Platelets
Platelets is called thrombocytes.
Thrombus means "clot", Cyte means "cells"
Platelets count; 25,0000-400000/mm3 of blood, more numerous than leukocytes.
Shape and size; Platelets are very small non-nucleated discs and are about one quarter the size of the RBCs (2-4 micrometer in dimeter).
Platelets exhibit many granules. As they have no nucleus so they are incapable of cell division.
Platelets help body from clots to stop bleeding. If blood vessels get damaged, it sends out signals that are picked by platelets.
Platelets then rush to the site of damage and form a plug, or clot, to repair the damage.
Function of Platelets
Main function of platelets is to prevent blood loss.
1. Repaire of injured blood vessels
2. Forming platelet plug, contribute fibrin formation, seal holes in blood vessel
3. To forming clot
4. Defense mechanism
Forming Platelet Plug
Platelet plug seal holes in small vessels, help to repair slightly damaged blood vessels
After entering blood stream, platelets begin to pick up and store chemical substance that can be released later to help seal blood vessel break.
Formation of Clot
Forming clot is maain function of platelets is to start process of blood clotting.
Granules contain chemicals that release to promote blood clotting.
Platelet contain variety of substance, including-
1. Platelet factors lll and lV
2. Serotonin
3. Platelet fibrinogen
4. Adenosine triphosphate (ATP)
Activated and initiate clotting mechanism.
Production of Thrombocytes/ Thrombocytopoiesis
Process of formation of platelets is called thrombocytopoiesis.
They originate from hemocytoblasts in bone marrow.
Formation of platelets takes about 10 days, newly formed platelets spend about 24-36 hours in spleen before being released into circulation.
The span of platelets is 7- 10 days. About 10% of platelets are utilized daily for minor hemorrhage and rest not used in process of hemostasis are destroyed by macrophages, mainly in the spleen.
Thrombocytopoiesis
Stem cell
Hemocytoblast in bone marrow
Megakayoblast
Promegakaryocytes
Megakaryocyte breakup
Platelets
Blood stream
(Formtion of platelets)
Hemostasis
Hemostasis is the cessation of blood loss from damaged vessel, prevent blood loss.
Arrest of bleeding is very important for maintenance of homeostasis.
Mechanism of Hemostasis
Hemostatic mechanism consists of 3 phase -
1. Vasocontrictive phase
2. Platelet plug formation
3. Blood clotting
1. Vasoconstructive Phase
Immediate but temporary closure of blood vessel resulting from contraction of smooth muscle with in wall of blood vessel.
When tissue or blood vessel is damaged and blood escapes from blood vessel.
Injury to blood vessel occurs.
Platelets migrates to blood vessels and connective tissue present in those blood vessels swell and adhere (stick) to collagen.
Which leads contriction to vessels.
Vasocontriction of vessel wall.
Narrow the opening of vessel.
2. Platelets plug formation
Clumping together of platelets, the key stages of this process are-
1. Platelet adhesion
2. Platelet release reaction
3. Platelet aggregation
Platelet plug formation is an accumulation of platelets that can seal up small breaks in blood vessels.
Platelet Adhesion
Initially platelet contract and stick to part of damaged blood vessels, such as collagen fibres of connective tissue.
Activated platelets, due to adhesion, platelets become activated and release chemicals serotonin, adenosin diphosphate (ADP).
Serotonin enhance vascular spasm.
Platelets Aggregation
Platelets aggregation, ADP helps in depositing of platelets over injured area. This process called platelets aggregation. Due to platelets aggregation, platelets become sticky and stickiness cause them to adhere to originate activated platelets.
So the damaged blood vascular wall activates increasing number of platelets that themselve attract more and more additional platelets, thus forming a platelets plug.
Escape from blood vessels of platelets.
Swell and adhere to the collagen in adjacent connective tissues.
Platelets become activated
Release chemicals like ADP, Serotonin
This attachment stimulates vasoconstriction.
Platelets become sticky and as more of them move into injured area they stick together.
Platelets aggregation (platelets plug formation)
Clog a small opening in the vessel.
3. Blood Clotting
Purpose of blood clotting is to plug the ruptured vessels so as to stop bleeding and prevent loss of vital body fluid.
Blood Clotting Mechanism
1. Coagulation factors/ clotting factors
Clotting involves several enzymes and other chemical.
Coagulation factors are proteins found within plasma.
Most coagulation factors synthesized in liver and released into blood plasma.
Some are released by platelets.
Thromboplastin is released from damaged tissue cells.
Fibrinogen is protein found in blood plasma. After injury, clotting factors are activated to produce a clot.
Factor I; Synthesized in liver and they are described as fibrinogen (protein, essential for prepare blood clot formation).
Factor ll; Synthesized in liver and described as prothrombin.
Factor lll; Activated during any injury and described as thromboplastin.
Factor lV; It is inherited by food we eat and these are calcium ions.
Factor V; Synthesized in liver and are also present in platelets.
Factor Vll; Synthesized in liver and act as prothrombin conversion accelerator.
Factor Vlll; Present in platelets and endothelial cell.
Factor lX; Synthesized in liver and is described as plasma thromboplastic component.
Factor X; Synthesized in liver, they are also found in serum and plasma and aids in conversion of fibrinogen into fibrin.
Factor XI; Synthesized in liver and described as plasma thromboplastin antecedent.
Factor XII; Synthesized in liver and are also present in platelets. This is activated when blood doesn't coagulate in normal period.
Factor XIll; Present in platelets and described as fibrin stabilizing factor.
Prothrombin activates in combination of activated factor V and X . There is no factor VI.
2. Blood Clot
Three stages of blood clotting are-
Stage 1; Formation of prothrombin activator
Stage 2; Conversion of prothrombin into enzyme thrombin
Stage 3: Conversion of Fibrinogen (soluble) to fibrin (insoluble)
Stage 1; Formation of Prothrombin Activator
Formation of prothrombin activator; prothrombin can be formed in 2 ways-
1. Intrinsic pathway
2. Extrinsic pathway
1. Intrinsic pathway (5-10 min) inside vascular system
After comming in contact with collagen fibres or rconnective tissues inside blood, it will lead to activation of-
Factor XII
Factor XI
Factor XII + Factor XI
Factor IX
Factor VIII
Factor VIII + Calcium ion + Factor III
Factor X
Factor V
Factor X + Factor V
Prothrombin activator/Prothrombinase
2. Extrinsic pathway (15-20sec)
Activated through tissue factor released by endothelial cell after external damage.
Tissue damage
Tissue thromboplastin activates -
(Factor III)
Factor VII
Factor X
Factor V
Factor X + Factor V
Prothrombin activator + Prothrombinase
Stage 2; Conversion of prothrombin into enzyme thrombin
Conversion of Prothrombin into enzyme thrombin
Prothrombin (Factor II) - Prothrombinase + calcium ions - Thrombin
Stage 3; Conversion of Fibrinogen (soluble) to fibrin (insoluble)
Conversion of fibrinogen (soluble) to fibrin (insoluble)
Thrombin acts as catalyst,
Fibrinogen (factror II ) - Thrombin - Insoluble fibrin.
Thrombin act as an enzyme by which fibrinogen is converted to fibrin, that stabilizes blood clots and promotes hemostasis.; process to prevent and stop bleeding.
Fibrin
Traps the blood cells
Forms the blood clot
Clot Retraction
Clot retraction is tightening of fibrin clot, with in few minutes after a clot is formed it begins to contract. This activity also assist the repair processes.
Fibrinolysis
Physiological mechanism that dissolves clots is called fibrinolysis.
After clot has formed, the process of removing it and healing the damaged blood vessels begins.
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