The Probert Encyclopaedia of Medicine

PLATELETS

Platelets are tiny specialized cells that are activated whenever blood clotting or repair to a vessel is necessary. Although they are often called cells, they are really fragments of other cells. They are made in bone marrow and are much smaller than red blood cells. A drop of blood contains some 15 million platelets. When a blood vessel is cut, platelets rush to the vessel and swell into odd, irregular shapes, grow sticky and clog at the cut, creating a plug. If the cut is too large for platelets, they send out signals to initiate clotting by releasing a hormone called serotonin, which stimulates blood vessels to contract thus reducing the flow of blood. Clotting is fundamentally a change of the soluble plasma protein fibrinogen into an insoluble, thread-like protein, called fibrin. More than a dozen factors are involved in this conversion. The fibrin strands mesh around the blood cells and then contract, squeezing a clear yellowish fluid called serum, and forming a solid clot. Clotting staunches bleeding and creates a scaffold on which to build new tissue.
Research Platelets