Blood Platelets

Platelets (plat'letz), or thrombocytes, are not complete cells. They arise from very large cells in the red bone marrow, called megakaryocytes, that fragment a little like a shattered plate, releasing small sections of cytoplasm— platelets—into the circulation. The larger fragments of the megakaryocytes shrink and become platelets as they pass through the blood vessels of the lungs.

Each platelet lacks a nucleus and is less than half the size of a red blood cell. It is capable of ameboid movement and may live for about ten days. In normal blood, the platelet count varies from 130,000 to 360,000 platelets per mm3.

Platelets help repair damaged blood vessels by sticking to broken surfaces. They release serotonin, which contracts smooth muscles in the vessel walls, reducing blood flow. Table 14.6 summarizes the characteristics of blood cells and platelets.

O What is the normal human blood platelet count? ^9 What is the function of blood platelets?

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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