Human Anatomy - Circulatory System

The circulatory system of the human body carries one of the most important substance in your body: the blood. Go on this tour of the human body to find out how it works.

This story was created for the Google Expeditions project by Vida Systems, now available on Google Arts & Culture.

Human Anatomy - Circulatory System by Vida Systems

At the center of it all is the heart, which is responsible for circulating this vital fluid throughout the body. 

Introduction to the Circulatory System

The circulatory system of the body keeps the blood flowing. Your heart, lungs, and blood vessels are responsible for circulating and dispersing oxygen-rich blood to all the tissues in your body, and recycling oxygen-depleted blood that your body has used.

The Heart Powers the Circulatory System

In order to push blood through the body, the human heart beats or contracts an average of 70 times per minute. It will beat more than 3 billion times during a person’s normal lifetime.

The Lungs Provide Oxygen for the Blood

Whether you are sleeping or vigorously exercising, through the process of respiration in the lungs, your circulatory system delivers oxygen to and removes carbon dioxide from the cells of your body.

The Circulatory System Helps Regulate Body Temperature

Directed by the brain, the circulatory system can help warm or cool the body as it transports blood. By dilating the blood vessels, heat from blood can be more readily released to the environment. To keep the heat in, blood vessels constrict.

Defining the Circulatory System

The circulatory system is made up of 3 independent systems that work together to move blood throughout the body. This includes the heart, the lungs, and a network of veins, arteries, and vessels that provide blood to the cells throughout the rest of the body. 

The Heart

The heart is the power center of the circulatory system. Even at rest, the average heart, about the size of a closed fist, easily pumps more than 5 liters of blood through the body each minute.

The Role of the Lungs

The pulmonary system connects the heart to the lungs. Capillaries on alveoli convert inhaled oxygen so it can be absorbed into blood. Oxygenated blood goes from the lungs to the heart for distribution. Oxygen-depleted blood moves from the heart to the lungs for more oxygen.

Arteries and Veins

Systemic circulation carries oxygenated blood (red) through the arteries to the cells of the whole body. After organs and tissues use the blood, veins return the oxygen-depleted, or deoxygenated, blood (blue) to the heart, which sends it back to the lungs for more oxygen.

The Structure of the Heart

This 360 degree gallery contains 4 illustrations of a heart; each one presents distinct features of the organ such as its layered construction and the locations of the vital veins and arteries. 

The pumping action of the heart sends oxygenated blood from the lungs to the body for use while simultaneously moving returning, deoxygenated blood from the body, and pumping it to the lungs for oxygenation.

The Heart’s Exterior

The workings of the heart are mostly hidden from view by the pericardium, the outermost layer which secures and protects the heart. The pericardium holds the heart in place while still allowing it enough room to beat and reducing friction between it and surrounding structures.

The Cardiac Muscle

With the pericardium out of the way, we can see the heart’s myocardium, or middle layer. The myocardium is the heart muscle; it contracts to pump blood out and then relaxes and refills with returning blood.

Within the Heart’s Chambers

The innermost layer of tissue that lines the chambers of the heart is called the endocardium. The heart is divided into 4 chambers: 2 atria, which receive blood, and 2 ventricles that pump blood.

The Network Hub

Underneath the cardiac tissue are blood vessels known as the coronary arteries and cardiac veins. These supply the heart tissue with blood. The larger, dark-red tunnels shown are the routes through which the heart pumps blood to and from the lungs and the body.

A Closer Look at Blood Vessels

Blood vessels are the tubular structures that carry blood through the tissues and organs. There are 3 major types of blood vessels. The arteries carry blood away from the heart. 

The capillaries enable the exchange of water and nutrients between the blood and the tissues. Veins carry blood from the capillaries back toward the heart.

Arteries Move Blood from the Heart

Blood traveling through the arteries is highly saturated with oxygen to supply the body's tissues. The red blood cells are responsible for carrying the oxygen to the tissues and carrying away the carbon dioxide.

Veins Bring Blood Back to the Heart

After the capillaries have furnished water and nutrients to all of the tissues of the body, veins carry the blood, now lacking in oxygen, back to the heart. The heart sends the blood to the lungs for oxygen replenishment.

Vessels Participate in Thermoregulation

The brain has the ability to contract and dilate the inner diameter of arteries. It uses the muscular layer of the vessel to control the blood flow to downstream organs, which is important for thermoregulation (the process of regulating body temperature).

A Deeper Look Into the Heart

Our heart is our body’s circulatory pump, taking deoxygenated blood to the lungs for oxygenation and then feeding the new oxygenated blood back to our tissues. 

Here we can see some of the chambers and valves of the heart which are constantly pumping blood through the heart and back into the body. 

The Aortic valve

The heart has 4 valves, which ensure that blood only flows in one direction. When the left ventricle contracts, the aortic valve opens and allows the blood to exit into the aorta. When the left ventricle stops contracting, the aortic valve closes. 

The Left Ventricle

The left ventricle is the largest of the heart’s chambers and it pumps blood to tissues all over the body. By contrast, the right ventricle only pumps blood to the lungs. 

Mitral Valve Leaflet

The mitral valve leaflet connects the left atrium with the left ventricle. When the left atrium contracts, it allows blood to flow from the left atrium to the left ventricle. It then closes to ensure that blood does not flow in the opposite direction. 

The Left Atrium

The left atrium is found on the left posterior side of the heart. This is the holding chamber for the oxygenated blood returning from the lungs. It then sends the blood to other areas of the heart.

Blood Groups

Blood is not all the same and every human belongs to one of 4 main blood groups: Type A, Type B, Type AB, and Type O. These groups differ due to antigens on the surface of red blood cells. 

Antigens, carbohydrates that cause an immune system to produce antibodies against it, are named, A, B, and D. Type A blood has A antigens, type B has B antigens, AB blood has both, and O blood has no antigens. Presence of D antigen indicates a positive blood type. 

Red Blood Cell - Type O

Blood group O carries 2 opposing blood type antibodies (against Type A and Type B) and no antigens. Type O is the universal blood donor as all blood groups can accept it. AB blood, containing both A and B antigens, is called the universal recipient. 

Red Blood Cell - Type A

Type A blood can only receive from A and O blood types. Every blood group further divides into positive and negative types. Positive signifies evidence of D antigen. 

Positive types can receive from either positive or negative, but negative types must only receive from negative. 

Plasma Blood Types

Plasma, the pale yellow liquid part of blood, contains proteins that allow the blood to clot. Plasma makes up 55% of the body’s total blood and a plasma transfusion is needed if someone has a blood clotting disorder or has suffered excessive bleeding. 

Plasma Type AB

Plasma blood transfusions almost act in the opposite way to red blood cell transfusion. AB plasma is the universal donor and O plasma is the universal recipient. This is because AB plasma does not have antibodies that may cross-react with recipient antigens. 

Pulmonary Circulation

Pulmonary circulation is the constant exchange of blood from the heart to the lungs that is happening in our bodies all the time. 

The arteries, capillaries, and veins transport our blood throughout our body and blood is always flowing in one direction thanks to the valves we have in our bodies.

Vena Cava, the Beginning of the Blood Circulation

Circulation of blood through the heart starts when blood returns to the heart through 2 big veins called the vena cava. The inferior vena cava carries blood from the lower body while the superior vena cava transports blood from the head, arms, and upper body.

Pulmonary artery

From the veins, blood is pumped from the right ventricle of the heart through the pulmonary artery to the lungs for oxygenation. The pulmonary artery is the only artery in the body that transports deoxygenated blood.


In the lungs, oxygen diffuses through the walls of the alveoli and capillaries into the red blood cells. This oxygen is then ultimately carried by the blood to the body tissue. 


Veins transport deoxygenated blood from the body’s tissues back to the heart. From there, blood flows through the pulmonary artery back to the lungs for oxygen, and the pulmonary vein carries the oxygenated blood back to the heart. 

Blockages Formed in Coronary Arteries

Blockages form in arteries when plaque builds up within them. This restricts blood flow and can be extremely dangerous. Coronary blockages affect the supply of oxygenated blood to the heart. When they are blocked they can cause angina or a heart attack. 

The Unobstructed Blood Vessel

All arteries are lined by a layer of cells called the endothelium which serves to keep the red blood cells, white bloods cells, and liquid plasma flowing freely through the arteries. 

Plaque Build-up

Damage to the endothelium can be caused by smoking, high blood pressure, or high cholesterol. Plaque may form and restrict blood flow. Plaque buildup can cause a rupture in the artery and the blood clot that forms over the rupture can cause a heart attack.

More Plaque Buildup

Plaque is a mix of cholesterol, cells, and debris that create a bump in the artery wall. As more cholesterol gets near the damaged endothelium, white blood cells stream in to digest the bad cholesterol, which causes the arterial bump to grow bigger. 

Completely Blocked Blood Vessels

Blocked arteries completely restrict blood flow to the heart. This doesn’t just happen in the arteries that carry blood from the heart, but all over your body. When blood can no longer flow through the arteries it causes a heart attack or stroke.

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