The big idea: In a bony fish, blood passes through the heart once on each circuit; in a mammal it passes through twice.
Double circulation — twice through the heart: Mammals have double circulation: blood passes through the heart twice on each full trip round the body.
Pulmonary circuit
- Right side of the heart → lungs → back to the heart
- Picks up oxygen
Systemic circuit
- Left side of the heart → body → back to the heart
- Drops off oxygen
The advantage: After the blood loses pressure in the lungs, the left ventricle re-pressurises it. So the body gets blood at high pressure, and oxygenated and deoxygenated blood are kept completely separate.
| Feature | Single circulation (e.g. fish) | Double circulation (mammals) |
|---|---|---|
| Times blood passes the heart per body circuit | once | twice |
| Two separate circuits? | no — one loop | yes — pulmonary (lungs) and systemic (body) |
| Pressure delivered to the body | lower (drops after the gills) | high (re-pressurised by the left ventricle) |
| Oxygenated and deoxygenated blood kept apart? | less so | yes — right side deoxygenated, left side oxygenated |
The points to remember
- Fish: heart → gills → body → heart: one loop, a single circulation.
- Mammal: heart → lungs → heart → body → heart: two loops, a double circulation.
- Fish heart: one atrium and one ventricle, carrying only deoxygenated blood.
- Mammal heart: two sides; deoxygenated blood on the right, oxygenated on the left.
- Both: gas exchange in capillaries (gills or lungs), and blood in closed vessels.
Real example: a trout. A trout's heart has just one atrium and one ventricle. It pumps blood through the gills, and the same push must then carry it on round the whole body.
Trap: Each point must compare both animals: 'gills v lungs', 'once v twice'. One animal alone does not score.
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How it is asked: Expect circuit diagrams to read (where is the blood oxygenated, where is pressure lowest) or a 'compare and contrast' of fish and mammal circulation.
IB-style questionCompare and contrast[3 marks]
Compare and contrast the circulation of blood in bony fish and in mammals.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.