In this explainer, we will learn how to describe the basic structure of the heart and explain how the heart is adapted for its function.
In an average person’s lifetime, their heart will beat over 3 billion times. This vital, hard-working organ pumps more than 2 000 gallons, or 7 500 litres, of blood in a day. Blood carries essential nutrients, gases, ions, cells, chemical messengers, fluids, wastes, and more throughout each part of the body. Failure of the heart to continuously and flawlessly carry out its function can quickly lead to serious illness or death.
The heart is a component of the circulatory system. As you can see in Figure 1, it sits near the center of the chest cavity, between the lungs, and it is surrounded by a tough, protective membrane called the “pericardium.” The heart pumps, or contracts: an action that pushes the blood through the vast network of blood vessels that make up the rest of the circulatory system. The heart itself is a complex organ. It is mostly made of specialized muscle tissue called cardiac muscle. This muscle is fed by arteries that provide a constant supply of blood rich in oxygen and nutrients. These nutrients will be converted into energy that allows the heart to keep contracting.
Example 1: Identifying the Primary Tissue of the Walls of the Heart
What type of body tissue are the walls of the heart primarily comprised of?
The heart is an organ in the circulatory system. It is responsible for contracting rhythmically, or pumping. This action pushes blood throughout the blood vessels that carry it to all the parts of the body. There are four main types of tissues in the human body. These tissues are the nervous tissue, connective tissue, epithelial tissue, and muscular tissue. Nervous tissue functions to carry impulses from place to place to facilitate rapid communication throughout the body. Connective tissue connects, binds, supports, and protects the structures of the body. Epithelial tissue lines the surfaces of organs and body structures. Muscular tissue functions to generate force that causes movement. We know that the primary function of the heart is to pump blood. We also know that it does so by contracting or beating, which generates the force that moves the blood through the blood vessels.
Using this information, we can conclude that the walls of the heart are comprised of muscle tissue.
The internal structure of the heart is divided into four chambers. There are two atria and two ventricles. The atria are thin-walled flexible chambers that receive blood as it enters the heart. The word “atrium” can also mean “entryway.” The two ventricles are thick-walled muscular chambers that pump blood out of the heart after it has passed through an atrium. Because the heart is divided into a left side and a right side, we call the four chambers the left atrium, right atrium, left ventricle, and right ventricle.
The heart also possesses four valves. The valves are special structures that prevent the blood from flowing backward. They ensure that the blood keeps moving in the right direction. There is a valve at the entrance and exit to each ventricle. The two valves that lead out of the heart are called the “semilunar” valves. The valve leading into the left ventricle is called the “bicuspid” valve (or mitral valve) because it has two flaps, and the one leading into the right ventricle is the “tricuspid” valve because it has 3 flaps. The flaps of the valves are attached by tendons inside the ventricles: 2 tendons for the bicuspid valve and 3 for the tricuspid. These tendons prevent the valves from turning inside out.
An atrium is a chamber of the heart with relatively thin flexible walls that receives blood as it enters the heart.
A ventricle is a chamber of the heart with relatively thick muscular walls that pumps blood out of the heart.
In diagrams, such as Figure 2, the right side of the heart appears on the left side of the diagram, and the left side of the heart appears on the right side of the diagram. This is because the heart is illustrated to appear as if it is in the chest of a person who is facing toward you.
Example 2: Identifying the Chambers of the Heart in a Diagram
The diagram provided shows a cross section of the human heart.
State the correct chambers indicated by the letters A, B, C, and D.
The heart is divided into a left and a right side. The sides appear on paper the opposite of what we might expect because they are diagrammed as if a person is facing out of the page, toward you. So, chambers A and D are on the right side of the heart, and chambers B and C are on the left. There are two types of chambers in the heart. There are thin, flexible atria. The atria receive blood as it enters the heart. There are also thick, muscular ventricles. The ventricles pump the blood out of the heart. The atria are located near the top of the heart, and the ventricles are located near the bottom.
This tells us that the labeled chambers are as follows:
A: right ventricle,
B: left ventricle,
C: left atrium,
D: right atrium.
You will notice that the right side of the heart is illustrated in blue and the left side of the heart is illustrated in red. These are common colors used to depict oxygenated and deoxygenated blood. As you know, our blood is never blue! Deoxygenated blood has a dark red color, while oxygenated blood tends to have a bright red color. This brighter red color comes from the fact that oxygen combines to the iron atoms contained in a specific molecule called hemoglobin in the blood.
The heart pumps blood rich in oxygen, also called oxygenated, to the tissues of the body where the oxygen is delivered to the cells that need it. The blood is then what we call “deoxygenated,” since most of the oxygen has been removed. The heart pumps deoxygenated blood to the lungs, where it absorbs oxygen from the air we inhale, as shown in Figure 3. There, the deoxygenated blood becomes oxygenated again.
The term oxygenated describes blood that has a high concentration of oxygen.
The term deoxygenated describes blood that has a low concentration of oxygen.
Our cells use oxygen to obtain energy from glucose through aerobic respiration. One waste product that results from this reaction is . A small proportion of all the produced by our cells can bind with hemoglobin in red blood cells, while the majority is transported out of the red blood cells in the form of bicarbonate ions in the plasma, which is the fluid that contains the blood cells and forms the blood. So, the deoxygenated blood pumped by the right side of the heart carries the and bicarbonate to the lungs where it is exhaled.
We can also tell by looking at the diagram in Figure 2 that blood within the heart does not pass between the left and the right side. In fact, the left side of the heart and the right side of the heart act as two parallel pumps. The left side of the heart pumps deoxygenated blood from the tissues of the body to the lungs to become oxygenated. The right side of the heart pumps oxygenated blood from the lungs to the tissues of the body where it becomes deoxygenated. Sometimes we refer to the pathway from the heart to the lungs and back as the “pulmonary circuit.” “Pulmonary” is a word that means “lungs.” The pathway from the heart to the body and back again is known as the “systemic circuit” since it feeds the body’s organ systems. These two circuits are illustrated in Figure 4.
Pulmonary means related to or referring to the lungs.
Systemic means having to do with the tissues and organs of the body.
The heart is connected to the rest of the circulatory system through four major blood vessels.
- The aorta is the arched blood vessel at the top of the heart. It carries blood from the left ventricle into the arteries that supply oxygenated blood to the tissues of the body.
- The venae cavae are a pair of large blood vessels vertically oriented on the left side of the heart. They collect deoxygenated blood from the veins of the body and transport it into the left atrium.
- The pulmonary arteries are a T-shaped set of blood vessels near the front of the heart. They carry deoxygenated blood from the left ventricle to the lungs.
- The pulmonary veins are a set of small blood vessels toward the rear of the heart. They carry oxygenated blood from the lungs into the right atrium.
Example 3: Identifying the Blood Vessels Connected to the Heart from a Diagram
The diagram provided shows a basic outline of the heart, with the major blood vessels indicated.
State the blood vessels indicated by each of the letters A, B, C, and D.
The heart is composed of 4 chambers that act as two parallel pumps. The right side of the heart pumps blood to the lungs. The left side of the heart pumps blood to the body. The upper chambers on each side, the atria, receive blood as it enters the heart and pass it on to the ventricles. The lower and more muscular chambers on each side, the ventricles, push the blood out of the heart and through the branched blood vessels that make up the circulatory system. On the right side, blood enters the heart through the vena cava, then exits the heart through the pulmonary arteries. On the left side of the heart, the blood enters the heart through the pulmonary veins, then exits the heart through the aorta.
This means that the labeled blood vessels are as follows:
A: vena cava,
B: pulmonary artery,
C: pulmonary vein,
If we only examine the systemic circuit, we might conclude that arteries always carry oxygenated blood and that veins always carry deoxygenated blood. However, this is not the case. In the pulmonary circuit, the veins carry oxygenated blood and the arteries carry deoxygenated blood. This is because, regardless of the oxygen content of the blood within them, arteries are the blood vessels that carry blood away from the heart and veins are the blood vessels that carry blood toward the heart.
A vein is a blood vessel that carries blood toward the heart.
An artery is a blood vessel that carries blood away from the heart.
Let’s imagine the path of a single red blood cell from the moment it enters the heart at the left atrium. Next, it will move through a valve into the left ventricle. Then, it will be pumped through another valve and into the aorta. The blood cell will encounter branches leading to smaller and smaller arteries until it drops off the oxygen it is carrying in the tissues of the body. The now deoxygenated red blood cell will enter larger and larger veins until it passes through the vena cava into the right atrium. From the right atrium, the blood cell will be pushed through a valve into the right ventricle, which will pump it through another valve and into a pulmonary artery. The pulmonary artery will branch into smaller and smaller vessels leading to one of the two lungs. The blood cell will absorb oxygen in the lung, becoming oxygenated. Then, it will be pushed into larger and larger vessels until it reaches the pulmonary veins and enters the heart at the left atrium again. The route blood takes through the heart and around the body is illustrated in the flowchart in Figure 6. A blood cell passes through this entire pathway in about one minute, on average.
Next, let’s learn how to calculate the cardiac output of the heart.
Cardiac output is a measure of the volume of blood the heart can pump in one minute. It is used to measure the heart’s performance. In order to calculate cardiac output, we need two values. We need to know the heart rate, or how fast the heart is beating. We also need to know the stroke volume, or how much blood the heart is pumping during a single beat. Cardiac output is calculated by multiplying the heart rate by the stroke volume. The equation is written out in the equation box, below.
Equation: Cardiac Output
How To: Calculating Cardiac Output
In order to calculate cardiac output, we must first be provided with the heart rate and the stroke volume for a patient.
Cardiac output, represented by “CO” or sometimes the variable “,” measures the volume of blood that the heart pumps in a minute.
The heart rate, represented by “HR,” measures the number of times a heart beats in a minute.
The stroke volume, represented by “SV,” measures the volume of blood that the heart pumps in a single beat.
Next, we apply the provided values to the equation:
For example, for a heart rate of 70 bpm, or beats per minute, and a stroke volume of 0.17 litres per beat, the calculations would be as follows: The beat units cancel out as follows: Multiply the values as follows:
Definition: Cardiac Output
Cardiac output is a measure of the volume of blood that the heart pumps in a minute.
Definition: Stroke Volume
Stroke volume is a measure of the volume of blood the heart pumps in a single beat.
Definition: Heart Rate (bpm)
Heart rate is a measure of the number of times the heart beats in a minute.
Example 4: Calculating Cardiac Output from Stroke Volume and Heart Rate
Calculate the cardiac output in cm3/min of a heart that has a stroke volume of 80 cubic centimetres per beat and a heart rate of 70 bpm, to the nearest whole number.
Cardiac output is a measure of the volume of blood that the heart will pump in a minute. In order to calculate cardiac output, we multiply the heart rate by the stroke volume. The heart rate measures the number of times that the heart beats in a minute. The stroke volume measures the volume of blood the heart pumps during a single beat.
The formula looks like this:
We can substitute the values provided in the question to get this equation:
By completing the calculations, we can conclude the following:
The heart is one of the most important organs in the human body. It has an essential function that it carries out alone for the duration of an entire lifespan. The heart is specifically adapted to carry out this function. Maintaining the health of the heart is essential to leading a long and healthy life.
Let’s review what we have learned about the heart in this explainer.
- The heart is composed of four chambers: two ventricles and two atria.
- Valves in the heart prevent the backward flow of blood.
- The four main blood vessels attached to the heart are the aorta, the vena cava, the pulmonary veins, and the pulmonary arteries.
- Blood travels through the circulatory system in two circuits: the pulmonary circuit where blood is pumped to the lungs and the systemic circuit where blood is pumped to the rest of the body.