Table of Contents
Definition of Platelet, Function
The cellular structure indispensable for coagulation (clotting), also called a thrombocyte.
Platelets, which are actually cell fragments rather than intact cells, separate from parent cells in the bone marrow called megakaryocytes, the largest cells in the bone marrow.
When platelets emerge into the circulation they become the smallest cell particles in the circulating blood.
Their small size permits them to travel into any blood vessel, even the tiniest arterioles and venules, to respond to bleeding. Platelets lack nuclei and thus, like erythrocytes, cannot divide. They live in the circulation for about 10 days.
Normal Platelet Count
The normal count of platelets in the blood is 130,000 to 400,000 per cubic milliliter.
The spleen holds about 30 percent of the blood’s platelets within its red pulp, releasing them into the circulation when needed to respond to bleeding. This containment helps reduce the risk for inadvertent agglutination as platelets swirl into contact with one another in the bloodstream.
Any breach in a blood vessel that allows blood to escape results in the release of the enzyme tissue factor (factor III), which attracts droves of platelets to the site. As the platelets agglutinate (come into contact with the damaged site and with one another) they release chemicals such as serotonin, thromboxane, and phospholipids that extend and focus the coagulation cascade.
For further discussion of blood and lymph structure and function, please see the overview section “The Blood and Lymph.”