Cardiovascular Physiology: Preload and Afterload

What is the difference between preload and afterload? This is one of most frequently asked questions and probably one of the most confusing topics that nursing students encounter during their lectures and clinical rotations.

To settle the confusion once and for all, we will provide you with the most uncomplicated answer.

The Slingshot Analogy

The heart is an intricate organ that causes quite a bit of bewilderment, especially with nursing students. And one of the topics that bring about such confusion is concerned with preload and afterload.

Consider this analogy – the heart is like a slingshot; a slingshot that requires pressure when pulling and energy upon release.

Preload: The Pulling Effect

The pulling action of the slingshot is called preload. Preload occurs so that there will be the pooling of blood that will either be pushed into the lungs or to the rest of the body. Preload is also referred to as the diastolic pressure inside the blood vessels.

Afterload: The Release

Afterload, on the other hand, is the action when the slingshot is released. Basically, afterload is the amount of pressure that the heart has to overcome to enter the next phase, whether the blood will go inside the lungs or the peripherals. Afterload is also known as the systolic pressure inside the blood vessels.

Quick Anatomy and Physiology

The heart has four chambers – right and left atria, and right and left ventricles. These chambers are also considered as rooms that have vital roles in the distribution and oxygenation of blood to various parts of the body.

Deoxygenated blood is received by the right atrium from the body and is pushed to the lungs by the right ventricle to be filled with oxygen. The left atrium will receive the oxygenated blood and will send it to the left ventricle. The left ventricle will distribute the oxygen-rich blood to the rest of the body.

Between the atria and the ventricles, the ventricles are considered as the essential rooms inside the heart because they’re responsible for the pulling and pushing of blood to the body.


So just remember this:

  • Preload – the stretching or pulling to fill the heart with blood
  • Afterload – the release or push of oxygenated blood by the left ventricle to the lungs and the rest of the body

That’s it for our simplified preload and afterload explanation that was brought to you by, the best student nurse website that has helped more 40,000 students all over the world.

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Cardiac Output: Preload and Afterload Part 2

Before going over the main topic – preload and afterload, we will first touch on cardiac output and stroke volume to get a clearer background as to how the mechanisms work.

Cardiac Output

Cardiac output is the amount of blood that is being ejected from the heart’s left ventricle to the aorta, then to the rest of the body in a minute. Normally, an average person has about five liters of blood circulating throughout the body’s systems. Therefore, if the pumping organ is compromised and there is no or insufficient cardiac output, blood that is responsible for transporting oxygen that sustains life, will decrease; thus, resulting in tissue death.

For instance, if a tourniquet is placed around the finger, blood supply is cut off. What follows is a series of manifestations for tissue death – turning pale, cold, and cyanotic. Cardiac output is the sum of a person’s heart rate times the stroke volume. But what is stroke volume?

Stroke Volume

Stroke volume is the amount of blood pumped in every beat – how much the heart is pumping in one clean push from the left ventricle. A typical example of how stroke volume is affected in cases of heart ailments is when left ventricular hypertrophy happens.

Left Ventricular Hypertrophy

For instance, if a client has a cardiac failure or congestive heart failure, there is increased pressure being back up from the rest of the body due to high blood pressure. The left ventricle is doing its best to keep up with the significant amount of pressure to pump out. When this happens, stroke volume is lessened because the left ventricle cannot pump all at once throughout the body. Swelling of the left ventricle occurs due to its effort of pushing against the resistance. Thus, the client is diagnosed with left ventricular hypertrophy. Remember, when stroke volume is compromised, cardiac output is compromised as well.

Brain Natriuretic Peptides

To measure the left ventricle’s hyperinflation, the client’s brain natriuretic peptides (BNP) are measured. Brain natriuretic peptide is compensatory mechanism released from the brain, to allow communication from the brainstem to the left ventricle, asking the heart to pump more blood to the rest of the body due to lack of oxygen. Inside the brainstem, there’s a vasomotor center that regulates and controls blood pressure.

The normal BNP is around 100; anything that goes beyond this number indicates a heart problem. Clients with congestive heart failure have a BNP of more than 300. Severe congestive heart failure clients can even reach over 25,000 of BNP.

Now that we’ve explained what cardiac output and stroke volume is, let’s go to preload and afterload.


Preload is the stretch in the ventricle, whether right or left. Ventricles stretch and squeeze to distribute blood adequately. However, if there’s too much pressure being backed up due to cardiac issues, the ventricles tend to stretch extensively, taking it longer to squeeze, resulting in an abnormal contraction.


Afterload is the degree of pressure inside the aorta to overcome the push of blood. Afterload is just a fancy term for how much pressure the ventricles need to exert during systole.

Going back to clients with congestive heart failure or even hypertension, the backing up of pressure will cause a wider stretch, increasing preload and afterload.

Decreasing Preload and Afterload

Nitroglycerin and Morphine are often given to bring down the preload and afterload; these two drugs, having a direct effect on the preload and afterload, are commonly given to clients with acute myocardial infarction.

Nitroglycerin dilates the pathways of the heart, relaxing the left ventricle. Morphine, a central nervous system opioid analgesic, also widens and relaxes the blood vessels by acting on the BNP of the brainstem.

Hopefully, we’ve shed some light on what is cardiac output, stroke volume, preload, and afterload, and how they are intertwined whenever there are heart issues like congestive heart failure.

The Pathophysiology of Cardiogenic Shock

Shock has different types, and the manifestations are different for every type. We’ve made a series of lectures talking about these types. Here, we’ll be talking about cardiogenic shock, what it really is, and how it differs from the other types of shock.

The important detail that you have to remember with cardiogenic shock is this: low pressure equals low perfusion.

For those who are unfamiliar with what perfusion is, it is the amount of oxygen being distributed around the body. Therefore, cardiogenic shock is decreased oxygen perfusion in the body.

Functions of the Heart

To fully understand what is happening with cardiogenic shock, we must first discuss, very quickly, the heart’s main function and how blood is pumped effectively in and out of the organ.

The heart’s primary function is to pump blood throughout the body. Blood is composed of a lot of components, and one of them is hemoglobin. Hemoglobin is a protein that holds oxygen and is responsible for transporting oxygen to different parts of the body.

Hematocrit is another blood component that is basically the liquid portion of the blood which helps hemoglobin to move around every system.

Decreased H&H

If there is decreased hemoglobin and hematocrit (H&H), there will be decreased perfusion since these two components are primarily tasked with transporting oxygen to all parts of the body. Insufficient hemoglobin and hematocrit are directly correlated with cardiac output.

H&H Ratio

If there is a decrease in hemoglobin, the hematocrit will also decrease because the two are intertwined with each other. If they go down, they go down together. The ratio is 1:3. One hemoglobin is equivalent to three hematocrit. For example, in your client’s laboratory values, if the hemoglobin is eight, the hematocrit will be 24.

Borderline H&H

If the hemoglobin value is eight or less, the client will immediately need a blood transfusion; therefore, the borderline value for hemoglobin around 10 g/dl.

Blood Flow

So, this is what happens with proper blood flow inside the heart and lungs:

The right side of the heart will push unoxygenated blood to the lungs to get oxygenated. Then, oxygenated blood will get pushed into the left atrium down to the bicuspid valve and into the left ventricle. The left ventricle is the area of the heart that pumps out all the blood to the rest of the body through the aorta. This is also called as the preload and afterload.

Stroke Volume

Now that we have a clearer picture of hemoglobin and hematocrit, the next thing that you have to know is the stroke volume. The stroke volume is referred to the amount of blood that comes out from the left ventricle in one pump. Stroke volume is composed of the preload and the afterload.

Question: How much blood is normally pushed out of the left ventricle?

Answer: About an ounce or two of fluid per stroke.

Preload and Afterload

Preload and afterload are phases of stroke volume. Preload refers to the time it takes for the left ventricle to be filled with blood, while afterload is the pumping of the blood out of the left ventricle. You can think of the preload and afterload as a slingshot. Preload is the pull while the release is the afterload. So, preload and afterload is equivalent to the stroke volume.

Cardiac Output

Cardiac output is simply the amount of blood being pumped out in 60 seconds or one minute. The normal cardiac output is between four to eight liters per minute. Cardiac output is also influenced by a client’s body build.

Blood Pressure

Blood pressure is a mixture of cardiac output (amount of blood coming out of the heart in a minute) and heart rate (how hard the heart pumps blood to get out of the heart).

Cardiogenic Shock

Knowing about cardiac output, stroke volume, and blood pressure will provide a better understanding of what happens in cardiogenic shock. Basically, cardiogenic shock is decreased pressure which equates to reduced perfusion.

Therefore, there’s decreased cardiac output which will result in decreased blood pressure. Instead of the normal cardiac output of four to eight liter per minute, there would be two liters per minute. And anything that’s less than two liters per minute can be categorized as cardiogenic shock.

Remember that shock, in any form, is decreased pressure which influences perfusion. If there is decreased perfusion, there will be decreased oxygen. A body that has decreased oxygen will become anxious and hypoxic, and the client will have really faint pulses.

Aside from those basic signs and symptoms, there are other manifestations that a client with cardiogenic shock will present. This will be tackled in our next lecture. Drop by SimpleNursing’s website and YouTube channel.

Cardiac Output: Stroke Volume, Preload, & Afterload Pt 5, with Mike Linares, is here once again to turn complicated, frustrating lectures into effortless, piece-of-cake study systems.

Right now, we will be discussing the following:

  1. Cardiac output (CO)
  2. Stroke volume (SV)
  3. Preload
  4. Afterload

Before we go into specifics, let’s first have a quick overview of how the heart functions regarding blood flow regulation.

Heart Regulation of Blood Flow

An average person has about five liters of blood that needs to be circulated throughout the body; therefore, as a pumping organ, if your heart cannot pump the required blood to the rest of the body, what happens? The body gets sick, gets impaired, or eventually dies.

The heart pumps blood throughout the body. Blood carries oxygen and plasma that helps infiltrate the veins and arteries, sustaining blood pressure, thereby sustaining life.

  • Insufficient blood flow = decreased oxygen distribution = tissue death

Think of it as putting a tourniquet around your finger, cutting the blood supply. Immediately, the finger starts to get pale, cold, and cyanotic. Technically, that’s what happens to all parts of the body if blood flow and oxygen are cut off.

Now that you have a better understanding of blood flow regulation by the heart, we go to our main topic.

Cardiac Output

Cardiac output is the amount of blood that’s ejected from the left ventricle, into the aorta of your heart, then out to the rest of the body in one minute.

                        Heart rate x Stroke volume = Cardiac output (one minute)

Stroke Volume

Stroke volume is the amount of blood in one clean pump. How is this seen or applied in the clinical setting?

A client with cardiac failure or congestive heart failure has increased pressure being backed up from the rest of the body because of high blood pressure, so left ventricle struggles to pump out blood to relieve pressure inside the heart. For this reason, stroke volume is decreased because the left ventricle is unable to pump blood efficiently.

Backing up of traffic (too much blood) causes the left ventricle to swell or inflate because it’s trying its hardest to push blood out, going against the resistance of high blood pressure.

  • Decreased stroke volume = compromised cardiac output = left ventricular hypertrophy

How Left Ventricle Hyperinflation is Measured

To determine left ventricular hyperinflation, the lab test of choice is the Brain Natriuretic Peptides (BNP).

When the cardiac output no longer sustains oxygen in the peripheral veins, the brain sends signals to the left ventricle.  Brain natriuretic peptides are compensatory mechanisms of the brain, communicating to the left ventricle, calling out its hyperinflation, and informing it that there is decreased oxygen level inside the body.

There’s a vasomotor center in the brainstem that controls blood pressure, the RAAS system of your kidneys, and the BNP. So BNP is basically telling the left ventricle, “Hey, we need you to take the pressure off.”

  • Normal BNP = less than 100

BNP as high as 300 or more is usually a sign of congestive heart failure (CHF).

Now, when the cardiac output is not meeting the required amount of blood in a minute, and the stroke volume is having a hard time getting pressure off from the heart because of too much resistance, that’s where preload and afterload come in.


Preload is, in simplest terms, the stretching of ventricles. So ventricles tend to stretch (fill with blood) and squeeze (push out blood). If there is too much pressure filling the ventricles, they tend to extend to the point of not having a proper contraction.

  • Too much stretch = unable to squeeze properly


Afterload is the degree of pressure inside the aorta to push or eject blood. Afterload is just a fancy word for the pressure required for the left ventricle to force blood out of the body. So, afterload is just the effort of the ventricle to squeeze. In cases of congestive heart failure (CHF) or hypertension, you have a back-up of pressure on the left ventricle causing it to stretch at great lengths causing a bigger preload and a struggling afterload.

How are increased preload and afterload managed in a hospital setting?

Clients with acute myocardial infarction (MI) are given nitroglycerin and morphine to bring down preload and afterload.

  • Nitroglycerin = relaxes smooth muscle to allow vasodilation
  • Morphine = a central nervous system (CNS) opioid analgesic that relaxes the heart

Hopefully, this was able to help you have a better grasp at one of the trickiest subjects of nursing.

For more useful tips and information, visit On this site, you can check out our Patho Bible – The Top 70 Diagnoses that are commonly seen in a clinical setting.

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