Trace single circulation through a fish
Follow one complete journey through the heart, gills and body.
Section B9.1 · Circulatory systems
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall gas exchange and capillaries.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where does fish blood gain oxygen?
What you will learn
- Count heart passages in a complete circuit
- A capillary region reduces pressure
Count heart passages in a complete circuit
In a fish’s single circulation, blood travels heart → gill capillaries → body capillaries → heart. It passes through the heart once during one complete journey around the two exchange regions. Single does not mean one vessel, one blood cell or only one exchange surface.
Blood gains oxygen at the gills and then delivers it to body tissues. The blood reaching the heart from the body has less oxygen than blood leaving the gills. Deoxygenated means relatively low oxygen, not necessarily no oxygen at all.
A capillary region reduces pressure
Pressure falls as blood passes through the narrow gill capillary network. Blood then travels to body tissues without first returning to the heart for another pressure-producing contraction. This sequence explains why the arrangement differs from mammalian double circulation.
A circuit can be traced from any starting point if its order is retained. Starting at the gills gives gills → body → heart → gills. Count one heart passage between the starting point and its first return; repeating the route twice would incorrectly count two circuits as one.
Sources for this lesson
Pause and explain
Which is one complete fish circuit?
Worked example
Starting at the body, write one complete fish circulation and count heart passages.
Show the worked solution
- Continue from body to heart, then gills.
- Return from gills to body; count the single heart encounter.
Answer Body → heart → gills → body; one passage through the heart.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
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.
Fill the route: heart → ___ → ___ → heart.
Give me a hint
Gas exchange occurs before delivery to the body.
Compare my reasoning
- After the heart, blood goes to gills.
- After gills, it travels to body tissues.
Heart → gills → body → heart.
Look for these in your work
- Correct order
- One heart passage in the circuit
Start a fish circuit at the gills rather than the heart. Write the route to its first return to the gills.
Give me a hint
Rotate the route; do not add another pump stage.
Compare my reasoning
- Gills supply the body.
- Body returns to heart, which sends blood to gills.
Gills → body → heart → gills.
Look for these in your work
- Preserve the route
- Return to the chosen starting point
Why is “single means only one capillary network” wrong for the supplied fish model?
Give me a hint
Identify both exchange regions.
Compare my reasoning
- Gill capillaries exchange gases.
- Body capillaries exchange materials with tissues, although the heart is passed once.
The circuit includes gill and body capillary networks; single refers to one heart passage per complete circuit.
Look for these in your work
- Name both networks
- Define single using heart passages
Common mistakes
- Inserting a second heart visit between gills and body.
- Defining single circulation as one blood vessel.
What can you explain now?
Which is one complete fish circuit?
Compare with the explanation
Heart → gills → body → heart
The heart is passed once per complete circuit.
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Make it stick
Trace a fish circuit and explain what the word single counts.
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.