Red blood cells are flexible discs with no nucleus, packed with hemoglobin. Their shape lets them squeeze through capillaries narrower than the cell itself.
Hemoglobin
Each hemoglobin molecule has four iron-containing heme groups, and each one can carry one oxygen molecule. Iron, vitamin B12 and folate are needed to make healthy red cells.
Loading and unloading
In the lungs, high oxygen levels fill hemoglobin almost completely. In working tissues, more carbon dioxide, more acid and a higher temperature make hemoglobin release oxygen more easily.
Carbon dioxide
Red cells also help carry carbon dioxide back to the lungs, mostly as bicarbonate made inside the cell.
Anemia
When hemoglobin is low, less oxygen reaches the tissues even if saturation is normal. Patients feel tired and breathless, look pale and have a fast heart rate.
At the bedside
Pulse oximetry shows the percent of hemoglobin carrying oxygen, not how much hemoglobin there is. Carbon monoxide binds hemoglobin tightly and can give a falsely normal reading.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).