Trace the closed double circulation
Trace blood through the pulmonary and systemic circuits and explain why the mammalian system is closed and double.
AS Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; microscope recognition, biological drawing and supervised practical competence require practical work, while human subject-teacher review remains pending.
Before you begin
- Know that mass transport moves substances between exchange surfaces and cells, and that diffusion occurs down a concentration or partial-pressure gradient.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What makes mammalian circulation double?
What you will learn
- State the parts of a closed double circulation
- Trace blood through heart, lungs and body without mixing the two circuits
Build the explanation
A mammalian circulatory system is closed: blood stays within the heart and blood vessels. It is double because blood passes through the heart twice during one complete circuit: once on the route to the lungs and once on the route to the body. The pathway is right heart → pulmonary vessels → lungs → pulmonary veins → left heart → aorta and systemic vessels → body → vena cava → right heart. Arteries carry blood away from the heart; veins return it. Arterioles and venules connect these larger vessels to capillary networks.
Apply it carefully
Keep the two circuits in order. The pulmonary artery carries blood from the right ventricle to the lungs; the pulmonary vein returns it to the left atrium. The aorta carries blood from the left ventricle to the body; the vena cava returns it to the right atrium. Vessel names follow direction, not oxygen content. The two sides are separated in the normal adult heart, so the systemic route receives oxygenated blood.
Pause and explain
Which order traces one full route from the right ventricle?
Worked example
Complete the route: right ventricle → ___ → lungs → ___ → left atrium.
Show the worked solution
- Blood leaves the right ventricle through the pulmonary artery.
- After gas exchange, it returns from the lungs through the pulmonary veins.
Answer Right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Trace blood from the right ventricle back to the right atrium, naming the two circuits.
Give me a hint
Write the lung route first, then the body route.
Compare my reasoning
- Right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium/left heart.
- Left ventricle → aorta → body capillaries → vena cava → right atrium.
Right ventricle → pulmonary artery → lungs → pulmonary veins → left heart, then left ventricle → aorta → body → vena cava → right atrium. The lung route is pulmonary and the body route is systemic.
Look for these in your work
- Pulmonary route in correct order
- Systemic route in correct order
- Both heart passages identified
A student calls the pulmonary artery a vein because its blood has low oxygen. Correct the explanation.
Give me a hint
Use direction to classify a vessel.
Compare my reasoning
- An artery carries blood away from the heart.
- The pulmonary artery leaves the right ventricle and carries blood to the lungs.
The pulmonary artery is an artery because it carries blood away from the heart, from the right ventricle to the lungs. Its oxygen content does not determine whether it is an artery or vein.
Look for these in your work
- Direction rule stated
- Pulmonary route applied
- Oxygen content separated from vessel name
A tracer is injected into the vena cava. Describe how it reaches the body tissues and returns to the injection region.
Give me a hint
Follow the blood through both heart pumps.
Compare my reasoning
- Vena cava → right atrium/ventricle → pulmonary artery → lungs → pulmonary veins → left heart.
- Left ventricle → aorta → systemic capillaries → veins → vena cava.
It enters the right heart, travels through the pulmonary artery to the lungs, returns by pulmonary veins to the left heart, then leaves through the aorta and passes through systemic vessels and capillaries before returning in veins to the vena cava.
Look for these in your work
- Both circuits included
- Lungs included between right and left heart
- Systemic capillaries and venous return included
Common mistakes
- Defining artery as oxygen-rich rather than blood moving away from the heart
- Skipping the second heart passage
- Reversing pulmonary artery and pulmonary vein
What can you explain now?
What makes mammalian circulation double?
Compare with the explanation
Blood passes through the heart twice per complete circuit
One passage pumps blood to the lungs; a second pumps it to the body.
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Make it stick
Why is mammalian circulation described as closed and double?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.