The key difference between preload and afterload is that preload is the amount of stretch during diastole when the ventricles fill with blood while afterload is the pressure against which the heart must work to eject blood during systole.
Stroke volume is one of the measurements that tells the amount of the blood pumps from each ventricle at each heartbeat. In simple words, it is the difference between the end-diastolic volume (EDV) and the end-systolic volume (ESV). End-diastolic volume is the filled volume of ventricle before contraction while end systolic volume is the volume of blood remaining in the ventricle after ejection. In a healthy individual, stroke volume is about 70 ml. Moreover, three prime factors regulate the stroke volume; they are the preload, afterload, and contractility. Preload is a volume while afterload is a pressure. Preload is the volume of ventricles at the end of the diastole. On the other hand, afterload is the pressure that needs to open the aortic valve to eject blood from the ventricle. This article aims at discussing the difference between preload and afterload.
What is Preload?
Preload, also known as end-diastolic volume, is the amount of blood in the ventricles at the end of the diastole. In simple words, it is the ventricular stretch at the end of the diastole. It relates with the ventricle filling or the ventricular end-diastolic volume and occurs before contraction of the heart. The heart gets prepared for a big squeeze at this moment. Preload directly interferes the stroke volume. When preload increases, stroke volume goes up. Increased preload occurs due to heart failure, renal failure, anaemia, pregnancy, etc.
On the other hand, decreased preload occurs due to diuretics, shock, haemorrhage, vasodilators, etc. Some factors affect the preload. They are venous blood pressure and the rate of venous return.
What is Afterload?
Afterload is one of the three factors that affect the stroke volume of the heart. It is a pressure or a force. Afterload can be defined as the pressure required in opening the aortic valve in order to eject blood from the ventricle. Generally, system and the blood in the aortic valve exert a pressure on the aortic valve. Hence, it remains closed.
During the systole, it is necessary to open the aortic valve in order to pump blood from the ventricle to other parts of the body. Therefore, the pressure is created to overcome the existing pressure from the other side. It is the afterload.
Furthermore, two things affect the afterload. They are systemic vascular resistance and pulmonary vascular resistance. Hence, afterload always should be greater than these two types of resistance to open the valves to eject blood from the ventricles. When the afterload is low, heart pumps more blood to the systemic circulation.
What are the Similarities Between Preload and Afterload?
- Preload and afterload are two major parameters that relate to the efficiency of our heart.
- They affect the stroke volume thereby affect the cardiac output.
- Hence, preload and afterload affect the overall function of the heart.
What is the Difference Between Preload and Afterload?
Preload and afterload are two factors affecting the stroke volume. Preload is a volume. Clinically, it is the end-diastolic volume which is the amount of blood in the ventricles at the end of the diastole. On the other hand, afterload is the pressure that generates by the heart in order to open the aortic valve and pump blood from the ventricles. Thus, this is the key difference between preload and afterload.
Moreover, a further difference between preload and afterload is that preload depends on the amount of ventricular filling while afterload depends on the arterial blood pressure and vascular tone. Besides, preload occurs during the diastole while afterload occurs during the systole. So, we can consider this is also as a difference between preload and afterload.
Below infographic on the difference between preload and afterload shows these differences comparatively.
Summary – Preload vs Afterload
Preload and afterload are two of the three primary factors that directly affect the stroke volume of the heart or the amount of blood pumps by the heart at each heartbeat. In summarizing the difference between preload and afterload; preload is the ventricular stretch at the end of the diastole. Clinically, it is the end diastolic volume. On the other hand, afterload is the pressure or the force that the heart must generate to overcome the resistance on the aortic valve to eject blood during systole. Both preload and afterload determine the efficiency of the heart.
1.“Afterload.” Wikipedia, Wikimedia Foundation, 23 Apr. 2018. Available here
2. Vincent, J L. “Understanding Cardiac Output.” Current Neurology and Neuroscience Reports., U.S. National Library of Medicine. Available here