Preload, Contractility and Afterload

Preload

Have you ever purchased lotion in a push pump like one of these bottles? The viscosityThe thickness or resistance to flow in a fluid, such as blood. of the lotion refuses to be transported by the push pump. It’s like the lotion is too thick and the tube is too small. Oh, that makes me think of this place where I used to get milkshakes. They would make them extra thick, which I love. But then they’d give me this cheap straw that had a diameter smaller than a normal one. 

The push pump is taking longer than expected to refill. You can push it a second time during the refill process. However, you won’t get another full pump of lotion.  How much you allow the push pump to refill affects the volume of lotion you will get.  

This is preload.  Preload is the degree to which the heart can stretch in diastole. Since this is diastole, we can think of the end diastolic volume.  Preload and the EDV increase together.  Until a point.  Preload is basically how much the heart’s willing to fill.  More is not necessarily better.  There’s a perfect ratio of preload to tension (force). How much movie popcorn CAN you put into your mouthThe opening of the digestive tract where food enters and mastication begins. until there is so much you can’t even chew?  There is an optional amount of preload.

Can I digress for a minute.  I swear it will be a minute.  There is a principle called the Frank-Starling principle.  I’m sure when they discovered it, it changed the world. It was groundbreaking. Can I just summarize this principle for you?  Ok, here goes: the heart pumps out all the blood it receives.  BOOM!  Genius.  Who would have ever suspected that what is filled into the heart is then pumped out?  Actuallym it will become important when we consider exercise.

Contractility

The heart can pump faster, but we should call that an increased heart rate.  The heart can also pump “harder.”  When someone says this, it’s hard to know what they really mean, but they are probably referring to contractilityThe ability of muscle tissue to shorten with force..  Have you ever worked out with someone who was in way way better shape than you?  Let’s say that you go jogging with this person.  About 5 minutes into the jog, they look at you and start making small talk.  You can’t answer because you feel as if you are dying.  No, I’m kidding, you can’t answer because your heart is pumping very fast.  You have a really fast heart rate because you and your heart are out of shape.  Adjusting the heart rate is all your heart can do it increase your cardiac output.  Your stupid friend though, they are so in shape that their heart doesn’t increase the rate of contraction. Instead, it adjusts the force of contraction.  Their heart can do this because they have trained it to do so.  Yours is not trained.  You increase the amount of blood that leaves the heart in one minute by adjusting the rate. You can also adjust the force. 

So, your EKG would have QRS waves that get closer together.  You end up having more QRS waves in one minute because your heart rate has increased.  Your friend does not have an increase in heart rate. However, the height of their QRS waves will increase.  The QRS wave is a reflection of stroke volume.  This is how your friend is meeting the demand, by pumping more blood with each contraction.  Again, their heart can do that because of the cardiovascular training they have done.  Hey, working out pays.

Right. So it’s the amount of force that’s produced. So if you were answering the question and you want to say the heart pumps harder, don’t say that. Say the heart contracts with more force. You can say that the heart increases its contractility. Let’s use that term contraction contractility. Right. Let’s not say pump harder. That’s not specific. The QRS wave on the EKG. Remember, our EKG looks like this little P wave and the QRS T wave over here. The QRS wave indicates the tension or contraction of the heart. It shows how much force is generated with each round of systole.

Afterload

Students often get preload and after load mixed up.  Preload is about blood coming into the heart in diastole.  Afterload is about blood leaving the aorta and pulmonary arteriesBlood vessels that carry oxygenated blood away from the heart (except pulmonary arteries, which carr.  During the isovolumetric contraction phaseThe period of increasing muscle tension during a twitch., the myocardium does not change shape.  It is building tension for ventricular ejection.  How much tension does it have to build?  Enough to pop open those semilunar valves.  Those valves are closed.  They are preventing blood from leaving.  On the other side of these valves is the blood in the aorta and pulmonary arteries.  This blood is surging backward ever so slightly against these semilunar valves.  The pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. that blood is exerting on those valves is 120 mmHg or systolic pressure.  If the heart builds enough tension to exert 121 mmHg of pressure on those semilunar valves, the blood gets ejected. It moves from the ventricles into the arteries. 

Ventricular Septal Defect

We know that the left and right ventricles must eject the same volume of blood. However, the left ventricle ejects that blood with more pressure.  When someone with a ventricular septal defect tries to generate that pressure during the isovolumetric contraction phase, the left ventricle tries to create pressure. Instead of creating pressure, it squirts blood into the right ventricle. Blood meant for the aorta is not getting there. This blood would have been measured as part of the stroke volume of the left ventricle. Instead, it is entering into the right ventricle. The stroke volume and cardiac output for the left ventricle decreases.  You might automatically think that the right ventricle will clear that blood volume. No.  The right ventricle’s myocardium doesn’t have the ability to generate the pressure required to inject that volume of blood. With every heartbeat blood meant for the aorta is just entering into the right ventricle.  This additional amount of blood in the right ventricle prevents blood from the right atrium from being pumped. It also stops blood from the vena cava from reaching the pulmonary artery.  This generates systemic congestion as blood backs up into the vena cava and then veinsBlood vessels that return deoxygenated blood to the heart (except pulmonary veins, which carry oxyge of the systemic circuitThe part of the circulatory system that carries oxygenated blood from the heart to the body and retu. Over time, the right ventricle will add myocardium. It does this to generate the force required to clear the additional blood volume. Although it’s unlikely someone would get to that point without intervention, it is improbable for someone to reach that stage without intervention. The right ventricle’s additional myocardium will merely shift the fluid overload to the pulmonary circuitThe circulation of blood between the heart and the lungs, where blood is oxygenated..