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What is Hemostasis?

By Rachel Burkot
Updated Mar 03, 2024
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Hemostasis is the process by which blood is changed to a solid state. It is what stops the bleeding after an injury to the blood vessels occurs. The blood vessels are protected by cells that prevent the formation of thrombin, a coagulation protein that catalyzes reactions in the bloodstream. When an injury permeates the cells and gets to the vessels, hemostasis occurs.

There are two phases of hemostasis. During the first, primary hemostasis, the vascular muscle contracts temporarily as soon as the cells are disturbed. This contraction slows the blood flow and either activates or speeds up the adhesion of platelets. During adhesion, proteins on the surface of each platelet stick to the von Willebrand factor, a protein found in blood plasma.

As platelets collect across the surface, they make contact with collagen, the main proteins in humans, and are thus activated. These platelets cover the surface and fibers, and the receptors of the platelet membranes grip the fibrinogen, a protein found in plasma and synthesized by the liver. When platelets and fibrinogen build up, they form a plug. This all happens within 20 seconds of the injury.

During secondary hemostasis, the clot is stabilized, but platelet secretions continue the contractions of the vascular muscle. Through the interaction of enzymes, platelet membranes and various coagulation processes, the plug becomes solid. The coagulation processes occur in the liver but circulate inactively throughout the body until something called a coagulation cascade begins. Throughout the cascade, a series of steps occurs in which one reaction leads to another until fibrinogen is converted to fibrin, a protein that forms the hemostatic plug or clot over an injury. The fibrin is mesh-like in texture at first, but when platelets and red blood cells combine with a dense grouping of fibers, a blood clot is formed.

Hemostasis and thrombosis are closely related, as thrombosis is the formation of the blood clot in a blood vessel. Thrombosis can occur in a vein or artery, and the clot itself is called a thrombus, which is Greek for lump or clump. Thrombosis in a vein can cause deep vein thrombosis, a condition that affects blood clotting in the legs. Coronary thrombosis is thrombosis that affects the arteries and can cause a heart attack when a blood clot cuts off the blood supply to the heart. A thrombus can be caused by an injury to a blood vessel, the disruption of regular blood flow, an inflammation or atherosclerosis.

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Discussion Comments

By anon216336 — On Sep 21, 2011

there are four stages of hemostasis, right?

By malmal — On Jun 06, 2011

Hemostasis is an extremely important function for the human body -- it's the reason we all don't bleed to death from a paper cut!

If your blood didn't clot from fibrin and platelets, hemostasis wouldn't be possible, and that means that blood would always stay liquid. If blood always stayed liquid, even the smallest cut or scrape would just keep bleeding at a steady pace as fast as it could escape the size of the hole, until you literally bled to death from a scrape or a tiny cut.

it sure makes a person grateful for the ways different cells work in the human body, doesn't it? Blood is really a pretty amazing substance. Most people tend to think of it as red blood cells and that's it, but there are so many different kinds of cells moving around through your body with every heartbeat that it's astounding.

Man, now I want to examine blood under a microscope again...

By hanley79 — On Jun 03, 2011

I can vividly recall learning about the process of hemostasis using a computer program called ADAM back when my family used Windows 3.1. If that sounds like the time of the dinosaurs to you, that's because in computer terms it is!

Anyway, in ADAM there were little animated segments that would illustrate clearly what each step of the process of hemostasis was.

They showed the red blood cells moving through an artery, then how they would slip through if there was a cut in the side of the artery.

Next they showed platelets (which looked like red blood cells only smaller and white) and fibrin (which looked kind of like yellow short hairs) moving through the blood with the red cells.

Finally, they showed how the platelets and fibrin stuck together to cover the cut and clot the blood -- hemostasis in action.

That program is the reason that I learned what platelets and fibrin were and could recite it back to people as a kid, surprising them.

My mom encouraged my siblings and I to use what we'd learned. We drew cut sections of arteries with fibrin strands and platelets and red blood cells, and to this day I still think of fibrin as yellow and platelets as white. I wonder what color they really are?

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