The Body's Transport Network
The circulatory system (also called the cardiovascular system) is the body's internal transport network. It delivers what cells need (oxygen, glucose, vitamins, hormones) and carries away what cells produce (carbon dioxide, water, urea). It consists of three main parts: the heart, blood vessels, and blood.
Humans have a double circulatory system: blood passes through the heart twice on each complete circuit — once to the lungs (pulmonary circuit) and once to the rest of the body (systemic circuit).
The Heart
The heart is a powerful muscular pump about the size of a fist, located slightly to the left of the centre of the chest. It beats about 70 times per minute at rest, propelling blood around the body.
The heart has four chambers:
- Right atrium — receives deoxygenated blood from the body
- Right ventricle — pumps deoxygenated blood to the lungs
- Left atrium — receives oxygenated blood from the lungs
- Left ventricle — pumps oxygenated blood to the rest of the body
Valves between the chambers and at the exits of the ventricles prevent blood from flowing backwards.
The left ventricle has a much thicker, more muscular wall than the right ventricle. This is because it must pump blood all the way around the body, while the right ventricle only pumps blood the short distance to the lungs.
Blood Vessels
Arteries — Carry blood away from the heart. They have thick, muscular, elastic walls to withstand the high pressure of blood pumped directly from the heart. Most arteries carry oxygenated blood (except the pulmonary arteries, which carry deoxygenated blood to the lungs).
Veins — Carry blood back to the heart. They have thinner walls than arteries and contain valves that prevent blood from flowing backward. Blood in veins is at lower pressure.
Capillaries — Tiny vessels, only one cell thick, that form dense networks throughout all tissues. Their thin walls allow oxygen, glucose, and other substances to diffuse out into cells, and for carbon dioxide and waste to diffuse in.
A useful memory trick: Arteries carry blood Away from the heart. Veins carry blood back to the heart: think of the V in veins pointing back toward the heart (like a return arrow).
Blood: What It Contains
Blood is a liquid tissue made of:
- Red blood cells (erythrocytes) — contain haemoglobin, which binds oxygen in the lungs and releases it in tissues. No nucleus; disc-shaped.
- White blood cells (leucocytes) — part of the immune system; fight infection.
- Platelets (thrombocytes) — tiny cell fragments that help blood clot when a vessel is damaged.
- Plasma — the pale yellow liquid (about 55% of blood volume) that carries dissolved substances: glucose, hormones, carbon dioxide, urea, and proteins.
Keeping the Heart Healthy
A healthy diet (low in saturated fats and salt), regular aerobic exercise, not smoking, and maintaining a healthy weight all reduce the risk of heart disease. Fatty deposits (atherosclerosis) can build up inside artery walls, narrowing them and increasing the risk of heart attack or stroke.
Real world: Feeling your pulse after exercise
After running, you can feel your heart beating faster by pressing two fingers on the inside of your wrist or neck (the carotid artery). This is your pulse — each beat is one contraction of the left ventricle forcing blood into the arteries. A resting pulse is about 60–80 bpm; during vigorous exercise it can exceed 180 bpm, delivering more oxygen to working muscles.
Real world: Blood donation and blood components
When you donate blood, the blood is often separated into components: red cells, platelets, and plasma — each given to different patients. Red cells for anaemia and surgery patients, platelets for cancer patients, plasma for burns victims. This real-world medical practice is possible precisely because blood is a mixture of distinct components with different functions.
Real world: Heart disease and artery health
Atherosclerosis — fatty deposits building up inside artery walls — is the leading cause of heart attacks globally. Risk is increased by a high-saturated-fat diet, smoking, lack of exercise, and high blood pressure. When a coronary artery (supplying the heart muscle itself) is narrowed, a blood clot can block it completely, causing a heart attack. News coverage of heart health campaigns directly relates to the biology of circulation.
Where do you see this in real life? Put your hand on your chest after exercise. Each heartbeat you feel is the ventricles contracting and pushing blood into the aorta. Count the beats for 15 seconds and multiply by 4 to get your heart rate in beats per minute — a direct observation of your circulatory system at work.
Practice Problems
answer="Right atrium → right ventricle → pulmonary arteries → lungs (picks up oxygen) → pulmonary veins → left atrium → left ventricle → aorta → arteries to head and brain." />
Why do capillaries have walls that are only one cell thick?