Our heart has 4 chambers. 2 atria one one each side, called the Right Atrium (RA) and Left Atrium (LA). And the we have 2 ventricles, Right Ventricle (RV) and Left Ventricle (LV). There are 2 valves between the atria and ventricles known as right and left atrio-ventricular valves. They are more commonly referred to as Tricuspid (right) and Mitral (left) valves (TV and MV). There are two more valves that will be mentioned as the discussion progresses. But here is a very simple diagram showing all the chambers and the valves :

Image source

In my last post, I discussed about how impulse is generated from the SAN over and over again to make the heart beat.
Now what exactly is a heart beat? It's actually the alternating contractions of the atrium and ventricles. In that post, I had also mentioned that the heart muscle cells are connected electrically via gap junctions, which means once the impulse is generated, it'll spread to all the connected cells instantaneously making all of them contract at the same time. Now if that's the case, why don't the atria and ventricles contract at the exact same time? Great question if that popped up in your head!!

Between the atria and the ventricles we have the atrio-ventricular septum. This septum is a fibromuscular band that does not contract electricity. Thus the impulse that's traveling along the atria can not pass into the ventricle as freely as we might think. There is only one gateway for the impulse to travel from the atria to the ventricle : Atrio-ventricular Node (AVN). But that still doesn't explain fully why the atria contracts first and then the ventricles contract. To understand this, we need to know a property of AVN that is quite different from the rest of the heart cells. Through most cells, the impulse conduction is pretty rapid. But the AVN is known as a slow conductor, which means the conduction velocity is much slower through the AVN and the impulse reaches the ventricles slower than the instantaneous transmission we've seen from the SAN to the rest of the atria.

This slow down gives the atria just enough time to finish their contraction before the ventricles. This is absolutely necessary because if the atria and ventricles are both contracting at the same time, that would be very counter-productive and the blood wouldn't really be able to move around efficiently anywhere. Imagine you're trying to squeeze out toothpaste from the tube and you press on both ends of the tube. Would you be able to squeeze any significant amount out? I didn't think so!!

After the impulse has crossed the AVN, it is rapidly conducted all over the ventricle via the Bundle of His - Right and left bundle branches - Purkinjie fibers. These are all very fast conductors and ensures, like the 2 atria, both the ventricles contract simultaneously as well.

Image source

Before we proceed, when it comes to cardiac contraction and relaxation, the technical terms are Systole and Diastole. But to make this post more understandable to a more wider audience, I'll stick to the terms contraction and relaxation.

2 different circulations

Our body in fact has 2 different circulation and there is no direction connection between the two. The Pulmonary circulation is involved with the blood going to the lungs and picking up oxygen, which will be supplied to the rest of the body via the Systemic circulation.

image from Kaplan USMLE Step 1 Physiology Lecture Notes

The Cardiac Cycle

First we'll take a quick look at the direction of blood flow within the heart and then we'll take a more detailed look and introduce the valves involved and their opening and closing at different times of the cardiac cycle.

Now, remember the 2 atria are contracting simultaneously, so are the 2 ventricles. But for the sake of simplicity and understanding we'll look at each chamber separately.

The LV is the chamber that pumps blood full with oxygen to the body via systemic circulation. The body tissues on receiving the blood, uses the oxygen for their designated functions. As, the oxygen is used up, the oxygen content of the blood keeps decreasing and needs to be refilled. This deoxygenated blood returns to the right side of the heart via Superior and Inferior vena cava.

The blood will first go in to the right atrium. The atrium will contract and push blood into the RV. From the right atrium, the blood goes to the RV via the tricuspid valve. After the RV has filled up with the blood from the RA and the RA has finished contracting, the impulse has now reached the ventricles and the ventricles begin to contract. (Again, remember that both the atria has contracted by now and both the ventricles will start contracting now, but we're looking at them one by one).

The blood from the RV has to exit via the Pulmonary artery to enter the pulmonary circulation. Now, as the ventricle begins to contract, you might raise the question, well why doesn't the blood just go back into the RA via the TV? That's an excellent question. As the pressure inside the RV keeps rising while contraction, there reaches a point where the pressure in RV exceeds the pressure in RA and at this point blood can potentially move back from the RV to the RA. But all the 4 valves of the heart are one way valves. So they are designed in such a way, that when the pressure in RV has gone over the pressure in RA, the TV will snap shut automatically and no blood can flow back into the RA. In the following image, the dotted lines represent atrial pressure and the solid curve represents the ventricular pressure.

image from Kaplan USMLE Step 1 Physiology Lecture Notes

This is the point where the TV closes. At this point the valve between the RV and pulmonary artery is still closed, because pressure inside the pulmonary artery is still higher than the RV pressure and if the valve was open, the blood from pulmonary artery would backflow. No backflow is allowed in the heart!!

The pressure keeps rising as the ventricles keeps contracting and at one point, the pressure inside the RV exceeds the pressure in the pulmonary artery and the pulmonary valve opens.

image from Kaplan USMLE Step 1 Physiology Lecture Notes

Blood is now ejected out from the RV into the pulmonary circulation. After the ventricular contraction finishes, the ventricles starts to relax. For a while the pressure inside the ventricle is similar to pulmonary artery pressure, so blood can't flow back into the ventricles, but the moment the pressure inside the ventricles fall below the pulmonary artery pressure, the pulmonary valve snaps shut to prevent backflow.

image from Kaplan USMLE Step 1 Physiology Lecture Notes

As the ventricles keep relaxing, the pressure keeps falling, but pressure inside RV is still higher than the RA, so the TV stays closed. Note here that, during the ventricular contraction, ALL the blood that was present is NOT ejected out into the pulmonary circulation. Some blood stays back and if the TV was open at this point, it could still backflow. Once the pressure drops below the atrial pressure, the TV will open and the cycle repeats.

image from Kaplan USMLE Step 1 Physiology Lecture Notes

Now let's continue our journey from the pulmonary artery. The Pulmonary artery takes the blood to the lungs where it is oxygenated, i.e. takes up fresh oxygen. The oxygenated blood will leave the lungs via the pulmonary veins and enter the Left Atrium. And now a very similar story to what we've seen in the right side, repeats itself. So I won't repeat the pictures again, I'll just quickly go over it. You can try to go back to the images and see if you can figure out which image fits which part of this description!! This will be a quick test to see if you really understood what's happening. If you're having trouble, leave a comment and i'll explain with the pictures. But it's pretty simple really, and I'm sure you can figure it out!!

Okay so as the left atrium contracts, blood goes into the LV via the mitral valve. After the LA has finished with it;s contraction, the LV starts to contract and as the pressure crosses the pressure in the LA, the mitral valve closes. The LV keeps contracting until the pressure goes above the Aorta. Aorta is the vessel via which blood from the left ventricle exits the heart to enter the systemic circulation. Once the pressure exceeds the pressure of the aorta, the aortic valve will open, causing ejection of the blood from the LV. Then the LV will start to relax. For a while, the pressure in the aorta and LV will fall simultaneously and be same. Once the pressure of the LV falls below that of the aorta, the aortic valves will close preventing blood flowing back into the LV. The LV pressure keeps falling, but for a while, it's still higher than the pressure in the LA. Once the pressure falls below that of LA, the mitral valves open and the cycle repeats itself, over and over and over

Pressure volume loop

Now we'll look at how the volume of blood within the left ventricle is changing along with the different stages of the cardiac cycle. But essentially the same in happening in RV too. This will also give us a final overview of the cardiac cycle.

image from Kaplan USMLE Step 1 Physiology Lecture Notes

Let's start from the position where the mitral valve closes. Do you still remember when the mitral valve closed? Yes, it closed when the pressure in the LV went above the pressure in the LA. Now recall that, after this, the LV is still contracting and the pressure is still increasing. The aortic valve is still closed, so even though the LV is contracting, noe blood is being pumped out. The volume stays the same. This part of the cardiac cycle is known as isovolumetric contraction.

Once the aortic valve opens, blood is ejected out, and this is the ejection phase. After LV has finished contraction, the pressure starts to fall and once the pressure falls below the pressure in the aorta, the aortic valve closes. The LV continues to relax and the pressure continues to fall. But it's still higher than the LA and the mitral valve is still closed. So, even though the LV is relaxing, no blood is entering the LV and the volume of the LV isn't increasing. This part of the cardiac cycle is known as the isovolumetric relaxation.

Once the mitral valves open, blood from the LA will start filling up the ventricle. This part is the filling phase of the cardiac cycle and the volume of the LV increases, and once the ventricles start contracting, the loop repeats.

Sources :
Kaplan USMLE Step 1 Physiology Lecture Notes, 2013
Guyton and Hall Textbook of Medical Physiology, Twelfth edition
Atrioventricular Node
Phases of the cardiac cycle

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