Video Transcript
In this video, we will explain the overall structure of the heart, trace the path that blood takes as it pumps throughout the body, and examine how the heart is adapted to its function. Then, we’ll learn how to calculate cardiac output before finally reviewing what we’ve learned.
While we popularly draw the heart shaped like this, yours actually looks a lot more like this. The heart is the main organ of your circulatory system. It’s responsible for pumping blood through your blood vessels to and from your lungs and the tissues of your body. The heart is the only place in your body where you’ll find cardiac muscle tissue. This amazing tissue allows your heart to be constantly, rhythmically, and automatically without tiring for your entire life span. Cardiac muscle is just one of several adaptations that allow the heart to carry out its important function, and we’ll learn more about these as we go along.
Now, we know this isn’t how your actual heart looks, but let’s use this simplified form for just a moment. The human heart is divided into four chambers. First, it’s divided into a left and a right side. You’ll notice that these are reversed from what you may expect. That’s because we diagrammed the heart as if it’s in the chest of someone who’s facing you. Each of the two sides are further divided into two chambers. The top chambers are called atria, the singular of which is atrium. And the lower chambers are called ventricles. One way to remember the difference is that atrium is a word that can mean entrance. Blood enters the heart through the two atria. And the ventricles in the lower part of the heart are V-shaped. V is for ventricle. So, the names of the four chambers are left atrium and left ventricle and right atrium and right ventricle.
To this simplified diagram of the heart, I’ve added a representation of the blood flow route. Here, we can note some important features. First, notice that blood does not flow between the left side and the right side of the heart. Within the heart, the blood is passed from the atrium to the ventricle on each side. This arrangement creates two distinct circuits or circular pathways: the systemic circuit where blood is pumped from the heart to the tissues of the body and back again and the pulmonary circuit, where blood is pumped from the heart to the lungs and back again. Pulmonary is a word that means lungs. In essence, your heart is two parallel pumping systems working in unison.
Now that we’ve learned the basics, let’s take a look at a diagram with a little more detail. So we’re already familiar with our four chambers. In this diagram, towards the top, we have the pulmonary circuit which includes the lungs. And towards the bottom, we have the systemic circuit which includes all the other tissues of the body. You’ll notice that we added the familiar red and blue colors to this diagram. The red color represents oxygenated blood or blood that has a high concentration of oxygen in it. The blue represents deoxygenated blood or blood that has a very low concentration of oxygen in it. It’s important to note here that all the blood in your body is actually red. We just used two different colors to denote the difference in oxygen concentration.
Also visible within this diagram are the four heart valves. Valves are special structures that keep blood flowing in one direction and prevent it from flowing backwards. The veins throughout your body also have valves inside of them. In this diagram, we’ll also take the time to label the four major blood vessels that enter and leave the heart. The first is the aorta. The aorta carries blood from the left ventricle to the arteries that supply oxygenated blood to the tissues of the body. In the tissues of the body, the blood drops off oxygen, which is needed to carry out cellular respiration, and then absorbs carbon dioxide, which has been generated by the cells as a byproduct of that process.
The blood then travels back to the heart through the veins of the body which converge in the vena cava. The vena cava are two large veins that carry blood back to the heart through the right atrium. This completes the systemic blood flow circuit. The blood moves from the right atrium to the right ventricle where it exits the heart again through one of the pulmonary arteries. The pulmonary arteries carry deoxygenated blood from the heart to the lungs. Our lungs are our primary organs of gas exchange. Here, the carbon dioxide that’s been carried from the tissues of our bodies is released into the air when we exhale. And oxygen is added from the lungs to the blood when we inhale. From the lungs, the freshly oxygenated blood travels back to the heart through one of the pulmonary veins where it enters the left atrium, is pumped into the left ventricle, and begins this journey all over again. On average, a blood cell can complete both these circuits from the heart to the body, the heart, the lungs, and back again in about one minute.
It’s also necessary to point out here that sometimes students get the mistaken idea that arteries only carry oxygenated blood and veins only carry deoxygenated blood. While this is true in the systemic circuit, the exact opposite is true in the pulmonary circuit. The correct distinction is that veins carry blood towards the heart, while arteries carry blood away from the heart. I like to try to recall this distinction by remembering that the A in arteries is the same as the A in the beginning of the word away.
Before we move on to our calculations, let’s see if we can correctly label the part of an anatomical heart diagram. Here, we have a cross section of an anatomical diagram of the heart. And in our blood flow route, I’ve added the common names of the four heart valves, in case you’re curious. So, let’s see if we can label the parts of the heart based on what we’ve learned. I like to start with the left ventricle. It has the thickest, most muscular walls because it has the most strenuous job, pumping blood from our heart through the tissues of our body, from the top of our heads to the soles of our feet and through every organ in between. The arched blood vessel at the top of the heart is the aorta. It connects the left ventricle to the arteries that supply oxygenated blood to the tissues of the body.
Here, on the right side of the heart, we find the vena cava. The vena cava carry deoxygenated blood from the tissues of the body into the right atrium. From the right atrium, blood flows into the right ventricle, just like in our previous diagrams. The T-shaped blood vessels close to the front of the heart are the pulmonary arteries. Deoxygenated blood travels through them to the left and right lungs. The smaller blood vessels we see on the back of the heart are the pulmonary veins. Freshly oxygenated blood travels through them from the lungs and back into the left atrium. From the left atrium, blood is pumped into the left ventricle, where it starts its journey all over again.
There, now, you learned to identify the four chambers of the heart, the four major blood vessels and to trace the flow of blood through the heart, the pulmonary circuit, and the systemic circuit of the circulatory system. That’s fantastic. Before we wrap up our lesson, we’ll learn to perform some calculations that help us to assess the heart’s performance.
As we’ve already learned, the heart’s function is to pump blood so that it circulates through the lungs and body, transporting important materials from place to place. In order to do that, the cardiac muscle of the heart contracts or beats rhythmically and constantly. Doctors sometimes assess the heart’s performance by measuring cardiac output. Cardiac output is a measure of how much blood your heart can pump in a minute. In order to calculate this value, we multiply your heart rate or the number of times your heart beats per minute by the stroke volume or the amount of blood pumped out of the heart with each beat.
Let’s demonstrate this by calculating the cardiac output of a typical biology student at rest and during moderate exercise. You may recall that all of the cells of your body need oxygen which they used to generate cellular energy during cellular respiration. The blood pumped by your heart is what delivers the oxygen from your lungs to the cells of your body that need it. Knowing that, compared to the heart at rest, would you expect cardiac output to increase or decrease while exercising? Absolutely, cardiac output should increase to supply more oxygen to your hard-working muscles.
Let’s see what the calculation say. First, we’ll calculate the cardiac output for our biology student at rest. From the table, we know that their heart rate is 65 beats per minute, and their stroke volume is 70 cubic centimeters per beat. When we multiply these values, the beats unit cancel out, leaving us with 4550 cubic centimeters per minute. Now, our biology student has been running for a little while. From the table, we can see that his heart rate is increased to 130 beats per minute, and his stroke volume has increased to 120 cubic centimeters per beat. Once again, we multiply those values together. The beats unit cancel out, giving us an exercising cardiac output of 15600 cubic centimeters per minute. As we expected, we see a significant increase over the cardiac output at rest.
Another interesting effect of regular cardiac exercise is that your heart becomes more efficient as you become more physically fit. An athlete will have a lower heart rate and a higher stroke volume than the average person, meaning that their heart is able to maintain the necessary cardiac output more easily. And that’s a great reason to get your daily cardiac workout.
Finally, let’s take a moment to summarize everything we’ve learned in this lesson. In this lesson, we learned about the root of the flow of blood through the heart. We learned about the four chambers, the four major blood vessels. And we learned about the mechanism and purpose of the gas exchange in the lungs and the tissues of the body. We reminded ourselves that deoxygenated blood may show up blue on a diagram, but it’s definitely also red. We learned that the word “pulmonary” is a word that means lungs. And we learned how to calculate cardiac output, given a person’s heart rate and their stroke volume.
