Circulatory System · 3D animation

One red blood cell, one lap

Speed 1×

Where the cell is

Right atrium

O₂

CO₂

100%

Deoxyhemoglobin · dark red

Drag the model to turn it and scroll or pinch to zoom; it keeps following the cell. The four blue dots stand for oxygen held by hemoglobin; each hemoglobin molecule carries up to four O₂, and one red blood cell holds about 270 million hemoglobin molecules. Simplified: at rest, blood heading back to the heart still carries most of its oxygen, but the animation empties the cell so the exchange is easy to see. Blood is never actually blue. Space plays or pauses, the arrow keys step through stages, F switches between following the cell and the whole body.