What is the double circulatory system? GCSE Biology
The double circulatory system is the way blood travels around the human body in two separate loops, with both loops passing through the heart. The first loop carries blood from the heart to the lungs and back (the pulmonary circuit). The second loop carries blood from the heart to the rest of the body and back (the systemic circuit).
This guide explains how the two loops work, why mammals need a double system rather than a single one, the structure of the heart that makes it possible, and the wording AQA examiners reward in your GCSE Biology answers.
Two loops, one heart
Blood passes through the heart twice for every full circuit of the body. The two loops never mix in a healthy adult.
High pressure to the body
The left side of the heart pumps oxygenated blood at high pressure so it reaches every cell. The right side pumps at lower pressure to the delicate lungs.
Better than a single system
Mammals need lots of energy. A double system delivers oxygen and glucose to cells faster than the single-loop systems found in fish.
What are the two circuits?
The two loops of the double circulatory system are the pulmonary circuit and the systemic circuit. Pulmonary means "to the lungs." Systemic means "to the rest of the body system." Together they make sure every cell in the body gets the oxygen it needs.
In the pulmonary circuit, deoxygenated blood is pumped from the right side of the heart to the lungs, picks up oxygen, and returns to the left side of the heart. In the systemic circuit, oxygenated blood is pumped from the left side of the heart around the body, drops off the oxygen, and returns deoxygenated to the right side of the heart.
An easy way to remember the circuits Right side of the heart equals lungs. Left side of the heart equals the rest of the body. Blood enters the right side deoxygenated and leaves the left side oxygenated. If you can say which side is doing which job, you have most of the marks for a definition question.
How blood flows through both circuits
AQA expects you to be able to trace blood through every chamber of the heart and name the major blood vessels. Following the blood step by step is the easiest way to learn it.
| Step | Where the blood is | Vessel or chamber |
|---|---|---|
| 1 | Deoxygenated blood returns from the body | Vena cava into the right atrium |
| 2 | Blood passes into the right ventricle | Through the tricuspid valve |
| 3 | Right ventricle pumps blood to the lungs | Pulmonary artery |
| 4 | Blood picks up oxygen | Alveolar capillaries in the lungs |
| 5 | Oxygenated blood returns to the heart | Pulmonary vein into the left atrium |
| 6 | Blood passes into the left ventricle | Through the bicuspid (mitral) valve |
| 7 | Left ventricle pumps blood to the body | Aorta |
| 8 | Blood drops off oxygen at body cells | Capillaries in tissues |
| 9 | Deoxygenated blood returns to the heart | Vena cava (back to step 1) |
Why do mammals need a double system?
Mammals are warm-blooded and need a lot of energy to maintain their body temperature and stay active. A double circulatory system delivers oxygen to cells faster than a single system because blood is pumped at high pressure to the body each time it leaves the heart.
In a single circulatory system like a fish has, blood passes through the gills and then straight to the body without being re-pumped. The blood slows down through the gill capillaries and never recovers its pressure. That works for fish but would not deliver enough oxygen for an active mammal.
| Feature | Single system (fish) | Double system (mammals) |
|---|---|---|
| Number of heart chambers | 2 | 4 |
| Times blood passes through the heart per circuit | 1 | 2 |
| Blood pressure to body cells | Low (already slowed by gills) | High (re-pumped by left ventricle) |
| Oxygen delivery rate | Slower | Faster |
| Energy demands supported | Low to medium | High |
The structure of the heart
The human heart has four chambers. The two atria sit on top and receive blood. The two ventricles sit below and pump blood out. The right side handles deoxygenated blood. The left side handles oxygenated blood. A thick muscular wall called the septum separates the two sides so the bloods do not mix.
The left ventricle has a much thicker muscular wall than the right ventricle. This is because it has to pump blood all the way around the body at high pressure, while the right ventricle only has to pump blood to the nearby lungs.
Left ventricle wall thickness: A reliable mark to pick up If you are asked why the left ventricle has a thicker wall than the right, the answer is: It pumps blood at higher pressure around the whole body, while the right ventricle only pumps blood to the lungs. That single sentence is worth two marks in most AQA mark schemes.
The role of valves
Valves keep blood moving in one direction through the heart. There are four valves in total. The tricuspid valve sits between the right atrium and right ventricle. The bicuspid (or mitral) valve sits between the left atrium and left ventricle. Two semilunar valves sit at the exits of the ventricles, in the pulmonary artery and the aorta.
Without valves, blood would flow backwards every time the heart muscle relaxed. AQA often asks students to label the valves on a heart diagram or explain what would happen if a valve became damaged.
Where students lose marks on circulatory system questions
Most lost marks are about being precise with the words "oxygenated" and "deoxygenated" and naming the correct chambers and vessels. AQA mark schemes are strict on terminology and will not give marks for vague answers.
Common mistakes that cost easy marks Mixing up the atrium and ventricle (atria are on top, ventricles are below). Mixing up the pulmonary artery and pulmonary vein (artery leaves the heart, vein returns to it). Saying the right side of the heart pumps oxygenated blood (it does not, it handles deoxygenated blood). Forgetting that the pulmonary artery is the only artery that carries deoxygenated blood. Saying the heart has "two chambers" instead of four.
Worked example: Why is the wall of the left ventricle thicker?
Exam question: Explain why the wall of the left ventricle is thicker than the wall of the right ventricle (3 marks).
Full-mark answer: The left ventricle pumps oxygenated blood to the whole body through the aorta. This requires high pressure to push blood through a much longer system of vessels. The right ventricle only pumps blood to the nearby lungs at lower pressure, so its wall does not need to be as thick.
That answer hits three mark-scheme points: Naming what the left ventricle pumps to, identifying the pressure difference, and explaining the reason for it.
Key facts to memorise for the exam
- Definition: Two loops of blood flow, both passing through the heart
- Pulmonary circuit: Right side of heart to lungs and back, deoxygenated to oxygenated
- Systemic circuit: Left side of heart to body and back, oxygenated to deoxygenated
- Four chambers: Right atrium, right ventricle, left atrium, left ventricle
- Left ventricle wall is thicker because it pumps blood at high pressure around the whole body
- Pulmonary artery: Only artery that carries deoxygenated blood (heart to lungs)
- Pulmonary vein: Only vein that carries oxygenated blood (lungs to heart)
- Mammals need a double system to deliver oxygen fast enough for high energy needs
