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.
The best known of all clotting diseases is hemophilia. It is an inherited disease from which only men suffer, although women may be carriers and pass it on to their sons. It is actually a relatively rare disease, affecting only one boy in 10,000. It is caused by the absence of one of the clotting factors, a ~plasma~ |protein| known as anti-hemophiliac globulin. The slightest injury to a person with this disease can lead to uncontrolled |bleeding|. Today, ~blood~ transfusions and injections of the missing factor, which can be extracted from ~plasma~, give hemophiliacs the semblance of a normal life.
