The third of the cellular elements within the blood are the platelets (thrombocytes). Platelets are actually small, round or oval cell fragments. They are about 2 to 4 mm in diameter and have no nuclei. Platelets are formed in the red bone marrow as pinched-off portions of the very large megakaryocytes. Each megakaryocyte, which is confined to the bone marrow, can produce up to 1000 platelets. Normally, there are approximately 300,000 platelets per microliter of blood. They are replaced about once every 10 days.

Platelets are essential for many aspects of hemostasis, or the cessation of blood loss. Several substances are found within the cytoplasm of platelets that contribute to the arrest of bleeding as well as vessel repair:

• Actin and myosin molecules, and thrombosthenin, are contractile proteins that enable platelets to contract.

• Fragments of the endoplasmic reticulum and the golgi apparatus produce enzymes and store calcium.

• Mitochondria and enzyme systems form ATP and ADP.

• Enzyme systems produce prostaglandins; these are substances involved with formation of platelet plugs as well as limitation of clot growth.

• Fibrin-stabilizing factor is a protein involved with blood coagulation.

• Growth factor facilitates vascular endothelial cell, vascular smooth muscle cell, and fibroblast multiplication and growth, leading to repair of damaged blood vessels.

These substances are discussed more fully in the following section.

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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