Chapter 4: The Cardiorespiratory System

Fitness professionals must have basic working knowledge of the cardiorespiratory system for a number of reasons. Program design principles are all based on the underlying anatomy of the human body. As such, knowledge of anatomy allows fitness professionals to make better programming decisions, and explain those decisions more effectively with the client.

Furthermore, anyone working with clients must understand the acute and chronic responses of the cardiorespiratory system to ensure they can safely train clients, monitor intensity, and respond to any emergencies.

This chapter covers the basic anatomy and function of the cardiorespiratory system and its responses to exercise.

Introduction to the Cardiorespiratory System

The cardiorespiratory system (CRS) plays an integral role in meeting the demands of the body. The cardiorespiratory system consists of two main pieces: the pulmonary system, which consists of the airways and lungs, and the cardiovascular system: the heart, blood vessels, and blood.3

In totality, the cardiorespiratory system pumps oxygenated, nutrient-rich blood through miles of blood vessels. Tissues take in the nutrients and oxygen from blood while excreting waste, which is then transported by the blood for excretion.

During physical activity, the increased demand for oxygen can prompt the cardiorespiratory system to work harder by increasing breath, heart rate, and blood flow. Exercise can elicit a physiological response to the CRS so that it meets the needs of the activity.

Components of the Cardiorespiratory System


the human heart

The heart is located within the mediastinum. This is a cavity within the thorax. It rests just above the diaphragm, posterior to the sternum. About two-thirds of the heart lies to the left side of the sternum line. The heart is enclosed by a double-walled sac called the pericardium.3

The superficial sac is the fibrous pericardium. This is a tough layer that anchors to surrounding structures and protects the heart. The serous pericardium is a thin, slippery layer that acts as a lubricant to prevent sticking.

The fluid used between these two layers is called serous fluid. This fluid helps the heart operate in a low-friction environment. The three layers of the heart are the epicardium, myocardium, and endocardium.

The epicardium is the layer of serous pericardium. This layer can be replaced with fat as individuals get older.

The myocardium is considered the muscle of the heart, while the endocardium lines the inside walls of the heart and blood vessels.

Cardiac muscle forms the bulk of what is considered the heart. Cardiac muscle is much different than skeletal muscle. Myocardium cells are held together by connective tissue that crisscrosses.

The muscle is arranged as bundles that connect the entire heart. This ensures that when the heart contracts, the most amount of blood possible is ejected from the heart.

Lastly, the endocardium covers the entire inside walls of the heart, including the valves, chambers, arteries, and veins.

Chambers and Vessels

The heart consists of four chambers: the left and right atria and left and right ventricles.

The chambers are separated by flow specific valves that open and close in perfect coordination during each cardiac cycle.

The atria are the chambers of the heart that receive blood. They are small appendages that are somewhat smooth on the outside.3 The right atrium has bundled up muscle tissue that forms ridges called pectinate which look like small comb bristles.

The two vessels are smaller and not as muscular as the ventricles because they only need to push blood downward into the ventricles. The ventricles on the other hand need to push the blood out to the entire body and to the lungs. Blood enters the right atrium via three veins: the superior vena cava, the inferior vena cava, and the coronary sinus.

The superior vena cava returns blood from the body region that is above the diaphragm. The inferior vena cava returns blood from the body region that is below the diaphragm. Lastly, the coronary sinus collects the blood that drains from the myocardium.

The ventricles are known as the discharging chambers. The ventricles are lined with trabeculae carneae, which look like irregular ridges. The walls of the ventricles are much thicker than the atria. The right ventricle pumps blood into the pulmonary trunk which takes the blood to the lungs. This is where carbon dioxide is offloaded and oxygen is loaded onto the hemoglobin. Hemoglobin is located on a red blood cell (erythrocyte).


Blood flows from the atria to the ventricles in one specific direction which is made possible by the four valves within the heart. These valves open and close in response to different pressures on either side.3 The AV or atrioventricular valve is located where the atria meet the ventricle. The right AV valve is known as the tricuspid valve.

The AV valve has three flaps of endocardium and connective tissue to give it rigidity. The left AV valve has two cusps and is also referred to as the bicuspid or mitral valve.

Attached to the flaps of the AV valves are the chordae tendineae. These are also known as the heart strings. These strings anchor the cusps and connect to the papillary muscle, which is two bands of muscle that protrude from the ventricle wall.

The aortic and pulmonary valves, also known as the semilunar valves, are located at the base of the aorta and pulmonary trunk and prevent backflow.3 Each valve has three cusps and is shaped like a half moon. Like the AV valves, the semilunar valves open and close in response to intraventricular pressure.

Because of the way blood flows through the heart, the right ventricle only pumps blood to the lungs for oxygenation. Since the lungs are in close proximity to the right side of the heart, the muscle wall of the right ventricle is thinner than that of the left ventricle.

The left ventricle has a very thick wall to sustain high pressures. This is because the left ventricle must pump the blood through the mitral valve into the aorta, ultimately dispersing the blood to the entire body.


The heart is equipped to depolarize and contract even when outside the body. Nerve fibers innervate the heart to alter its rhythm based on oxygen needs.3 Heart rhythm can also be altered when the autonomic nervous system elicits its fight or flight response.

Specific cells within the heart called cardiac pacemaker cells can depolarize using potassium and sodium ions (K+ and Na+). The Sinoatrial (SA) node is also known as the pacemaker. The SA node generates impulses that are sent to the AV node.

The atrioventricular node or AV node then receives the signal about 0.1 seconds later. This delay is critical in heart contraction because it allows the atria to contract and then enables the ventricles to contract. After the signal is received at the AV node, it then goes to the AV bundle. This branches out into the myocardium. The signal then travels into the subendocardial network which depolarizes the cells and gets them ready for a contraction.

Electrocardiogram (ECG)

An electrocardiogram (ECG) is a graphic representation of heart activity. An ECG shows the action potential that is generated by nodes.3 It does not show the physical actions that are happening in the heart.

An ECG appears visually as lines on a graph that go up and down. There are three waves that are very distinguishable in a normal ECG. The first wave is the P wave.

This typically happens when the atria depolarize. After that there is the QRS complex. This is typically the large, tall section of the line. This is what happens on an ECG when the ventricle depolarizes and then proceeds to contract. The last part of the wave is the T wave. This is when the ventricle repolarizes.

Cardiac Output

Cardiac output or CO is the amount of blood pumped out of the heart in one minute. To find out what the volume per minute (CO) is take the total volume per stroke, and the stroke volume, and multiply it by the heart rate.

The formula is as follows: CO = HR X SV.

Notice that cardiac output is dependent on heart rate and stroke volume. If one variable is manipulated then cardiac output will change. Cardiac reserve is the difference between cardiac output maximally and at rest.3

Stroke volume is found by taking the end diastolic volume and subtracting the end systolic volume. Diastolic volume is the amount of blood left in the heart in a resting state. Systolic volume is the amount of blood left in the ventricle after it has contracted.

The contraction phase happens when blood is ejected and is known as systole. The relaxation phase, where the chambers refill with blood, is called diastole.

It should be noted that there’s a small amount of blood left in the ventricles after the heart has contracted called end-systolic volume.


Blood is the bodily fluid that runs through the cardiovascular system and delivers nutrients and removes waste, among other functions.

Blood is composed of three main parts.3

The first is plasma, a fluid that consists primarily of water with proteins, mineral salts, fats, sugars, hormones, and vitamins dissolved or contained in it as well. Plasma is the fluid that transports the other components of blood, which make up around 55 percent of blood volume.

Red blood cells, or erythrocytes, make up 44 percent of total blood volume and are the densest component in blood. White blood cells (leucocytes) and platelets (thrombocytes) make up most of the final 1-2 percent of blood volume.

Blood has many functions in our bodies including:

  • Transporting oxygen and nutrients for uptake in cells
  • Removing metabolic waste for excretion
  • Regulating body temperature
  • Preventing further blood loss via clotting
  • Preventing infection via antibodies and cells carried in blood

Red blood cells are also called erythrocytes. These are microscopic discs that use hemoglobin to transport O2 and CO2. Red blood cells are made within red bone marrow that is stimulated by the kidney.

Finally, there are white blood cells, which are the body’s defense against bacteria. These cells increase during bacterial infections.

White blood cells are also attracted to inflammation.

The following are a few of the major specific functions of blood:

  1. Blood delivers oxygen and nutrients from both the lungs and digestive track.
  2. The blood oversees transporting metabolic waste products from cells to the lungs or kidneys which will then be excreted as urine.
  3. Blood helps maintain homeostasis by maintaining body temperature. This occurs as blood brings heat from inside the body towards the skin for better dissipation into the environment.
  4. Blood transports hormones from organs to the target destination.
  5. Blood helps maintain normal pH levels in body tissue by providing buffers to keep things from getting too acidic.
  6. Blood helps prevent blood loss from wounds by initiating clot formation.
  7. Blood helps prevent infection by delivering antibodies and white blood cells to infected tissue.

Blood Vessels

There are three major types of blood vessels: arteries, capillaries, and veins.

As the heart contracts, it pumps blood into large arteries that leave the ventricles of the heart.3 After the blood leaves the main arteries, it is pumped into smaller arteries, which are called arterioles. Arterioles then feed into the capillary beds, which are found in the organs and tissues.

Once the oxygen and carbon dioxide exchange takes place, the blood travels to the venules. Venules are considered the smallest veins. As the blood travels toward the heart, it merges into larger and larger veins. Once the deoxygenated blood meets the heart, it enters the atrium.

Every artery and vein consist of a tunica intima, which is the inner layer of the vessel, tunica media (found in the middle), and tunica externa (the outermost layer).

One major structural difference between veins and arteries is that veins contain valves to prevent the backflow of blood into the tissue and encourage blood to travel toward the heart. Arteries do not need valves because the pressure from the heart contraction pushing the blood towards the organs and tissues prevents any backblow.

Healthy arteries are able to handle the intense pressure of blood being pumped from the heart. 

Arteries can develop issues as a result of lifestyle factors and genetics, which results in plaque buildup and hardened arteries. Also known as atherosclerosis, the hardening of the arteries poses serious health risks, including stroke and death.

Some common causes of the start of atherosclerosis include blood-borne chemicals, hypertension, bacterial infections, and smoking. Once the arterial wall is damaged, it forms a fatty streak, which then turns into a fibrous plaque and then the plaque can become unstable. Once the plaque becomes unstable it may rupture.

Atherosclerosis is especially problematic for someone with hypertension. The hypertensive pressure beats on the plaque and can cause it to come loose from the wall. This commonly results in the formation of a blood clot. Atherosclerosis accounts for about half the deaths in the developed world.

Lungs & Respiratory Pump Structures

The respiratory system consists of the airways, lungs, and respiratory muscles and provides oxygen to the body while expelling carbon dioxide.

The respiratory system has four major functions: providing pulmonary ventilation, external respiration, transport of respiratory gasses, and internal respiration.3

Pulmonary ventilation (breathing) is the process of air moving in and out of the lungs.

Air is first drawn through the nostril down through the nasal cavity, then it travels to the pharynx.

The pharynx leads into the trachea, then the carina of trachea, which splits the airway and leads air to the left and right bronchi.

External respiration is the process of oxygen diffusing into the blood and carbon dioxide diffusing into the lungs. Once oxygen diffuses into the blood, the cardiovascular system takes over to transport the oxygen to the specific tissue where it is needed.

Lastly, internal respiration happens when oxygen diffuses from the blood to the tissue and carbon dioxide diffuses from the tissue to the blood. The main anatomical structures that are part of the respiratory system include the nose, the nasal cavity, the paranasal sinuses, the pharynx, larynx, trachea, bronchi, and the alveoli.

The respiratory system is split into an upper and lower section. The lower respiratory system starts at the larynx and ends at the alveoli. The alveolar sac is where the capillaries line the sac to get blood as close to the air as possible. The alveoli are the location in the human body where gas exchange from the blood to the air takes place.

Muscles of Respiration

The diaphragm is one of the main muscles used in inspiration. The diaphragm has a natural cone shape. Inspiratory muscles called intercostals are located between the ribs which help lift the rib cage and lower the rib cage during expiration.

During inspiration, the diaphragm is drawn downward, and the rib cage is drawn upward. As the thoracic cavity volume increases, the pressure inside the lungs decreases. Air then flows into the lungs because of the pressure gradient.

When someone breathes out, the inspiratory muscles lower the rib cage by relaxing, and the diaphragm moves inferiorly. This reduces the lung cavity volume which then raises the inside lung pressure, forcing the air out of the lungs, through the respiratory tract, and into the environment.

Cardiorespiratory System Function

To initiate the entire process of delivering oxygen to the body, the diaphragm contracts and moves downward. The extra space created by the diaphragm contraction also increases the lung capacity.

This increase in capacity decreases the pleural pressure in the lung. Because air moves from high pressure to low pressure, air from the environment moves into the lungs.

Air then passes through the nostril, enters the nasal cavity, and gets passed down the pharynx, through the larynx and then into the trachea. Once through the trachea, the air is passed into the right and left lungs by the bronchi and bronchioles.

The air ultimately ends up in the alveoli, which are terminal branches of the lung with thin membranes separating the air from blood in the capillary bed of the lungs. The alveoli are grouped into alveolar sacs, and capillaries wrap around each alveolus.

Oxygen from the air then diffuses across the membrane and into the blood, while carbon dioxide diffuses into the lungs to be exhaled.

Air passes into the bronchioles, which end in a terminal bronchial and a respiratory bronchial. Air passes into the alveolar duct, which fills the alveoli. A group of alveoli is called an alveolar sac. This is where the capillaries are wrapped around each alveolus.

The oxygen has been extracted from the air and diffuses across the membrane onto a red blood cell. Once the red blood cell leaves the capillary beds of the lungs, it travels through pulmonary veins which then leads into the left atrium.

Note that pulmonary veins contain oxygenated blood which flows into the left atrium, unlike normal veins which take deoxygenated blood from the body into the right atrium.

Once in the left atrium, the blood travels through the mitral (bicuspid) valve. It flows to the left ventricle and then when the heart contracts the blood is pushed through the aortic valve which leads to the aorta.

The aorta and other supporting arteries are considered elastic arteries or conducting arteries. The elastic arteries then flow into muscular arteries or distributing arteries. Once in the muscular arteries, the blood then flows into the arterioles.

Arterioles are the smallest artery branch. The blood moves through the arterioles into the capillaries, which are arranged in capillary beds.

Capillaries are very thin and in some cases one endothelial cell makes up the circumference of the capillary wall. This is where the red blood cell delivers its oxygen because of the oxygen gradient across the cell membrane.

Red blood cells have a natural affinity for carbon dioxide, which gets loaded onto the red blood cell to replace the oxygen, with some carbon dioxide dissolving directly into the blood.

The blood then carries red blood cells heads from the capillaries to the venules when leaving the tissue. Venules form veins that get larger as they get closer to the heart. Veins do not have as much smooth muscle as arteries therefore veins rely more on pressure gradient.1

Once the deoxygenated blood reaches the heart it enters through the superior vena cava or the inferior vena cava. The superior vena cava collects all the deoxygenated blood from the upper body. The inferior vena cava collects all the deoxygenated blood from the lower half of the body.

The blood flows into the right atrium and then through the tricuspid valve which brings the blood into the right ventricle.

Once the blood is in the right ventricle it is then pumped through the pulmonary valve which then enters the left pulmonary artery and the right pulmonary artery.

The pulmonary arteries head back to the lungs where carbon dioxide is exchanged with oxygen via the alveoli to start the cycle again.

Cardiorespiratory System Responses to Exercise

As stated above the SA node and AV node can initiate the electrical impulse and thereby cause the heart to beat. All of this keeps the heart beating at a consistent rate. The sympathetic and parasympathetic nervous system as well as a few hormones can speed up or slow down the heart rate.1

The sympathetic nervous system stimulates the release of catecholamines: epinephrine and norepinephrine. These act to increase SA node activity which will increase heart rate.

The parasympathetic nervous system stimulates the vagus nerve which releases acetylcholine. This is a hormone that has a depressing effect on the SA node, which decreases its firing activity. With a decreased firing activity this will decrease the heart rate. A decreased heart rate is known as bradycardia. An increase in heart rate is known as tachycardia.

Typically, high performing athletes have a low resting heart rate. In the clinical sense, when a patient has a low heart rate it can be an indication of some sort of cardiac problem.1 In a healthy athlete a low resting heart rate can indicate increased levels of efficiency on the heart, nervous system, and lungs.

Acute Responses to Exercise

During acute bouts of exercise oxygen consumption (VO2) increases to meet the high demands of working muscle and other tissues.1 As exercise intensity increases there will be a greater demand for energy and therefore a greater demand for oxygen.

As mentioned above, cardiac output is the product of heart rate and stroke volume. To meet the demands of an acute exercise bout, the body must increase cardiac output.

During the early stage of exercise, heart rate and stroke volume will increase to increase cardiac output. In this state, most of the circulating blood is diverted from things like digestion to working muscles where it’s needed most in that moment.

Chronic Adaptations to Exercise

If an individual stays compliant with an exercise program for a long period of time they may experience long-term adaptations to the cardiorespiratory system.

With a prolonged cardiorespiratory exercise program, adaptations include increased stroke volume, higher cardiac output, a decrease in systolic and diastolic blood pressure, and an increase in left ventricular muscle mass.1

It should be noted that the respiratory system is not usually a major limiting factor in increasing exercise efficiency. This is because, in general, lung capacity changes very little from exercise.

Long term endurance training tends to simulate hypervolemia. This is where the body increases blood volume to help the cardiorespiratory system become more efficient by being able to supply more oxygen. Most of the blood volume increase comes from plasma and very little comes from red blood cells.

Improvements to VO2 max occur with prolonged aerobic training and reflect improvements in the ability of the heart to pump blood and the ability of the muscles to use the oxygen provided.

Environmental Factors Affecting CRS

Severe environmental conditions can cause the cardiorespiratory system to work harder to meet the body’s oxygen demands. Exercising in the heat causes the body to shuttle more blood to the skin to help dissipate body heat.

This is typically shown when someone gets hot, they end up turning a shade of pink or red. As blood flow to the skin is increased, vasculature can become engorged and cause blood pooling.2 Blood pooling reduces the venous return and can pause the cardiovascular system in order to work harder in filling the heart.

Dehydration caused by extreme heat can result in a decreased amount of plasma volume. This means less blood is available for working muscles and to maintain normal body functions. The body copes with this by increasing the heart rate which may not be enough to maintain cardiac output.

Exercising in a cold environment can cause heat loss during exercise. Long duration of exercise in the cold can increase the risk of hypothermia. This is especially true when core body temperature lowers.

To counteract this, the body tries to create heat by shivering and through the vasoconstriction of blood vessels in the skin. The respiratory rate is usually higher and maximal oxygen consumption may be slightly lower.

Altitude is another factor that can affect the cardiorespiratory system.

When someone increases their elevation the pressure of oxygen becomes reduced.2 This makes it harder for oxygen to diffuse into the tissue because of the lack of pressure gradient. To compensate for this, breathing rate is usually increased with altitude.


The cardiorespiratory system contains the pulmonary system and the cardiovascular system. The pulmonary system contains the lungs and airways and is responsible for providing oxygen to the body while expelling carbon dioxide. It also provides ventilation, respiration and exchange of gases. The cardiovascular system contains the heart, blood vessels, and blood. The heart pumps blood throughout the body, which helps to deliver nutrients and oxygen and to remove waste alongside many other functions.

During exercise the consumption of oxygen increases to meet the energy needs of the body and cardiac output goes up. Regular cardiovascular training will increase VO2 max, because of improved performance in cardiorespiratory efficiency.

Fitness professional should also be aware that environmental factors can impact the cardiorespiratory system, like dehydration and altitude, and prepare accordingly with monitoring fluid intakes or modulating exercise intensity to ensure sessions are safe and productive.


  1. Chandler TJ, Brown L. Conditioning for Strength, and Human Performance. Routledge; 2019.
  2. Magyari P. ACSM’s Resources for the Exercise Physiologist: A Practical Guide for the Health Fitness Professional. Philadelphia: Wolters Kluwer; 2018. 
  3. Marieb EN, Hoehn K. Human Anatomy & Physiology. Pearson; 2016. 

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