A red blood cell is a delivery truck for oxygen. Watch it pick up O₂ in the lungs, where it turns bright red. Then watch it drop it off in the tissues, where it turns darker. It goes round and round.
The oxygen loop
Oxygen transportlungs ↔ tissues
How it works
Hemoglobin has 4 protein chains. Each chain has an iron-holding heme. So each hemoglobin carries up to 4 O₂. A red cell has about 270 million hemoglobins, so one cell can hold ~1 billion oxygen molecules.
📷 Reference image
A real labeled figure. Compare it with the interactive above.
Oxygen Transport — OpenStax, CC BY 3.0 (via Wikimedia Commons)
