What is the Difference Between Preload and Afterload

The main difference between preload and afterload is that preload is the pressure that occurs at the initial stretching of the cardiac myocytes, whereas afterload is the force that occurs when the heart pumps blood.

Preload and afterload are two pressures that occur in the heart. Both occur at the ventricles of the heart. 

Key Areas Covered

1. What is Preload
– Definition, Process, Importance
2. What is Afterload
– Definition, Process, Importance
3. Similarities Between Preload and Afterload
– Outline of Common Features
4. Difference Between Preload and Afterload
– Comparison of Key Differences

Key Terms 

Afterload, Preload

What is Preload

Preload is the amount of sarcomere stretch on the cardiac muscle cell at the end of the ventricular filling. It occurs during diastole. The relaxed ventricle fills during the diastole. During filling, the walls of the ventricle undergo stretching. Also, it increases the length of the sarcomere. Additionally, the end-diastolic volume can define preload. It estimates the volume of a healthy, filled left ventricle. However, the volume of the right ventricle is difficult to calculate. The volume of the left ventricle, or the end-diastolic pressure, is important to determine the end-diastolic pressure. According to Boyle’s law, pressure is inversely proportional to the volume.

Figure 1: Diastole

Furthermore, the Swan-Ganz catheter can directly measure the end-diastolic pressure of the right ventricle. In the left ventricle, the end-diastolic pressure equals the pulmonary wedge pressure. Therefore, it is equal to the pressure of the left atrium. Also, the pressure of the left atrium and left ventricle are equal. Therefore, the pulmonary wedge pressure is equal to the left ventricle pressure in a healthy heart. This pressure is known as preload. 

What is Afterload

Afterload is the pressure produced by the heart to eject blood during systole. It is proportional to the average atrial pressure. Also, it is measured as millimeters of mercury (mm Hg). Importantly, afterload determines the cardiac output. Cardiac output is the product of heart rate and stroke volume. However, afterload equals the aortic pressure for the left ventricle and pulmonic pressure for the right ventricle. However, the systolic pressure should be greater than the systemic and pulmonary pressure to open the aortic and pulmonic valves, respectively.

Figure 2: Systole

Moreover, the quantitative determination of the afterload is determined by the wall stress of the ventricles. However, afterload is affected by many factors. Disease conditions can increase the left ventricular afterload, increasing blood pressure. For example, systolic hypertension increases the left ventricular afterload. Similarly, pulmonary hypertension increases the blood pressure on the right side of the heart. The natural aging process also increases the blood pressure.

Similarities Between Preload and Afterload

• Preload and afterload are two pressures that occur in the heart’s ventricles.
• They are important for the transport of blood in the heart.

Difference Between Preload and Afterload

Definition

Preload refers to the volume in a ventricle just before the start of systole. In contrast, afterload refers to the resistance the heart must overcome to open the aortic valve and push the blood volume into the systemic circulation.

Occurrence

Preload occurs before the filling of the ventricle, while afterload occurs after filling the ventricle.

Heart Phase

Preload occurs during the diastole, while afterload occurs during the systole.

Importance

Preload is the pressure in the left ventricle before it fills with blood, while afterload is the pressure after filling the left ventricle from blood.  

Conclusion

In brief, preload and afterload are two pressures that occur on the left ventricle. Preload is the contractile pressure on the left ventricle when it is not filled with blood. Afterload occurs when the left ventricle is filled with blood. It provides pressure to leave the blood from the left ventricle. Therefore, the main difference between preload and afterload is their occurrence. 

References:

1. PRELOAD, AFTERLOAD AND CONTRACTILITY. Preload, afterload and contractility. (n.d.). 

Image Courtesy:

1. “Heart diasystole” By Wapcaplet, Reytan. – Own work (CC-BY SA 3.0) via Commons Wikimedia
2. “Heart systole” By Wapcaplet, Reytan, Mtcv- Own Work (CC-BY SA 3.0) via Commons Wikimedia