DENNIS KUNKEL MICROSCOPY / SCIENCE PHOTO LIBRARY DENNIS KUNKEL MICROSCOPY / SCIENCE PHOTO LIBRARY
Human blood - red blood cells, 3 T-lymphocytes and activated platelets, composite coloured scanning electron micrograph (SEM). Human red blood cells are the most abundant type of cell in human blood, accounting for 40% of the blood volume. Each cubic millimetre of blood contains around five million of these tiny, flexible disc-shaped cells. The red colour comes from the iron-containing protein haemoglobin, which picks up oxygen in the lungs and distributes it around the body. Because the cells have no nucleus and are subjected to constant physical action, they last only four months before being destroyed and broken down. T lymphocytes are involved in the specific immune response and are composed mainly of precursor T cells and B cells (pre-T cells and pre-B cells). Platelets are cell fragments in the blood that play an essential role in blood clotting and wound repair. Platelets can also activate certain immune responses. Magnification: x800 when shortest axis.
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