JOHN BAVOSI / SCIENCE PHOTO LIBRARY JOHN BAVOSI / SCIENCE PHOTO LIBRARY
Red blood cells. Artwork of red blood cells (erythrocytes) and platelets (thrombocytes, blue). Red blood cells contain the oxygen-carrying pigment haemoglobin (seen in the red blood cell at centre). On average there are 350 million haemoglobin molecules in each red blood cell. Haemoglobin has four globin polypeptide chains (pink/red), each with an iron-containing haem group (green). The haem groups bind reversibly to oxygen in the lungs, where it is high in concentration, and release it in tissues, where the concentration is low. They also carry carbon dioxide to the lungs for exhalation. Platelets are cells that are involved in blood clotting.
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