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What is hemostasis?
a sequence of responses that stops bleeding
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What are the 3 mechanisms that reduce blood loss in hemostasis?
1. vascular spasm
2. platelet plug formation
3. blood clotting
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When hemostasis is successful, what does it prevent?
hemorrhage
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What is hemorrhage?
the loss of a large amount of blood from the vessels
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Hemostatic mechanisms can prevent hemorrhage from smaller blood vessels, but extensive hemorrhage from larger vessels usually requires what?
medical intervention
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What is vascular spasm?
When arteries or arterioles are damaged, the circularly arranged smooth muscle in their walls contracts immediately.
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How long does vascular spasm reduce blood loss?
for several minutes to several hours, during which time the other hemostatic mechanisms go into operation
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What probably causes vascular spasms?
damage to the smooth muscle, by substances released from activated platelets, and by reflexes initiated by pain receptors
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What clotting factors are in platelets?
ADP, ATP, Ca2+, and serotonin
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What is platelet-derived growth factor?
A hormone within platelets that can cause proliferation of vascular endothelial cells, vascular smooth muscle fibers, and fibroblasts to help repair damaged blood vessel walls.
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What is platelet adhesion?
Initially, platelets contact and stick to parts of a damaged blood vessel, such as collagen fibers of the connective tissue underlying the damaged endothelial cells
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What is platelet release reaction?
Due to adhesion, the platelets become activated, and their characteristics change dramatically. They extend many projections that enable them to contact and interact with one another, and they begin to liberate the contents of their vesicles.
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What is platelet aggregation?
The release of ADP makes other platelets in the area sticky, and the stickiness of the newly recruited and activated platelets causes them to adhere to the originally activated platelets
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what is a platelet plug?
Eventually, the accumulation and attach-ment of large numbers of platelets form a mass called a platelet plug
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When is a platelet plug very effective?
In preventing blood loss in a small vessel
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What is a platelet plug reinforced by?
fibrin threads formed during clotting
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Can a platelet plug stop blood loss completely?
yes, if the hole in a blood vessel is not too large
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what is serum?
blood plasma minus the clotting proteins
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Normally, blood remains in its liquid form as long as it stays within its vessels. But, what happens if it is drawn from the body?
it thickens and forms a gel...the gel separates from the liquid...the liquid is serum and the gel is called a clot
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What does a clot consist of?
a network of insoluble protein fibers called fibrin in which the formed elements of blood are trapped
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What is the process of gel formation where a series of chemical reactions culminate in the formation of fibrin threads?
clotting or coagulation
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If blood clots too easily, the result can be...
thrombosis
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What is thrombosis?
clotting in an undamaged blood vessel
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when can hemorrhage occur?
if the blood takes too long to clot
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What are the clotting (coagulation) factors?
calcium ions
several inactive enzymes that are synthesized by hepatocytes (liver cells) and released into the bloodstream
various molecules associated with platelets or released by damaged tissues
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How are most clotting factors identified?
By roman numerals that indicate the order of their discovery
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How many stages are in clotting?
3
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What are the 3 stages of clotting?
1. Two pathways, called the extrinsic pathway and the intrinsic pathway, lead to the formation of prothrombinase. Once prothrombinase is formed, the steps involved in the next two stages of clotting are the same for both the extrinsic and intrinsic pathways, and together these two stages are referred to as the common pathway.
2.Prothrombinase converts prothrombin (a plasma protein formed by the liver) into the enzyme thrombin
3.Thrombin converts soluble fibrinogen (another plasma protein formed by the liver) into insoluble fibrin. Fibrin forms the threads of the clot
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Is blood clotting a positive or negative feedback?
positive feedback
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What is the outcome of the first stage of blood clotting?
the formation of prothrombinase
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How soon after a trauma does the extrinsic pathway of blood clotting occur?
within a matter of seconds
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Why does extrinsic pathway have its name?
because a tissue protein called tissue factor (TF), also known as thromboplastin, leaks into the blood from cells outside blood vessels and initiates the formation of prothrombinase
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What is thromboplastin?
a tissue protein called tissue factor (TF) that initiates the formation of prothrombinase
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What is TF made of?
a complex mixture of lipoproteins and phospholipids released from the surfaces of damaged cells
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What does TF ultimately activate?
clotting factor X
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What is the intrinsic pathway?
When the activators are either in direct contact with blood or contained within the blood, outside tissue damage is not needed
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Is the intrinsic pathway simpler than the extrensic pathway?
NO
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How long does the intrinsic pathway take?
more slowly than the extrinsic pathway, usually requiring several minutes
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What marks the beginning of the common pathway?
the formation of prothrombinase
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What is the 2nd stage of blood clotting?
pothrombinase and Ca2+ catalyze the conversion of prothrombin to thrombin
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What is the 3rd stage of blood clotting?
thrombin, in the presence of Ca2+, converts fibrinogen, which is soluble, to loose fibrin threads, which are insoluble. thrombin also activates factor XIII (fibrin stabilizing factor), which strengthens and stabilizes the fibrin threads into a sturdy clot
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Thrombin has 2 ___ ___ effects
positive feedback
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In the first positive feedback loop of thrombin, which involves factor V, what happens?
it accelerates the formation of prothrombinase. Prothrombinase in turn accelerates the production of more thrombin, and so on
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What happens in the 2nd positive feedback loop of thrombin?
thrombin activates platelets, which reinforces their aggregation and the release of platelet phospholipids
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