Trace the two circuits of a mammal
Distinguish the lung circuit from the body circuit and count both heart passages.
Section 9.1 · Circulatory systems
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Recall single circulation and gas exchange.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does pulmonary refer to in this route?
What you will learn
- The pulmonary circuit returns to the heart
- Join the circuits without mixing them
The pulmonary circuit returns to the heart
In mammalian double circulation, the right side of the heart pumps blood to the lungs. Gas exchange increases its oxygen content, and the blood returns to the left side of the heart. The pulmonary circuit is heart → lungs → heart.
The left side then pumps blood to the rest of the body. Body tissues receive oxygen and nutrients; blood returns to the right side. The systemic circuit is heart → body → heart. Pulmonary means associated with the lungs; systemic refers here to the body circuit.
Join the circuits without mixing them
One complete journey can be written right heart → lungs → left heart → body → right heart. Blood passes through the heart twice, once in each side, before returning to its starting point. The two circuits are connected in sequence, not two independent supplies of blood.
The sides are separated so oxygen-rich and oxygen-poor blood are largely kept apart in the normal adult model. Diagram colours are conventions; all blood is red, with different shades. A blue line is not evidence that the blood is literally blue.
Pause and explain
Why is mammalian circulation called double?
Worked example
Write one mammalian circuit beginning immediately before blood enters the right heart.
Show the worked solution
- Follow right heart to lungs and then left heart.
- Continue to body and return to right heart.
Answer Right heart → lungs → left heart → body → right heart; two heart passages in one complete circuit.
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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.
Complete right heart → ___ → left heart → ___ → right heart.
Give me a hint
Identify gas exchange and tissue delivery.
Compare my reasoning
- The right heart supplies the lungs.
- The left heart supplies the body.
Right heart → lungs → left heart → body → right heart.
Look for these in your work
- Correct sequence
- Keep the two heart sides distinct
Define pulmonary and systemic circuits using a start and return at the heart.
Give me a hint
Both circuits return to the pump.
Compare my reasoning
- Pulmonary goes to lungs and back.
- Systemic goes to body tissues and back.
Pulmonary: heart → lungs → heart. Systemic: heart → body → heart.
Look for these in your work
- Name both destinations
- Include return to heart
A route is drawn body → right heart → lungs → left heart → body. Is it a complete double circulation even though it does not start at the heart?
Give me a hint
Count passages before the starting point is reached again.
Compare my reasoning
- It returns to the body after traversing lungs.
- It includes right and left heart passages.
Yes; starting point is arbitrary, and the route includes two heart passages before returning to body.
Look for these in your work
- Preserve order
- Count both heart passages
Common mistakes
- Calling the pulmonary circuit the body circuit.
- Treating blue diagram blood as literally blue.
What can you explain now?
Why is mammalian circulation called double?
Compare with the explanation
Blood passes through the heart twice per complete circuit
It is pumped to lungs and then pumped to body.
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Make it stick
Trace both circuits without relying on red and blue colours.
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.