Platelets, or thrombocytes, are minute fragments of cells consisting of a small amount of cytoplasm surrounded by a cell membrane. They are roughly disk shaped and average approximately 3 mm in diameter.
The life expectancy of platelets is approximately 5 to 9 days. Platelets are produced within the marrow and are derived from megakaryocytes, which are extremely large cells with diameters up to 100 mm. Small fragments of these cells break off and enter the circulation as platelets.
Platelets play an important role in preventing blood loss. This prevention is accomplished in two ways: the formation of platelet plugs, which seal holes in small vessels; and the formation of clots, which help seal off larger wounds in the vessels.
If the number of platelets is too low, excessive bleeding can occur. However, if the number of platelets is too high, blood clots can form (thrombosis), which may obstruct blood vessels and result in such events as stroke, heart attack, pulmonary embolism or blockage of blood vessels to other parts of the body, such as the extremities of the arms or legs.
Ex. 23. Answer the following questions:
1. What is platelet? 2. What are the shape of platelet? 3. What is the life expectancy of platelets? 4. Where are the platelets produced? 5. What is the major function of platelets?
Ex. 24. Read and retell the following text:
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