The Body's Transport System
Every cell in your body needs a constant supply of oxygen, glucose, and other nutrients to survive. At the same time, cells produce waste products — like carbon dioxide — that must be removed. The circulatory system is the body's transport network, delivering supplies and collecting waste around the clock.
The circulatory system has three main parts: the heart (the pump), the blood vessels (the tubes), and the blood (the transport medium). Together they form a closed loop that blood travels through continuously.
The Heart
The heart is a muscular organ about the size of your fist, located slightly left of centre in your chest. It is made entirely of cardiac muscle, which contracts about 70 times per minute at rest — around 100,000 times per day without stopping.
The heart has four chambers: the right atrium and right ventricle on one side, and the left atrium and left ventricle on the other. The right side receives deoxygenated blood from the body and pumps it to the lungs to pick up oxygen. The left side receives oxygenated blood from the lungs and pumps it out to the rest of the body. Valves inside the heart make sure blood only flows in one direction and cannot flow backwards.
Your heart beats about 3 billion times over an average lifetime. If you laid all the blood vessels in your body end to end, they would stretch about 100,000 km — that is enough to circle the Earth about 2.5 times!
Blood Vessels
There are three types of blood vessel. Arteries carry blood away from the heart. They have thick, muscular walls to withstand the high pressure of blood being pumped from the heart. The main artery leaving the heart is the aorta.
Veins carry blood back towards the heart. The pressure is lower, so vein walls are thinner. Many veins contain valves to prevent blood from flowing backwards. Capillaries are the tiniest vessels — so narrow that red blood cells must pass in single file. Capillaries have walls just one cell thick, allowing easy exchange of substances between blood and body cells.
Memory trick for vessels: Arteries carry blood Away from the heart. Veins carry blood back Via the veins (returning). Capillaries are tiny and make contact with every cell.
Blood and What It Carries
Blood is a liquid tissue made up of a watery fluid called plasma and three types of cells. Red blood cells contain haemoglobin, a protein that binds to oxygen in the lungs and releases it in body tissues. White blood cells are part of the immune system, fighting infections and foreign invaders. Platelets are tiny cell fragments that clump together to seal wounds and stop bleeding.
Blood also transports dissolved nutrients (like glucose and amino acids), hormones, heat, and waste products like carbon dioxide, all dissolved in plasma.
Students often mix up arteries and veins by saying "arteries carry oxygenated blood." This is usually true, but not always — the pulmonary artery carries deoxygenated blood from the heart to the lungs. The key rule is: arteries carry blood away from the heart, regardless of whether that blood is oxygenated or not.
Real world: CPR (cardiopulmonary resuscitation)
When a person's heart stops, blood circulation ceases and brain cells begin dying within 4 minutes. CPR — chest compressions and rescue breaths — manually pumps blood around the body, maintaining oxygen supply to the brain until a defibrillator can restart the heart's normal rhythm. The technique directly mimics what the heart's left ventricle does naturally with every beat.
Real world: blood pressure monitors
A sphygmomanometer (blood pressure cuff) measures two pressures: systolic (when the ventricles contract, e.g. 120 mmHg) and diastolic (when they relax, e.g. 80 mmHg). High blood pressure (hypertension) means artery walls are under excessive stress with each heartbeat. Doctors use this reading daily to diagnose cardiovascular risk — applied knowledge of heart anatomy and blood vessel function.
Real world: exercise and pulse rate
During exercise, muscles need more oxygen and produce more CO₂. The heart rate increases to circulate blood faster and deliver more oxygen. A trained athlete's heart muscle is stronger and pumps more blood per beat (higher stroke volume), so their resting heart rate can be as low as 40 bpm, while an untrained person averages 60–80 bpm. Monitoring pulse rate is a direct window into cardiovascular health.
Practice Problems
How many chambers does the human heart have, and what are they called?
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What is the job of red blood cells?
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Name the three types of blood vessel and describe one difference between arteries and veins.