National Geographic: Heart 101
Want to learn more about the heart and see how it works? Watch this video.
Circulatory system
The circulatory system transports blood, oxygen and nutrients around the body. Understanding how it works helps us see how the body maintains its energy levels, responds to changes and stays health. Use this resource to explore the components and functions of the circulatory system.
The circulatory system or cardiovascular system consists of the heart, blood vessels and blood. They work together to sustain life by delivering vital substances to cells and removing waste products.
Blood
Blood is the fluid in our circulatory system. It is responsible for transporting oxygen, nutrients and hormones around the body. Its composition is summarised in the table.
| Component | Proportion |
|---|---|
| Red blood cells | \(44\%\) |
| White blood cells | \(<1\%\) |
| Platelets | \(<1\%\) |
| Plasma | \(55\%\) |
Keep reading for more detail.
Red blood cells
The red blood cells or erythrocytes (“erythro“ meaning “red“ and “cyte“ meaning “cell“) are responsible for transporting oxygen from the lungs to the rest of the body, and returning carbon dioxide waste to the lungs for removal from the body. They are often abbreviated to RBC.
Did you know?
The size of RBCs can differ between animals. Elephants and capybaras have RBCs larger than \(110\) MCV (where MCV is mean corpuscular volume, a measure of red blood cell size). Compare this to a sheep's which is closer to \(20\) MCV.These cells contain a protein called haemoglobin, which contains iron that binds to oxygen. The presence of iron gives RBCs their colour.
RBCs have a biconcave shape where they curve inwards on both sides. This increases their surface area for gas exchange and allows them to move easily through blood vessels.
White blood cells
The white blood cells or leukocytes (“leuko” meaning “white” and “cyte” meaning “cell”) are cells of the immune system. There are many types of leukocytes, like granulocytes, monocytes and lymphocytes, which all play different roles in the immune response. They are critical for helping the body fight infection and other diseases.
You can learn more about the immune response by going to Immunology.
Platelets
Platelets or thrombocytes (“thrombo” meaning “clot” and “cyte” meaning “cell”) are small cell fragments that play a central role in clotting.
When an injury occurs, they gather at the site and stick together to form a sort of plug that prevents excessive bleeding. They also release chemicals that activate during the clotting process to stabilise this plug. This gives the body some time to repair the tissue.
Plasma
Plasma makes up the biggest component of the whole blood volume. It is mostly made of water and serves as a medium for carrying nutrients, dissolved gases like carbon dioxide (and small amount of oxygen), hormones, and proteins like antibodies and clotting factors.
Plasma helps maintain blood pressure, regulate body temperature and balance pH levels to make sure that our cells function properly.
Blood vessels
The blood vessels are the network of pathways that carry blood throughout the body. There are three main types.
- Arteries transport oxygen-rich blood away from the heart to the body's tissues.
- Veins carry oxygen-depleted blood back to the heart for reoxygenation.
- Capillaries are the tiny vessels where gas exchange occurs between the blood and tissues.
In the figure shown, arteries branch into smaller arterioles, which then branch into capillaries. The arteries and veins join at the capillary bed, then transition into venules, which then join up to form veins.
Together, these vessels ensure that blood flows efficiently, delivering vital substances and removing waste from the body.
The heart
The heart is a muscular organ that pumps blood throughout the body to distribute oxygen, nutrients and hormones.
Animated Human Heart (inside) by raito via Sketchfab, licensed under Standard Sketchfab License
An interactive three-dimensional model of a beating human heart. A section of the front of the heart has been removed to show the atria, ventricles and valves inside. The walls of the heart are shown expanding and relaxing as the muscles pump and valves open and close to push blood around the body.
It has four chambers:
- the atria – two upper chambers
- the ventricles – two lower chambers.
The heart works in a coordinated way, with the right side of the heart involved in the pulmonary circulation and the left side involved in the systemic circulation.
Gas exchange occurs at the capillary beds, which are networks of tiny blood vessels where oxygen is delivered to cells and carbon dioxide is picked up for removal. These capillary beds are most dense around major organs, like the heart, brain, liver and kidneys.
Pulmonary circulation
In the pulmonary circulation, blood travels between the heart and lungs. It allows us to exchange carbon dioxide for oxygen.
- Oxygen-poor blood returns from the heart through veins in the right atrium, particularly via the vena cava (the largest vein in the body).
- The blood moves to the right ventricle.
- The right ventricle pumps the blood to the lungs through the pulmonary arteries for reoxygenation.
Systemic circulation
In the systemic circulation, blood travels between the heart and the rest of the body. It involves the distribution of nutrients and oxygen to cells throughout the body, and collection of waste products and carbon dioxide for removal.
- Oxygen-rich blood returns to the left atrium.
- The blood moves to the left ventricle.
- The left ventricle pumps the oxygenated blood through the aorta (the largest artery in the body) to supply the entire body.
The atria receives blood and the ventricles pump it out.
Did you know?
The rhythmic thumping of your heart is made by the valves in your heart opening and closing.
Blood in the circulatory system is kept moving in one direction by valves. The heart's natural pacemaker, known as the sinoatrial node regulates the heartbeat, coordinating contractings of the heart muscles to keep a steady rhythm.
Watch this video to learn more about the heart and see it in action.
Veins are blood vessels containing blood flowing to the heart. While arteries have blood flowing from the heart. The blue is representative of blood vessels containing deoxygenated blood, while the red blood vessels have oxygenated blood fresh from the lungs.
Now, let's follow the path of the blood through the heart. The superior vena cava receives blood from the head, neck, upper limbs and chest. Meanwhile, the inferior vena cava receives blood from the trunk, viscera and lower limbs. Both the superior and inferior vena cava end up in the right atrium, one of the four chambers of the heart.
The heart not only has four chambers, it also have four valves. The purpose of the valves is to keep blood moving in the right direction and not flow backwards.
Blood exits the right atrium through the tricuspid valve so called because it has three flaps, and enters the right ventricle. The blood exits the right ventricle through the pulmonary valve and enters the right pulmonary artery.
Again, it is an artery because blood is flowing away from the heart, but it is blue because it lacks oxygen.
The pulmonary artery then splits into the left and right pulmonary arteries, which go to each respective lung. In the lungs, gas exchange occurs. The blood discards carbon dioxide and picks up oxygen.
Now, blood comes back from the lungs through the pulmonary veins, entering the left atrium. Next, the blood is pumped into the left ventricle through the mitral, or bicuspid valve.
Finally, the oxygenated blood leaves the left ventricle through the aortic semi-lunar valuve, entering the aortic arch.
The aorta, which is the largest of all the arteries, distributes the oxygenated blood to the rest of the body. The aortic arch has three major branches, which supply the head and arms with blood.
Then the aorta curls downward behind the heart, forming the descending aorta, which descends through the chest and continues down through the abdomen. In the abdomen, the descending aorta splits to the supply the pelvis and legs with blood.
Summary
The components of the circulatory system and their functions are outlined in the table.
| Component | Function |
|---|---|
| Blood | Transports oxygen, nutrients and hormones around the body |
| Blood vessels | Carries blood throughout the body |
| Heart | Pumps blood around the body |
Exercise
See how well you understand the circulatory system with a quick quiz.
