What is the Difference Between Preload and Afterload?

Preload and afterload are terms used to describe the forces that affect the heart's ability to pump blood efficiently. They are related to the blood volume present in the heart and the pressure against which the heart has to pump blood, respectively.

Preload refers to:

• The initial stretching of the cardiac myocytes (muscle cells) prior to contraction.
• The amount of ventricular stretch at the end of diastole, also known as left ventricular end-diastolic pressure (LVEDP).
• The blood volume present in the heart's ventricles when it is ready to pump.

Afterload refers to:

• The force or load against which the heart has to contract to eject blood.
• The pressure against which the heart has to pump blood.

Changes in preload and afterload can affect cardiac output, which is the amount of blood pumped by the heart per minute. If preload is high and afterload is low, the heart will have an easier time pumping blood throughout the body and maintaining adequate blood flow. On the other hand, if preload is too high and afterload is too low, the ventricles may fill up faster than they can pump out blood, potentially causing heart failure. If preload is too low and afterload is too high, the ventricles may have trouble emptying themselves properly, leading to high blood pressure.

Comparative Table: Preload vs Afterload

The main difference between preload and afterload lies in their definitions and the fact that they occur during different phases of the cardiac cycle. Here is a comparison table highlighting the differences between preload and afterload:

In summary, preload is the ventricular stretch at the end of diastole, while afterload is the pressure that the heart needs to work against during systole to pump blood into the systemic circulation. Preload is a volume and occurs during diastole, while afterload is a pressure and occurs during systole.