output up to a point. At very fast rates the ventricles have too little time to fill, so stroke volume and output fall.
| Factor | Raised by | Lowered by |
|---|---|---|
| Preload | IV fluids, fluid overload | Bleeding, dehydration, vasodilators |
| Afterload | Hypertension, aortic stenosis, vasoconstriction | Vasodilators, sepsis |
| Contractility | Adrenaline, dobutamine | Heart attack, heart failure, beta blockers |
At the bedside
In a patient with low blood pressure, ask which part is failing: the tank is empty (preload), the pump is weak (contractility) or the pipes are too wide (low afterload). The answer guides the choice of fluids, inotropes or vasopressors.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).