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Biology
Two Circulatory Loops
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The blood journey through the human body
Building the circulation diagram…
Oxygen-rich bloodOxygen-poor bloodGas exchange (capillaries)O₂ particleCO₂ particle
Model notes
- This diagram illustrates the path of blood and is not to anatomical scale or position: the heart is drawn large so its four chambers can be labelled, and the body outline is only for orientation.
- The body has two lungs and many organs, but the blood drop follows one representative route (through the left lung and the lower body); a real heart pumps blood to every region at once.
- The drop moves at a pace set for each structure, slowing down in the heart, through the valves and in the capillaries so it can be observed. This pace does not represent real blood speed, pressure or heart rate.
- Red is oxygen-rich blood, blue is oxygen-poor blood and purple is blood exchanging gases in the capillaries. Real oxygen-poor blood is dark red, not blue, and should not be called “dirty blood”.
- The heart valves are drawn opening as the drop passes and closing afterwards to stress one-way flow; in a real heart the valves open and close with every contraction and relaxation of the atria and ventricles.
- The O₂ and CO₂ particles only show the direction of diffusion in the lung capillaries and the organ capillaries, not the number of molecules.