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 healthy. 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
\(55\%\)
Click to reveal more detail about each component.
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. Image by qimono via Pixabay
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 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 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 a 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.
A diagram of a blood sample tube with the composition of blood shown.
Plasma, 55%
Proteins, 4%
Water, 50%
Other solutes, 1%
White blood cells and platelets, less than 1%
Platelets, less than 1%
Leukocytes, less than 1%
Red blood cells, 45%
Erythrocytes, 45%
Blood vessels
The blood vessels are the network of pathways that carry blood throughout the body. There are three main types.
Arteries generally transport oxygen-rich blood away from the heart to the body's tissues.
Veins usually 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. Blood vessels, adapted from image by Kelvinsong via Wikimedia Commons, licensed under CC BY-SA 3.0
Blood vessels
A diagram of the blood vessels showing how they are structured and connected.
An "Artery" is shown on the left. It branches into "Arterioles" and then "Capillaries".
An arrow towards the artery shows the flow of blood from the heart.
A "Vein" is shown on the right. It branches into "Venules", and then also "Capillaries".
An arrow away from the vein shows the flow of blood to the heart.
The capillaries meet in the middle to form a continuous network of vessels.
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.
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.
Chambers of the heart, adapted from image by ZooFari, licensed under CC BY-SA 3.0
Chambers of the heart
A diagram of the heart with the chambers visible.
Right atrium
Right ventricle
Left atrium
Left ventricle
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. Circulation, adapted from image by OpenStax College via Wikimedia Commons, licensed under CC BY 3.0
Blood circulation
A diagram showing the pathway of blood circulation. Blue pathways leading to the heart show deoxygenated blood and red pathways leading from the heart show oxygenated blood. The capillary beds are shown in purple to indicate gas exchange.
The "Pulmonary circulation" involves the lungs, heart and pulmonary artery and vein. There is an arrow from the heart through the pulmonary artery to the lungs, then back to the heart via the pulmonary vein.
The "Systemic circulation" involves the other major organs in the body. Arrows show movement of blood from the heart through the aorta to the capillary beds in the upper body, then back into the heart through the vena cava.
Blood from the aorta also goes to the liver through the hepatic artery, then returns to the heart through the hepatic vein. Blood goes to the stomach and intestines, which gets filtered into the liver through the hepatic portal vein.
The kidneys receives blood from the renal artery, which is returned to the heart via the renal vein.
The lower body also has arteries and veins that transport blood to and from the lower body.
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.
In pulmonary circulation, blood travels between the heart and lungs. It allows us to exchange carbon dioxide for oxygen.
Oxygen-poor blood returns from the rest of the body to the right atrium of the heart, 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.
Pulmonary blood vessels work in the opposite way than the rest of the blood vessels in our bodies. The pulmonary artery carries oxygen-depleted blood, while the pulmonary veins carry oxygen-rich blood.
In 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 the collection of waste products and carbon dioxide for removal.
Oxygen-rich blood returns to the left atrium via pulmonary veins.
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 contractions 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 has 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
Carry blood throughout the body
Heart
Pumps blood around the body
Exercise
See how well you understand the circulatory system with a quick quiz.
Read the scenario and use the information provided to answer the questions in the quiz.
Zainab is a Year 11 student studying VCE Physical Education. Her class is completing a practical looking at how different types of exercise affect heart rate.
In groups of four, the students take turns completing 10 minutes of jogging, cycling or skipping rope, with 20 minutes of rest in between exercises. They record their resting heart rate and heart rate after 10 minutes of exercise in the table. To measure heart rate, they use a wristband.
