The heart and circulatory system
Double circulation, heart structure, and the three types of blood vessel.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 9.1.1Describe the structure and function of the heart and the double circulatory system.
- 9.1.2Relate the structure of arteries, veins and capillaries to their functions.
6 minute read
The heart and circulatory system
Mammals have a double circulatory system: blood passes through the heart twice for each complete circuit of the body.
- Pulmonary circulation — heart → lungs → heart, picking up oxygen.
- Systemic circulation — heart → body → heart, delivering it.
The advantage is pressure. Blood is re-pressurised after the lungs, so it can be pushed round the whole body quickly, giving a high delivery rate of oxygen and glucose — necessary for warm-blooded animals with high energy demands.
Structure of the heart
Four chambers: two thin-walled atria receiving blood, two thick-walled ventricles pumping it out. Valves between them prevent backflow. The left ventricle wall is much thicker than the right, because the left must pump blood all the way around the body while the right only pumps to the nearby lungs. This is a favourite exam question and the answer is always about the distance and therefore the pressure required.
The three vessels
| Artery | Vein | Capillary | |
|---|---|---|---|
| Direction | away from heart | towards heart | between them |
| Wall | thick, muscular, elastic | thinner | one cell thick |
| Lumen | narrow | wide | very narrow |
| Valves | no | yes | no |
| Pressure | high | low | falling |
Arteries have thick elastic walls to withstand and smooth out high pressure surges. Veins have valves because the pressure is too low to prevent backflow on its own. Capillaries are one cell thick to allow rapid exchange by diffusion over a short distance.
Think of it like this
The double circulation is a relay with a re-fuelling stop. Rather than one exhausted runner circling the whole track, the baton returns to the centre to be re-launched at full speed for the long lap.
Worked examples
Method, step by step
Explain why veins contain valves but arteries do not.
- 1Blood in arteries is at high pressure, having just left the heart, so it cannot flow backwards.
- 2Blood in veins is at much lower pressure, far from the pumping action of the heart.
- 3Without valves, low-pressure blood could flow backwards, so valves ensure one-way flow back to the heart.
Veins carry blood at low pressure, so valves are needed to prevent backflow and keep blood moving towards the heart. Arteries carry high-pressure blood that cannot flow backwards, so valves are unnecessary.
Common misconceptions
- Saying arteries always carry oxygenated blood. The **pulmonary artery** carries deoxygenated blood to the lungs — the rule is about direction (away from the heart), not oxygen.
- Thinking the left side of the heart is "stronger because it is more important". It is thicker specifically because the systemic circuit is longer and needs higher pressure.
- Forgetting valves in veins. Their presence is a standard mark, explained by the low pressure in veins.
In the exam
- For "explain why the left ventricle wall is thicker", the marking points are: pumps blood to the whole body / a much greater distance, so needs to generate a higher pressure.
- Capillary questions almost always want "one cell thick, so a short diffusion distance" plus "large surface area" — the same logic as the small intestine and alveoli.