A red blood cell is a delivery truck for oxygen. Watch it load O₂ in the lungs (turning bright red) and drop it off in the tissues (turning darker), on a loop.
The oxygen loop
Oxygen transportlungs ↔ tissues
How it works
Hemoglobin is 4 protein chains, each with an iron-containing heme — so one RBC's hemoglobin carries up to 4 O₂ each, and a single red cell holds ~1 billion oxygen molecules.
📷 Reference image
A real labeled figure to compare with the interactive above.
Oxygen Transport — OpenStax, CC BY 3.0 (via Wikimedia Commons)
