Topic 9.1 · Supplement
Single and Double Circulation
The word single or double here is not describing how many hearts an animal has, or how many chambers sit inside it. It is describing something much narrower and much easier to get exactly right: how many times blood passes through the heart during one complete circuit of the body.
Keep that one unit of comparison — one complete circuit — fixed in your head before you look at either system, because almost every mistake in this section comes from comparing the wrong thing: chamber counts, heart size, or number of hearts, instead of the number of times blood passes through the heart on its way around the body once.
Fish: single circulation
In a fish, blood passes through the heart once per complete circuit. The route is heart → gills → body tissues → heart. Deoxygenated blood leaves the heart and travels to the gills, where it becomes oxygenated; it then continues straight on to the rest of the body without returning to the heart first, before eventually making its way back to complete the circuit.
Mammals: double circulation
In a mammal, blood passes through the heart twice per complete circuit, because two connected circuits share the same pump: pulmonary circulation (heart → lungs → heart) and systemic circulation (heart → body → heart). Deoxygenated blood is pumped from the right side of the heart to the lungs; oxygenated blood returns to the left side of the heart and is pumped out again, this time to the rest of the body. Written as one continuous route, that is heart → lungs → heart → body → heart.
Why double circulation is an advantage
After blood has passed through the narrow capillaries of the lungs, its pressure has dropped considerably — that is simply what happens to fluid pressure moving through a fine capillary network. In a fish, that lower-pressure blood then has to travel all the way around the rest of the body with no further boost. In a mammal, that same blood is returned to the heart first and pumped again before it sets off around the body, so it reaches the tissues at a considerably higher pressure than it would otherwise have. Higher systemic pressure means faster, more efficient delivery of oxygen and other transported substances to every part of the body — which is the actual advantage, not simply “mammals need more oxygen” stated on its own.
There is a second benefit built into the same arrangement: because the heart is divided by a septum into separate left and right sides, oxygenated blood returning from the lungs never mixes with deoxygenated blood returning from the body before it is sent out again. Tissues around the body therefore receive blood with a consistently high oxygen concentration, rather than a diluted mixture.
Seeing the two side by side
| Feature | Single circulation (fish) | Double circulation (mammals) |
|---|---|---|
| Heart passages per complete circuit | 1 | 2 |
| Route | heart → gills → body → heart | heart → lungs → heart → body → heart |
| Re-pumped after gas exchange? | No | Yes |
| Typical systemic pressure | Lower, since it is never boosted again | Higher, since the heart pumps it a second time |
Don’t confuse
“Double” does not mean two separate hearts working independently — pulmonary and systemic circulation are two connected halves of one circulatory system, driven by one heart. Nor does it mean blood travels around the body twice; it means blood passes through the heart twice while completing one circuit. And single circulation is not somehow “less one-way” than double circulation — both systems still rely on valves to keep blood moving in a single direction, exactly as described on the circulatory system page.
The chamber structure and valve action that make the mammalian side of this comparison possible are covered in full on the mammalian heart page.