Cardiac preload is one of the main factors that influence how much blood the heart pumps out with each heartbeat, or stroke.
Now, remember that the heart has two upper chambers: the left atrium, which receives oxygenated blood from the lungs via the pulmonary veins; and the right atrium, which receives deoxygenated blood from all of our organs and tissues via the superior and inferior vena cava.
From the atria, the blood flows into the lower chambers of the heart: the left ventricle, which pumps oxygenated blood to all our organs and tissues via the aorta; and the right ventricle, which pumps the deoxygenated blood back to the lungs via the pulmonary arteries.
Alright, now, each heartbeat consists of two phases: systole, which is when the heart contracts and pumps the blood out of the ventricles; and diastole, which is when the heart relaxes and ventricles fill with blood.
And as the left ventricle fills with blood during diastole, the pressure within it rises.
The pressure at the end of diastole is called the left ventricular end-diastolic pressure, which is a key determinant of cardiac preload.
So, cardiac preload can be defined as the ventricular wall stress at the end of diastole.
And it can be calculated using the law of Laplace, which states that wall stress = pressure (P) x radius (R) / 2 x wall thickness (W).
Another way to say this is that cardiac preload is directly proportional to the end-diastolic pressure and radius of the left ventricle, and indirectly proportional to two times the ventricular wall thickness.