Platelets are essential for blood clotting

Besides producing erythrocytes and leukocytes, the stem cells in the bone marrow produce cells called megakaryocytes. Megakaryocytes are large cells that remain in the bone marrow and continually break off cell fragments called platelets. A platelet is just a tiny fragment of a cell without cell organelles, but it is packed with enzymes and chemicals necessary for its function: sealing leaks in blood vessels and initiating blood clotting (Figure 49.16).

Damage to a blood vessel exposes collagen fibers. When a platelet encounters collagen fibers, it is activated. It swells, becomes irregularly shaped and sticky, and releases chemicals called clotting factors, which activate other platelets and initiate the clotting of blood. The sticky platelets also form a patch at the damaged site.

The clotting of blood requires many steps and many clotting factors. The absence of any one of these factors can impair clotting and cause excessive bleeding. Because the liver produces most of the clotting factors, liver diseases such as hepatitis and cirrhosis can result in excessive bleeding. The sex-linked genetic disorder hemophilia (see Chapters 10 and 17) is an example of a genetic inability to produce one of the clotting factors.

Blood clotting factors participate in a cascade of chemical reactions that activate other substances circulating in the blood. The cascade begins with cell damage and platelet activation and leads to the conversion of an inactive circulating enzyme, prothrombin, to its active form, thrombin. Thrombin causes molecules of a plasma protein called fibrinogen to polymer ia)

An injury to the lining of a blood vessel exposes collagen fibers; platelets adhere and become sticky.

Platelet M

An injury to the lining of a blood vessel exposes collagen fibers; platelets adhere and become sticky.

Platelet M

Red blood cell

Collagen fibers

Clotting factors

1. Released from platelets and injured tissue

2. Plasma proteins synthesized in liver and circulated in inactive form

Platelet Plug

Platelet plug

Red blood cell

Collagen fibers

Platelet plug

Clotting factors

1. Released from platelets and injured tissue

2. Plasma proteins synthesized in liver and circulated in inactive form

Prothrombrin circulating in plasma

Thrombin

49.16 Blood Clotting (a) Damage to a blood vessel initiates a cascade of events that produces a fibrin meshwork. (b) As the meshwork forms, red blood cells are enmeshed in the fibrin threads, forming a clot.

Fibrinogen circulating in plasma

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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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