LEarning centre

Topic Selector

CARDIOLOGY BASICS

Use the interactive tool below to see how the heart works. For additional reading, click on any of the topics below to learn the fundamentals of cardiology.

 
Click the thumbnail to navigate to the "How The Heart Works" tool!
 

The Cardiovascular System

The cardiovascular system, often referred to as the "circulatory" system, is analogous to a network of one-way streets, due to the way it is organized. It is a sealed, air-tight system through which oxygen and nutrients are transported throughout the body. 

The cardiovascular system consists of the heart, blood, and vessels. The heart pumps blood through various types of vessels, to deliver nutrients to organs, tissues, and cells of your body. All three of these components must be in order, to ensure you can lead a safe and healthy life.

 

Watch this video to familiarize yourself with how the cardiovascular system is organized.

Basics of the Human Heart

HOW THE HEART BEATS

STRUCTURE & FUNCTION

The heart is a muscular organ about the size of a fist that sits in the middle of your chest. It pumps and supplies nutrient filled blood to the rest of your body.

 

The heart has four chambers. The upper two chambers of the heart are known as the left and right atria (plural for atrium). The lower two chambers are referred to as the left and right ventricles. The tissue separating the left and right sides of the heart is known as the septum.

Blood enters the heart through vessels known as veins, the largest of which is the superior vena cava. Blood leaves the heart through vessels known as arteries, the largest of which is the aorta. Click here to learn more about vessels.

In a normal heart, the electrical impulse that starts the heartbeat begins in a group of cells called the SA node which is located in the right atrium.

 

The SA node is often called the pacemaker of the heart. Just like a spark plug in a car engine, the SA node produces the electrical signals that make the heart pump.

 

The SA node generates a number of signals each minute in response to the body's needs. The resting heart rate is usually about 60 to 80 beats per minute for the average person.

Once produced, the electrical signals travel to the lower chambers of the heart called the ventricles, making them contract. As the ventricles contract, they pump blood out of the heart to be carried through vessels and supply your body with nutrients.

 

Basics of Blood

Blood may seem simple - it's red and it's liquid and we have lots of it! While those observations are valid, there is more to blood then what meets the eye. Watch this video to learn about the basics of blood.

COMPOSITION

Blood is approximately half liquid, and half solid. The liquid component is called the plasma and contains a combination of various nutrients, salts, hormones, blood proteins etc.

 

The solid component consists of three types of blood cells. Erythrocytes (red blood cells), Leukocytes (white blood cells), and Platelets. These blood cells are critical for the normal functioning of your body. Blood cells ensure that the tissues and organs of your body are receiving adequate amounts of nutrients and oxygen, and that waste products are being readily removed. Furthermore, your blood cells play a role in your immunity by attacking and preventing the spread of harmful bacteria or viral infections that may enter your body.

ERYTHROCYTES (RED BLOOD CELLS)

PRIMARY FUNCTION: OXYGEN DELIVERY 

Commonly known as the “Red Blood Cells”, erythrocytes are involved in delivering oxygen to all of the tissues and organs in the body - including the heart.

 

In addition to oxygen delivery, erythrocytes are responsible for the uptake and removal of carbon dioxide from tissues to exchange them at the level of the lungs.

LEUKOCYTES (WHITE BLOOD CELLS)

PRIMARY FUNCTION: IMMUNE SYSTEM 

Commonly known as the “White Blood Cells”, Leukocytes comprise less than 1% of the total blood volume.

 

While the red blood cells are primarily responsible for delivering oxygen to your tissues, the white blood cells are a critical component of the immune system. They protect your body against attacks from harmful bacterial and viral infection or foreign substances. In addition,  white blood cells are responsible for removing damaged and dead cells within the body.  

 

Basics: Blood Vessels

ARTERIES

Arteries carry oxygenated blood, pumped from the heart, towards the rest of the body. They are thick and elastic, designed to withstand the high pressure of blood exiting the heart. Arteries branch out into smaller vessels called arterioles before reaching the capillaries.

The two leading causes of deathmyocardial infarction (heart attack), and stroke, may directly result from an arterial system that has been slowly and progressively compromised by years of deterioration.

CAPILLARIES

Capillaries are the smallest blood vessels in the body. Their walls are merely one cell thick. This enables nutrients to easily enter and exit the blood. A capillary bed (pictured on the left) is aligned with tissues in the body, where oxygen, water, and other nutrients are exchanged (pictured below).

See the oxygen and nutrient exchange section for more details on this.

VEINS

Veins carry de-oxygenated blood from the body, towards the heart. Smaller capillaries converge into venules which then converge into large veins.

The two leading causes of deathmyocardial infarction (heart attack), and stroke, may directly result from an arterial system that has been slowly and progressively compromised by years of deterioration.

 

Basics: Adult Circulation

Blood is distributed throughout the body by the heart. In simple terms, the heart is a pump where each contraction causes blood to circulate throughout the body. There are two main circulations that are fueled by the heart - systemic and pulmonary.

 

Systemic circulation refers to the delivery of blood from your heart to the rest of your body.

 

Pulmonary circulation refers to the delivery of blood to the lungs so that it can be oxygenated.

All blood first enters the right side of the heart, specifically into the right atrium, through either the superior or inferior vena cava. When the right atrium contracts, the tricuspid valve opens to allow blood to flow into the right ventricle. The right ventricle pumps blood into the pulmonary artery to start the process of pulmonary circulation. The blood makes its way into the lungs where it picks up oxygen and disposes carbon dioxide.

The blood returns from pulmonary circulation into the left atrium. When the left atrium contracts, the mitral valve opens to allow blood to flow into the left ventricle. When the left ventricle contracts (known as systole), the blood is then pumped through the opening of the aortic valve into the body's largest artery: the aorta. From here the blood is released into the rest of the body for systemic circulation and eventually returns to the heart via the vena cavas.

 

Toronto General Hospital 

 200 Elizabeth Street | 6E - 249

Toronto, Ontario M5G 2C4

Tel: 416-340-3835

Fax: 416-340-3000

  • Cardiology UHN Facebook
  • Cardiology UHN Twitter
  • Cardiology UHN YouTube

© Structural and Congenital Heart Disease Intervention at Peter Munk Cardiac Centre | University Health Network | All Rights Reserved.

CardiacClinic.ca and affiliated websites do not provide personal medical advice, diagnosis, or treatment. Read our Terms and Policies.