Anatomy & Physiology

Anatomy & Physiology Study Guide

18.6 Platelets Platelets are flattened discs that appear round when viewed from above and spindle-shaped in section or a blood smear. They average about 4 µm in diameter and are roughly 1 µm thick. Platelets are a major participant in a vascular clotting system that also includes plasma proteins and the cells and tissues of the blood vessels. Platelets are continuously replaced. Each platelet circulates for 9–12 days before being removed by phagocytes, mainly in the spleen. Each microliter of circulating blood contains 150,000– 500,000 platelets; 350,000/µL is the average concentration. One-third of the body’s platelets at any given moment are held as reserve supply in the spleen and other vascular organs, rather than in the bloodstream. During a circulatory crisis, such as severe bleeding, these reserves are mobilized. An abnormally low platelet count (80,000/µL or less) is known as thrombocytopenia . The functions of platelets include: • The release of chemicals important to the clotting process: By releasing enzymes and other factors at the appropriate times, platelets help initiate and control the clotting process. • The formation of a temporary patch in the walls of damaged blood vessels: Platelets clump together at an injury site, forming a platelet plug, which can slow the rate of blood loss while clotting occurs. • Active contraction after clot formation has occurred: Platelets contain filaments of actin and myosin. After a blood clot has formed, the platelet filaments contract to shrink the clot and reduce the size of the break in the vessel wall. Platelet Production Thrombocytopoiesis , or platelet production, occurs in the bone marrow. Normal bone marrow contains some megakaryocytes , which are enormous cells (up to 160 µm in diameter) with large nuclei from which platelets originate. Bone marrow contains many megakaryocytes that manufacture structural proteins, enzymes, and membranes. They then begin shedding cytoplasm in small membrane-enclosed packets. These packets are the platelets that enter the bloodstream. After a mature megakaryocyte loses all its cytoplasm, it produces about 4000 platelets before the nucleus is engulfed by phagocytes and broken down for recycling. 18.7 Hemostasis The process of hemostasis, the cessation of bleeding, halts the loss of blood through the walls of damaged vessels. At the same time, it establishes a framework for tissue repairs. Hemostasis consists of three phases: the vascular phase, the platelet phase, and the coagulation phase. However, the boundaries of these phases are somewhat arbitrary. In reality, hemostasis is a complex cascade in which many things happen at once, and all of them interact to some degree. The Vascular and Platelet Phase When the wall of a blood vessel is severed, it triggers a contraction of the smooth muscle fibers in the vessel wall. This local contraction, called a vascular spasm, decreases the diameter of the vessel at the site of injury to attempt to slow or stop blood loss. The vascular spasm lasts about 30 minutes,

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