The Circulatory System: Everything You Need to Know | The Daily Dose
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The Circulatory System: Everything You Need to Know

August 03, 2020 9 min read

Diagram of the human circulatory system

Dr. Sandra El Hajj - MSc, N-MD, DHSc

At a glance

Your circulatory system, also known as the cardiovascular system, is a group of organs synchronized together to serve a vital function. Comprising the heart, vessels, and the lymphatic circulation, it is in charge of delivering oxygen and nutrients to all organs of the body, and getting the body rid of toxins and carbon dioxide. Many illnesses could affect the proper functioning of the circulatory system, such as high blood pressure, atherosclerosis, angina, arrhythmia, blood clots and varicose veins. Understanding these conditions can help you prevent them.

The human body is a complex organism made of more than 100 trillion cells. While there are many body parts that perform specific functions, others have far more complicated roles in the proper functioning of our inside! We surely use our ears to hear, our eyes to see and our muscles to lift things; but, when it comes to the circulatory system, that’s where the real fun begins.

What is the circulatory system?

The circulatory system is also known as the cardiovascular system. It is an amazing network comprising the heart and blood vessels where blood circulates. These vessels work to provide oxygenated blood and nutrients to our organs and rid the organs and tissues of carbon dioxide and waste products. The circulatory system helps keep our cells healthy and alive by doing this exchange. By listening to all the signals sent by the different organs of the body, the circulatory system knows how hard it needs to pump the blood. The circulatory system is essential to being alive (1).

Parts of the circulatory system

The circulatory system is made of many parts, each accomplishing a different function to make sure the body is healthy and well. There are four elements in this complex system: The heart, the arteries, the veins and the capillaries. Amazingly, your heart can pump up to five liters (1.3 gallons) of blood every single minute. And, by the time you are 70 years old, it will have beaten up to 2.5 billion times (considering an average resting heart rate between 60 and 100 beats per minute.

The heart

The circulatory system’s work revolves around the heart. When you understand this powerful muscle and its functions, you understand the entire circulatory system. What makes this organ special is that it is located in the center of the circulatory system. It is made of many different layers.

The pericardium is the wrap around the heart that keeps it in its place. It contains fluids that act as lubricants preventing friction of the cavities. The pericardium itself is made of two layers: The visceral layer that directly covers the heart, and the parietal layer that is the sac containing the fluid.

The heart wall is the second section and is made of three layers:

  • The epicardium lubricates and protects the outside of the heart.
  • The myocardiumis the muscular layer of the heart and constitutes the thickest part of the heart.
  • The endocardium is the endocardium that is the inner smooth lining part of the heart that prevents the blood from sticking to the heart and clotting.

The heart is made of four chambers: twoatria (plural of atrium) and twoventricles. The atria are small cavities that are located on the left and right sides of the heart. They are the receiving chambers for the blood. Ventricles, on the other hand, are large muscular pumping chambers that are in charge of pushing the blood outside the heart into the blood vessels. In general, the right side of the heart is smaller than the left side. The right side has fewer muscles as they send out the blood into the lungs while the left side sends it to the entire body, reaching the extremities.

The heart contains fibrous flaps between the different chambers and within veins. These are calledvalves and are in charge of preventing a backflow of blood. In other words, they ensure the blood is flowing in one direction. There are theatrioventricular valves that are located between atriums and ventricles, and thesemilunar valves that are found between the arteries and ventricles(2).

The blood vessels

The blood vessels are small tubes that transport the blood from and to the heart. Within your body, there are more than 60,000 miles of blood vessels. There are three types of tubes.

  • The veins are in charge of delivering blood poor in oxygen back to the heart to get its needed oxygen.
  • The capillaries that are microscopic vessels connecting the arteries to the veins. It is in between the capillaries that oxygen, nutrients and waste are exchanged between the organs, tissues and the blood. Capillaries are the most abundant blood vessels in the body and the smallest.
  • The arteries are the tubes in charge of transporting blood that is rich in oxygen to the organs and tissues. The arteries start off with the aorta (biggest artery) that branches out into smaller arteries.

How does your circulatory system work?

The circulatory system is a vital system. Its functions are optimal and necessary for our life and well-being. Why is it vital? It is simply because it transports oxygen from the air and lungs to every cell of our body. It also brings in nutrients to make sure our organs and tissues have what they need to perform their functions. The circulatory system conversely transports back the waste products and carbon dioxide from these cells and tissues and helps the body expel it(3).

Your blood works within a specific loop. It is a simple circle where all the functions are accomplished when you are healthy.

Step 1: It all starts when the unoxygenated blood comes towards the right side of the heart through the veins.

Step 2: The heart pumps this unoxygenated blood to the lungs where they discharge the carbon dioxide and pick up some oxygen.

Step 3: The blood is now rich in oxygen and is ready to return to the left side of the heart where it gets pumped through the arteries.

Step 4: This newly oxygenated blood is ready to nourish the body’s cells and tissues. From the arteries, it goes into much smaller tubes called the capillaries. This is the area where the oxygen and nutrients get released to feed your organs. Once oxygen-free, the blood acts as a carrier once again, picking up carbon dioxide and waste products.

Step 5: This loaded blood returns then to the heart using the veins and the loop goes on again with the same cycle over and over.

This cycle can be affected by many stimuli, which may affect the volume of blood propelled and the speed of blood flow. Some of these factors include hormones and electrolytes.

How does the heart pump blood?

Your heart has one main function, that is to pump blood into the circulatory system. This normally is the result of two actions that happen back to back: Contraction and relaxation. Contraction is referred to as systole and relaxation is referred to as diastole. These two happen in rhythmic patterns allowing the blood to get pumped. So, together, they form the cardiac cycle commonly referred to as the heartbeat, which takes about one second.

The number of heartbeats that occur within a minute depends on several factors including your physical activity. So, when you are resting, your heartbeats are slower than when you are exercising. On average, the heart beats up to 100,000 times every day, averaging to nearly 3 billion beats in a lifetime(4)!

Diseases of the circulatory system

Just like for every organ of the body, the heart and its vessels can face many common conditions that can affect its proper functioning (5). Here are the most common ones:


When plaque starts building up on the walls of your arteries, you are starting to develop atherosclerosis. There are many factors that could lead to the buildup of plaque. These include: Having high blood pressure, high cholesterol levels, using tobacco, being diabetic, following an unhealthy diet, not working out properly and being obese among many other issues.

Atherosclerosis is a progressive condition. It happens gradually, making arteries narrower and leading to less oxygen reaching organs and tissues. If atherosclerosis happens at the levels of the heart arteries, it is referred to as coronary artery disease. When it affects the arteries of the legs, feet, arms and hands, it is referred to as peripheral artery disease.

High blood pressure

Blood pressure is the force exerted by your blood on the walls of your blood vessels (arteries). When your blood pressure is high, this could lead to damage to the heart and vessels. High blood pressure could also have a negative impact on the brain, kidneys and eyes.


When you start feeling chest pain occurring due to a depleted oxygen supply to the heart, you are dealing with angina. This is normally the result of coronary artery disease that may cause the narrowing of arteries due to a buildup of plaques.


Arrhythmia is a very common illness affecting the lives of millions of Americans, every year. It occurs when the heart rhythm is not normal. Your heart may be beating too slow or too fast. Your heart beats may even be irregular. Arrhythmias have many risk factors, such as having changes in the heart muscle, an injury caused by a heart attack, an electrolyte imbalance in the blood or other heart diseases like coronary artery disease.

Varicose veins

Veins contain valves, as explained earlier. These veins keep blood that is depleted from oxygen from flowing back to the heart. When these valves start failing, blood begins to collect in the veins, causing a bulge that can be swollen and painful. These are called varicose veins and tend to often appear on the lower legs.

Blood clots

Blood clots occur when blood begins to coagulate, resulting in clumped formations that may seem to be made of gel-like particles. These clots get stuck in blood vessels, preventing blood from flowing properly in these. When clots start forming due to the thickening of platelets and plasma proteins from the blood, it might lead to many cardiovascular conditions such as heart attack, stroke, pulmonary embolism or even deep vein thrombosis.

To go into more detail, when a blood vessel gets damaged, small parts of the blood, referred to as platelets, get released and begin sticking on the walls of the affected area to form a protective patch and keep your blood from escaping the affected vessel. When this happens, your blood proteins begin sending signals and forming long strands of fibrin to create a thicker and more durable protective patch that traps more cells and platelets. When these plaques happen in the heart or brain and burst, they may lead to heart attacks and strokes.

These clots can also form when blood is not properly flowing and starts pooling in the vessels, which leads to platelets sticking together and slowing blood flow.

Heart attacks

A heart attack happens when one of the blood vessels, through which the blood flows, becomes blocked. This leads to less oxygen being provided, which creates an imbalance in the supply/demand of the oxygen supply. As a result, part of the heart becomes deprived of oxygen and starts losing its functions or even gradually dies.


A stroke is the result of a low oxygen provision to the brain. When a blood vessel that normally provides oxygen to the brain gets blocked, oxygen and nutrients no longer reach the brain leading to its death. The process happens gradually. When brain cells get damaged, they cannot be replaced, and the damage to the brain becomes irreparable. Only by quickly restoring a proper blood flow would you be able to restore brain function.

Heart failure

Heart failure occurs when the heart cannot pump the necessary amount of blood to keep the tissues and organs well-oxygenated, or effectively manage the waste and CO2 removal process. Heart failure can be systolic or diastolic. Systolic heart failure is the result of the heart not pumping enough blood, while the diastolic heart failure is when the heart pumps normally but it does not relax sufficiently.

Heart valve problems

This problem occurs when the valves controlling blood flow in the blood vessels and heart are leaky or blocked. This would make the heart less efficiently pumping the blood.

Heart inflammation

When the inner lining of the heart becomes inflamed, the inflammation is referred to as endocarditis. When the inflammation happens in the outer sac of the heart, it is referred to as pericarditis; when the muscles of the heart itself are inflamed, it is referred to as myocarditis.


This is when the artery walls become weak and start bulging out. It could happen to either the big arteries or the small ones. When the aneurysm is in the large arteries and it ruptures, it could be deadly.

Congenital heart disease

This condition affects kids who are born with some kind of abnormalities in their heart and blood vessels that may have happened during the formation of the heart.


This is a condition that affects the walls of the blood vessels. It happens when these walls become inflamed and could lead to further complications such as aneurysms.

In summary

Your heart is a major organ in your body. when it faces malfunctions, it has the ability to affect the entire body. The goal of your heart is to provide oxygen and nutrients to all your cells while getting the body rid of waste products and carbon dioxide. Your circulatory system comprises the heart along its veins and lymphatic circulation. Altogether, they are synchronized to work effectively. Many conditions may affect your heart and arteries that could be a potential threat, leading to serious health conditions. Among these are arrhythmias, high blood pressure, blood clots, varicose veins, heart failure, stroke, and others. Understanding these malfunctions can help prevent them. There are many tips to follow that can help keep your heart well and healthy, which you canread more about here.

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