MARY MARTIN / SCIENCE PHOTO LIBRARY MARY MARTIN / SCIENCE PHOTO LIBRARY
Colour enhanced composite of scanning electron micrographs (SEM) of normal human blood cells. One can see red blood cells (red), white blood cells (yellow) (including a neutrophil, a monocyte, and a lymphocyte), and many small disc-shaped (mostly inactive) platelets (purple). Blood circulates through the heart, arteries, capillaries, and veins. It is composed of red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes) suspended in plasma, which is a straw-coloured fluid composed of proteins, water, and salts. Erythrocytes are made in the bone marrow. Mature red blood cells are non-nucleated. In place of a nucleus, they contain haemoglobin. The iron in haemoglobin is an important protein which allows the cell to carry.
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