Cardiac Cycle

Ventricular filling

I like to start with filling the heart called ventricular filling.  The AV valves are open and the SL valves are closed.  Blood is filling into the atria. It presses on the AV valves like a trap door. This opens them so blood can get into the ventricles and stretch and fill them. This process doesn’t fill the ventricles completely. This passive action of blood flowing back to the heart fills only about 80% of the volume of the ventricles. To get the remaining 20% into the ventricles, the atria have to contract.

Atrial systole

The red arrows indicate the contraction of the atria. They empty it and push the blood down into the ventricles on both sides of the heart. This is called atrial systole. It might be a little confusing because you’re thinking, “Wait, it’s contracting?” Yes, but it’s only the atria contracting to push the last 20% into the ventricles. A pacemaker node fires an action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission.. This node isn’t cardiac muscle but nervous tissue in the heart. It causes the atria to contract. The contraction pushes the rest of the blood down into the ventricles to fill them completely. This is indicated by the P wave on the EKG. Abnormalities with the P wave correspond to atrial issues.

Isovolumetric contraction

Still, we passively filled the ventricles. In this picture down here, we pushed the last remaining 20% into the ventricles. Now the ventricles are full, and usually, students say, “Oh, the ventricles will contract and push blood to the heart.” Not yet. This picture shows something called isovolumetric contraction. “Iso” means the same, and “volumetric” means volume. This means there is no change in volume. A contraction is happening, as indicated by the black arrows, but that contraction isn’t moving any blood. Instead, it’s building up enough pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. to pop open the semilunar valves. No blood is flowing; we are contracting the ventricles, and all valves are closed.

So, there’s no movementA fundamental property of life involving motion of the body or its parts., but we’re building up pressure. The heart has to generate pressure above 120 millimeters of mercury. Recall from what you know about blood pressure. The blood in your systemic arterial circuit has a pressure of 120. That pressure also pushes back against the heart when all of these valves are closed. That pressure is actually pushing back. The heart has to generate enough pressure to overcome this and push blood into the vessels. The number it needs to reach is 121 millimeters of mercury. Think of it as a doorway. The valve of the heart is like a doorway, and someone inside is pushing to get out. They have to generate enough force to push that doorway open.

Ventricular ejection

OK, so we filled the heart, we filled the ventricles, we’re building up pressure, and all valves are closed.  Once we build up more pressure than there is in the aorta, we can pop open those semi-lunar valves. We can then forcefully eject blood into the pulmonary artery and the aorta. This is what is called ventricular ejection. This is the big event of systole. The SL valves are open. We popped them open with our pressure. The AV valves are closed. Ventricular ejection is represented by this big, huge QRS wave here.   There is an abundance of myocardium when they contract. Their contraction is so forceful that it’s reflected by this big, huge QRS wave here on the EKG.  While the ventricles contract, the atria are actually starting in on diastole.  This is why we get such a big wave with all three of those points: Q, R, and S.  We have a lot of myocardium in the ventricles. They are having action potentials. However, not all of them have action potentials at the same time. This creates the spiked wave on the EKG.

Isovolumetric relaxation

The last thing that happens is that the heart starts to relax again. At first, all of the valves are closed.  Blood is returning from the veinsBlood vessels that return deoxygenated blood to the heart (except pulmonary veins, which carry oxyge into the atria.  When there is enough blood in the atria, the AV valves open like trap doors. Blood begins filling into the ventricles.  This is the last step in this sequence. It leads us to the first step. We described this with the ventricles passively filling.  The T wave on the EKG kinda shows this. However, it’s better represented by the interval between S and T on the EKG.  The ST interval is a sharp indicator for how much your heart is filling.  If you can’t fill, you can’t pump. Remember, some books start with this as step number one and then go to ventricular filling. I kind of like this story better this way. I don’t know.