Curriculum fit: GCSE Biology and Combined Science (transport in humans). US: high school life science (structure and function of body systems).
On this page
What Is It?
The circulatory system carries blood around the body to deliver oxygen and nutrients and to remove waste such as carbon dioxide. Humans have a double circulatory system: one loop takes blood to the lungs, and the other takes blood around the rest of the body.
The heart is a muscular pump with four chambers. Blood flows through arteries, veins and capillaries, which have different structures suited to their jobs.
Key Formulas and Ideas
- Right side of the heart pumps deoxygenated blood to the lungs; left side pumps oxygenated blood to the body.
- Arteries carry blood away from the heart (thick, elastic walls). Veins carry blood to the heart (valves). Capillaries exchange materials (walls one cell thick).
- Cardiac output = stroke volume × heart rate.
Worked Examples
Trace the Blood
Describe the path of blood from the body back to the body, via the lungs.
- Deoxygenated blood returns to the right atrium, then the right ventricle.
- The right ventricle pumps it to the lungs through the pulmonary artery.
- Oxygenated blood returns to the left atrium, then the left ventricle.
- The left ventricle pumps it to the body through the aorta.
✓ Answer Body → right side → lungs → left side → body
Cardiac Output
Stroke volume is 70 cm³ and heart rate is 72 beats per minute. Find the cardiac output.
- Cardiac output = stroke volume × heart rate.
- 70 × 72 = 5040.
✓ Answer 5040 cm³ per minute
Vessel Structure
Why do veins have valves?
- Blood in veins is at low pressure.
- Valves stop blood flowing backwards.
✓ Answer To prevent backflow
Common Mistakes to Avoid
- Saying arteries always carry oxygenated blood. The pulmonary artery carries deoxygenated blood.
- Mixing up left and right sides of the heart.
- Forgetting the units for cardiac output.
- Writing that capillaries have thick walls.
Practice Questions
Frequently Asked Questions
Why is the left ventricle wall thicker?
It pumps blood around the whole body, so it needs to generate more pressure.
What do red blood cells do?
They carry oxygen using haemoglobin and have no nucleus, which leaves more room for oxygen.
Keep Going With These Guides
Aerobic and Anaerobic Respiration: Equations and Differences
Compare aerobic and anaerobic respiration in animals and yeast, with the equations, oxygen debt and practice questions to test yourself.
GCSEHigh SchoolAPDiffusion, Osmosis and Active Transport: Differences Explained
Compare how substances move into and out of cells, what affects the rate and how to calculate percentage change in a potato practical.
GCSEHigh SchoolAPHomeostasis: Blood Glucose, Temperature and Negative Feedback
Learn how the body keeps conditions steady, how insulin and glucagon control blood glucose and what happens in diabetes, with examples.
GCSEHigh SchoolAPSources and Review
Reviewed by StudyMorrow editorial review on 5 Nov 2026.