The Human Heart and the Cardiovascular System

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Updated on: Educator Review By: Michelle Connolly

All organs in our body work together to keep us alive and healthy. But how do they communicate? Which organ delivers blood to other organs so we can move and run and do our daily tasks? The answer is the heart! The heart doesn’t really look like the heart we draw on a piece of paper. It is actually more complex than this. The heart is a part of a system called the cardiovascular system. ‘Cardio’ means heart and ‘vascular’ is for blood vessels. The cardiovascular system is much like a network of veins and arteries that deliver and receive blood and the heart is the center of this network. We are going to explore together what the heart consists of, what it does, how it functions, what veins and arteries are, what blood is, and how to keep our heart happy and healthy!

What is the heart and what is its function?

Our heart is basically a muscle. Our whole body is covered in muscles that allow us to move throughout the day. Our muscles do not move on their own. For instance, we must think of moving our hands first and then the brain gives a signal to the hand to grab a pen. But our heart does not need all this much thinking. The heart is a special muscle because it moves on its own. Your heart is about the size of your fist and it is protected by your ribcage. 

Your heart is responsible for pumping blood to your whole body through veins and arteries. This blood carries oxygen and nourishment to the rest of the body and also helps clean any waste. With every pump, the heart carries and receives blood to and from the lungs. This means that every time your heart is filled with blood, it beats, or ‘squeezes’ the blood out to be delivered.

Parts of the Heart

We will now take a look at what the heart really looks like. The heart is divided into four parts. Each part is called a ‘chamber.’ You can think of it as a blood chamber. Each side of the heart has two chambers: one is at the top and the other one is at the bottom. On the left side, the chamber that is at the top is called the left atrium and the one at the bottom is called the left ventricle. On the right side, the chamber at the top is called the right atrium and the one at the bottom is called the right ventricle

Extra information! The plural form of the word atrium is atria

The atria are responsible for collecting blood that is coming from the body and lungs. The two ventricles are responsible for pumping to squirt out blood to the lungs and the rest of the body. There is an area between the left and right sides of the heart that is called the septum. This area exists to separate the two sides so the oxygenated blood doesn’t mix with deoxygenated blood.

Between an atrium and a ventricle is something called the valve. There are two valves between an atrium and a ventricle. A valve is like a door. It opens and shuts to let blood in and out with every heartbeat. There are four valves in your heart. Each side has two valves. On the right side, there is the tricuspid valve and the pulmonary valve. On the left side, there is the mitral valve and the aortic valve. These valves control the flow of the blood. 

Extra information! If you see the word ‘pulmonary,’ it most likely has to do something with the lungs. 

To deliver and return blood to and from your whole body, the heart is connected with blood vessels called veins, arteries, and capillaries. Next, we will explore these vessels in detail and how the blood flows inside of them. 

Veins, Arteries, and Capillaries

Blood vessels take the form of tubes. There are three kinds of blood vessels, and they are: veins, arteries, and capillaries. Arteries carry the blood from the heart to the rest of your body. Veins return the blood from the body to your heart. The walls of the arteries are thicker than those of veins and capillaries. Veins have valves to keep the blood from flowing back to where it came from. Capillaries are the tiniest and they connect the arteries and the veins to the body tissues.

Keep in mind that the blood that is carried by the arteries is called oxygenated (oxygen-rich) blood, and the blood that is carried by the veins is called deoxygenated (oxygen-poor) blood.   

Why are arteries thicker than veins?

The answer to this question is pressure. When the heart pumps blood into the arteries, it does that at high pressure so blood can reach every part of the body quickly, and this is why arteries need thick walls to withstand this pressure. Veins return the blood to the heart at lower pressure, and this is why their walls are thinner.

Major Blood Vessels

All blood vessels are surely important. But here we will talk about the main ones that branch out from the heart and cover all of our body. The major blood vessels that are connected to your heart are the aorta, the superior vena cava, the inferior vena cava, the pulmonary arteries, the pulmonary veins, the coronary arteries, and the coronary veins. 

The Aorta

The aorta is the main artery that carries blood from your heart to your muscles, organs, and every bit of your body. It delivers the oxygenated blood to the rest of the major arteries to your brain, muscles, cells, and other organs.

The Superior Vena Cava

The superior vena cava, also known as ‘precava,’ is one of the largest veins in the body. It carries blood from the head, neck, arms, and chest to your right atrium. It sets above the heart, and that is why it’s called ‘superior.’ 

The Inferior Vena Cava

The inferior vena cava, also known as ‘postcava,’ carries blood from the legs, feet, and organs in the abdomen and pelvis to the right atrium of your heart. It is under the heart, and that is why it’s called ‘inferior.’

Pulmonary Arteries

The main pulmonary artery carries deoxygenated blood from the right ventricle to the lungs. 

Pulmonary Veins

The pulmonary veins do the exact opposite of what the pulmonary arteries do. They carry the oxygenated blood from the lungs back to the heart.

The Coronary Arteries

The heart is like any other organ that needs to be supplied with oxygenated blood so it can function. The tissues of the heart need to have the nutrients required to keep beating and send and receive blood to and from the body. The coronary arteries do just that. They are wrapped around the outside of the heart. They deliver oxygenated blood to the heart to work.

The Coronary Veins

The coronary veins carry away the deoxygenated heart to the right atrium for it to get oxygenated and return to the heart through the coronary arteries. 

What is blood?

Do you know the little blood that comes out when you fall and get scratched? They are just a few drops of the blood that is circulating our whole body. Blood is the carrier of oxygen, hormones, and nutrients to every part of the body so they can keep working and help us stay alive and healthy. Think of blood as a delivery person. Blood carries nutrients and oxygen just like a delivery person carries your hot pizza to your doorsteps. Isn’t it fascinating? 

Blood actually has two jobs. It does deliver oxygen to the rest of the body, but it also carries away the carbon dioxide and other waste products to the lungs, kidneys, and digestive system to be filtered inside them and then removed from the body. Your blood is also a fighter of its own. It can fight infections and cuts so your skin can heal faster. 

Now you would wonder what blood consists of. Blood has two components: blood cells and plasma.


When plasma is detached from red and white blood cells and platelets, it will look like a yellowish fluid. Plasma makes up about 55% of your blood, making it the largest component. It is the part that carries nutrients, hormones, and proteins to the rest of the body. Plasma also helps to remove waste from cells and ultimately the body. 

Blood Cells

There are different types of blood cells, and each type has its own job to do. 

Red Blood Cells

Red blood cells look like a flattened donut that is not hollow. Red blood cells have a protein called hemoglobin. Red blood cells get oxygen from lungs and the hemoglobin carries the oxygen. As the blood circulates inside the body, hemoglobin releases the oxygen to the body parts that need it. Extra fact: hemoglobin carried with oxygen is what gives our blood the color red!

cardiovascular system,heart,the conduction system LearningMole

Red blood cells only live for 4 months. They are created in the bone marrow every day to replace those who are dead or lost. So, if you got a paper cut, don’t worry! Your bone marrow will reproduce new red blood cells for you.

White Blood Cells

Remember when we said that your blood is a fighter? White blood cells are what makes it this way. White blood cells are a part of your strong immune system. When you get a cold, your white blood cells start to be on alert to fight the foreign visitor, which is the virus. They start to produce antibodies to fight off the infectious disease and make sure that it doesn’t affect you if it visits again. 

White blood cells’ lifespan varies from hours to years. They constantly get produced in the bone marrow and other body parts like lymph nodes. More white blood cells get produced when there is an infection or a virus, that’s why a person with a cold would have more white blood cells than a healthy person. 


These are oval-shaped, and they are very important in healing wounds and cuts. When you have a cut, platelets and some proteins in your plasma run fast to the location of the cut and help in the clotting process. Platelets survive for about 9 days, and they are constantly being created in the bone marrow. 

What does clotting mean? Clotting happens when a blood vessel breaks. Your platelets start to gather and make a clot so the bleeding would stop. After the injury has healed, your body will naturally dissolve the clot.

Blood Types

You might think that since everyone’s blood is red, it is the same for everyone. This is not true. Each one of us has a unique type of blood. There are eight blood types. To describe these types, we use the letters A, B, and O. Each of these letters stands for a certain protein found in red blood cells. We all have different proteins in our blood. 

A blood type should consist of a letter or two, and a negative or a positive. The negative or the positive depend on having a certain protein called Rh. If you are positive, you have this protein in your blood. If you are negative, you don’t.

The eight blood types are: 

  • A+
  • A-
  • B+
  • B-
  • O+
  • O-
  • AB+
  • AB- 

How does blood circulate inside our body?

We now know every part of the cardiovascular system and what they do. But how do they all work together? 

Let’s trace down the steps of how every part of our body gets oxygenated blood. 

Let’s start with a random part of your body. The hand, for example. Let’s say that the blood in your hand has deoxygenated blood. 

1- The deoxygenated blood travels through the inferior vena cava. 

2- Blood then enters the right atrium.

3- The right atrium contracts and the tricuspid valve opens, allowing the blood to enter the right ventricle. 

4- The right ventricle then contracts, and the pulmonary valve opens, allowing the blood to move to the pulmonary artery. 

5- The pulmonary artery takes the deoxygenated blood to the lungs where red blood cells in the blood will take the oxygen from there and release carbon dioxide. When we exhale, we breathe out the carbon dioxide. When we inhale, new oxygen enters our lungs, so the blood gets supplied with oxygen.

6- The blood is now oxygenated! 

7- The oxygenated blood travels through the pulmonary vein to the left atrium. 

8- The left atrium contracts, and the mitral valve opens, allowing the blood to enter the left ventricle. 

9- The left ventricle then contracts, and the aortic valve opens, allowing the oxygenated blood to move to the aorta. 

10- The aorta carries the oxygenated blood to the rest of the body parts. 

To make this easier, try to track down the blood path this way:

Deoxygenated blood → inferior vena cava → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary artery → lungs → oxygenated blood → pulmonary vein → left atrium → mitral valve → left ventricle → aortic valve → aorta → body organs, tissues, and cells

What’s fascinating is that all of this happens in less than a minute in each heartbeat!

the heart and the cardiovascular system
The flow of blood in the heart

The Heart’s Electrical System (the Conduction System)

We know that when the heart contracts and relaxes with each heartbeat. Have you ever wondered how it does this? What gives it the signal to contract and then relax? We will learn about this through the heart’s electrical system, or what we call in science ‘the conduction system.’

The conduction system is a network of nodes, specialized cells, and electrical signals that make the heart beat. Nodes are groups of cells that can be either nerve or muscle tissues. There are two types of cells that control the heartbeat: 

  • Conducting cells that carry the electrical signals
  • Muscle cells that control the heart when it contracts

Parts of the Conduction System

The conduction system is made up of specialized cells and nodes that work together to keep your heart beating. These are the:

  • Sinoatrial node
  • Atrioventricular node
  • Bundle of His (atrioventricular bundle)
  • Purkinje fibers

Extra information: a node in the heart is a type of tissue that behaves as both muscle and nervous tissue. 

The Sinoatrial Node

The sinoatrial (SA) node is also known as the ‘natural pacemaker’ of the heart. It is located at the upper part of the right atrium, near the start of the superior vena cava. It sends the electrical signals that start the heartbeat. 

The autonomic nervous system is what controls how fast or slowly your SA node sends the signals for your heart to beat. The autonomic nervous system sends hormones to the SA node depending on what you’re doing. For example, if you’re resting, it sends hormones that make your heart beat slowly. If you’re nervous or running, it sends hormones that make your heart beat fast. 

The Atrioventricular Node

The atrioventricular (AV) node plays a crucial role in controlling the heartbeat. It delays the SA’s electrical signal. It does this so the atria have time to be empty of blood before the contraction stops. The AV node is located in the central area of the heart, near the tricuspid valve.

The Bundle of His

The bundle of His is also known as atrioventricular bundle. This bundle is a branch of fibers that stick out from the atrioventricular node. This bundle of fibers is responsible for receiving electrical signals from the AV node. It then carries these signals to the Purkinje fibers. 

The bundle of His is located in the septum area and it further branches into two branches:

  • Left bundle branch: it is the branch that’s responsible for sending electrical signals through the Purkinje fibers to the left ventricle.
  •  Right bundle branch: it is the branch that’s responsible for sending electrical signals through the Purkinje fibers to the right ventricle.

The Purkinje Fibers

The Purkinje fibers are specialized nerve cells that send electrical signals quickly to your left and right ventricles. After the Purkinje fibers send signals, the right ventricle contracts, making the blood flow to the pulmonary arteries, and the left ventricle also contracts, making the blood flow to the aorta. 

A summary of how the heart expands and contracts

  1. The sinoatrial node creates an electrical signal.
  2.  The electrical signal travels to the atria, telling them to contract.
  3. The electrical signal reaches the atrioventricular node and tells it to delay the contraction of the atria for about tenth of a second until the atria empty the blood into the ventricles
  4. The electrical signal reaches the bundle of His to get carried to the Purkinje fibers.
  5. The Purkinje fibers carry the signal to the ventricles, signaling them to contract and pump blood into the pulmonary artery and the aorta.

What’s totally amazing is that this all happens in a blink of an eye! Our heart does its best all day to keep relaxing and contracting so we can move and do our daily activities.

How do we keep the heart healthy and happy?

Our heart does its best every second of the day to keep us alive. Here are things you should do to keep your heart healthy and happy.

  • Exercise. You should move your body every day to keep the heart muscle healthy. You can walk, run, play a sport, or even dance! 
  • Eat your veggies and fruit every day. Vegetables and fruits contain vitamins and fibers which are important for your heart. 
  • Don’t drink soft drinks and juice. You can drink fresh juice like orange juice without any sugar.
  • Don’t eat too many snacks. One snack a day is enough.
  • Look out for saturated fats. Saturated fats are bad for you because they raise your cholesterol level. A high cholesterol level means more fats in your blood, and this is not healthy. You can read food labels to see if your favorite snack is relatively healthy or not.
  • Don’t be around someone when they smoke. Smoking is bad for your blood vessels. Inhaling cigarettes’ smoke is also bad for you.

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