Editing Anatomy and Physiology (section)

=== Cardiac Output ===

Cardiac output equals '''heart rate multiplied by stroke volume''', normally about 4 to 8 liters per minute in an adult. Stroke volume depends on:
* '''Preload''': the volume stretching the ventricle at end diastole. Increased by fluid overload, decreased by hemorrhage, dehydration, and venous dilators.
* '''Afterload''': the resistance the ventricle pumps against. Increased by hypertension, vasoconstriction, and aortic stenosis.
* '''Contractility''': the force of contraction, improved by positive inotropes and reduced by ischemia, acidosis, and hypoxia.

The '''Frank-Starling law''' states that greater stretch produces stronger contraction, up to a point beyond which the failing ventricle cannot keep up.

Blood pressure is a product of cardiac output and systemic vascular resistance. '''Mean arterial pressure''' of roughly 65 mmHg or more is generally needed to perfuse the vital organs.