Cardiac output (CO = heart rate × stroke volume) must be continuously adjusted to maintain adequate blood pressure and match metabolic demands. Stroke volume is determined by three physiological factors: preload (the degree of ventricular filling and stretch), contractility (the intrinsic force of contraction independent of loading), and afterload (the resistance against which the ventricle ejects blood). The Frank-Starling mechanism states that within physiological ranges, increased preload increases stroke volume by optimizing sarcomere length and cross-bridge overlap. Sympathetic stimulation increases contractility by enhancing intracellular calcium handling.
Use echocardiography or cardiac catheterization to measure stroke volume and observe how controlled changes in preload (fluid administration or withdrawal), afterload (vasopressor or vasodilator drugs), or contractility (dobutamine or esmolol) affect it.
Cardiac output is not simply determined by heart rate; stroke volume changes are equally important, especially during exercise where both increase to produce large increases in CO.
From your study of the cardiac cycle, you know that the heart fills during diastole and ejects blood during systole. Cardiac output is simply the volume of blood the heart pumps per minute, calculated as heart rate multiplied by stroke volume — the amount ejected with each beat. A resting heart rate of 70 bpm and a stroke volume of 70 mL gives a cardiac output of about 5 L/min, which is roughly the entire blood volume circulated every minute. The question this topic addresses is: how does the heart adjust this output to match the body's changing metabolic demands?
The answer lies in three determinants of stroke volume. Preload is the degree to which the ventricle is stretched by blood at the end of diastole — essentially, how full the chamber is before it contracts. The Frank-Starling mechanism explains why preload matters: when more blood fills the ventricle, the cardiac muscle fibers are stretched to a more optimal length, producing more forceful contractions and a larger stroke volume. Think of it like stretching a rubber band — within a physiological range, more stretch means more snap-back force. This mechanism is intrinsic to the heart muscle itself and requires no neural input. It is what allows the heart to automatically match its output to venous return: if more blood flows back to the heart, the heart pumps more out.
Contractility (also called inotropy) is the force of contraction independent of how much the ventricle is stretched. Two hearts can have the same preload but different contractility. Sympathetic stimulation increases contractility by triggering norepinephrine release, which activates beta-1 adrenergic receptors on cardiac myocytes. This increases intracellular calcium availability during each contraction cycle — more calcium means more cross-bridge cycling between actin and myosin, producing a stronger contraction. The Frank-Starling curve effectively shifts upward: at the same preload, the heart ejects more blood. Conversely, heart failure is a state of reduced contractility where the curve shifts downward.
Afterload is the resistance the ventricle must overcome to eject blood, determined primarily by arterial blood pressure and vascular resistance. Higher afterload means the ventricle must generate more pressure before the aortic valve opens, leaving less energy for ejection and reducing stroke volume. Think of afterload as pushing against a heavy door — the harder it is to open, the less you get through. In clinical practice, chronically elevated afterload (as in uncontrolled hypertension) forces the heart to work harder with every beat, eventually leading to pathological hypertrophy. During exercise, cardiac output can increase four- to fivefold through the coordinated increase of heart rate (sympathetic activation and vagal withdrawal), preload (increased venous return from the muscle pump and venoconstriction), and contractility (sympathetic drive), while afterload may actually decrease slightly as skeletal muscle vasodilation reduces total peripheral resistance.