BLOOD PLATELETS Non nucleated Biconcave discs 2 3

• • BLOOD PLATELETS Non nucleated. Biconcave discs. 2 -3 m in diameter. Circular Present in clusters its count in peripheral blood is about 250. 000/m. L. its life span is usually 7 -10 days. About 60 -75% of the platelets are circulating in the blood while the reminders are mostly in the spleen.

• Thrombocytosis: It is the increase in the number of platelets, this occurs physiologically in pregnancy and after hemorrhage. • Thrombocytopenia: It is the decrease in the number of platelets, this occurs after aplastic anemia, pernicious anemia, and during menstruation. • Thromboathenia: It is the decrease in the functions of platelets.

The cytoplasm contains many active factors including: • Contractile proteins: • A skeleton of microtubules • Glycogen granules which are utilized for energy production • Lysosomes containing hydrolytic enzymes. • endoplasmic reticulum and the Golgi apparatus which store large quantities of calcium ions. • Mitochondria and enzyme system those are capable of forming ATP and ADP. • Enzyme system that synthesizes prostaglandin which are local hormones those cause many different types of vascular and local tissue reactions. • Many granules:

• 2 types of granules • A) Dense granules which contain non protein substances such as ATP, ADP, serotonin and calcium. • B) Specific (alpha) granules: which contain protein substances such as, fibrinogen, heparin antagonist, (PF 4) platelet derived growth factor (PDGF), which stimulates wound healing through stimulation of growth and multiplication of vascular endothelial cells, smooth muscle cells and fibroblasts.

Functions of platelet • Haemostasis. • Phagocytosis: viruses and antigenantibody complex are phagocytosed by platelets • Serotonin storage

HAEMOSTASIS • Haemostasis is the arrest of bleeding. • Four major systems are involved maintaining Haemostasis: • I- Vasoconstriction of blood vessels in • II- Platelets plug formation. • III- facilitation if initial vasoconstriction. • IV- Fibrin Forming system

1 - Vasoconstriction of blood vessels The vascular system acts to prevent bleeding by: 1 - Contraction of vessels (vasoconstriction): and reflex stimulation of adjacent vessels. 2 - This reflex contraction is due to myogenic (direct effect on the vessel wall) and due to sympathetic stimulation. 3 - Diversion of blood flow around damaged vessels 4 - Initiation of contact-activation of platelet with subsequent aggregation. This mechanism may be sufficient to stop bleeding from small vessel.

II- Role of platelets in haemostasis • Platelets are intimately involved in primary haemostasis, which is the interaction of platelets and the vascular endothelium. • The functions of platelets in haemostasis is platelet plug formation.

-Platelet Plug Formation • Various processes are involved in the initial formation of a platelet plug: • A- Platelet adhesion, • B- Platelet release factors, and • C- Platelet aggregation.

Adhesion and release factors • Exposure of subendothelial connective tissue, such as collagen fibers, initiates platelet adhesion. • When platelets adhere to the collagen they swell up, becomes irregular and spread radiating processes (pseudopodia). • These pseudopodia, secret the contents of various intracellular granules. • These secreted substances: ADP, thromboxane A 2, collagen, restocetocin and cathepsin G……. . ect

Aggregation • These released substances stimulate other platelets to adhere to the first one and causing platelet adhesion. • During platelet aggregation, the injured platelet changes shape from discoid to spherical with pseudopodia formation. • Initial aggregation of platelets is caused by adenosine diphosphate (ADP), which is released from adherent platelets or endothelial cells. • This aggregates forming a mass or plugs and seals the injured sites. • Platelet plug can`nt support stoppage of bleeding in large arteries.

Coagulation of blood • It is a process by which fluid blood is converted into a semi-solid jelly like mass called a clot. • It is a process of plasma {many coagulation proteins normally present in the plasma in an inactive state (coagulation factor) mediate this system}. • It started by formation of prothrombin activator as a result of vascular injury. • Coagulation process is an enzyme cascade and it is a complex phenomena.

• Coagulation process involves a series of biochemical reactions, one reaction trigger (activate) the next. • This means the activation of one clotting factor leads to activation of the next factor. • This process requires plasma proteins as well as phospholipids and calcium. • The clotting factor are listed blow:

• • • • F I fibrinogen F II prothrombin F III tissue factor F IV calcium F V labile factor F VI not present F VII stable factor F VIII antihaemophilic factor F IX Christmas factor F X Stuart power F XI factor XI plasma thromboplastin F XII Hageman Factor. F XIII fibrin stabilizing protein

• The formation of prothrombin activator depend upon two pathways: the extrinsic pathway and the intrinsic pathway, • Both of which share specific common coagulation factors with the common pathway. • Both pathways require initiation, which leads to subsequent activation of various coagulation factors in a cascading, waterfall. • According to the cascade theory, each coagulation factor is converted to its active form by the preceding factor in a series of biochemical chain reaction

EXTRINSIC PATHWAY • When blood is exposed to tissue extracts, blood clotting is rapidly initiated via the extrinsic pathway by the interaction of tissue factor (glycoprotein) and FIII (thromboplastin). • tissue factor in presence of Ca++ combine with factor VII and activate it. • Activated factor VII in presence of Ca++ activate factor X. • The activated factor X in the presence of tissue phospholipids form prothrombin activator.

INTRINSIC PATHWAY • The intrinsic pathway starts when blood comes in contact with foreign surface (exposed collagen from blood vessel wall), factor XII activated and platelet factor III released. • Activated factor XII in the presence of Ca++ activate factor XI which in turn in presence of Ca++ activate factor IX which in turn in presence of Ca++ activate factor VIII. • In the presence of Ca++ activated factor VIII activate factor X. • Activated factor X in the presence of platelet factor III form prothrombin activator.

COMMON PATHWAY • The common final pathway involves the conversion of: – Prothrombin (Factor II) to thrombin in the presence of prothrombin activator. – Thrombin in the presence of Ca++ convert fibrinogen to a soluble fibrin (threads of fibrin) – Soluble fibrin in the presence of Ca++ and FXIII form insoluble fibrin clot (Mesh). – This mesh entrap RBCs, WBCs and platelet to form clot. – The blood clot adheres to the injured vascular walls and plugging it permanently.

Intrinsic Pathway • (1 -6 minutes) Extrinsic Pathway • (15 seconds)

Fate of the clot • Few minutes after formation of the clot, the clot start to shrink and release serum. • This process is supported by platelet which adhere to clot thread and extent their pseudopodia, which contract and pull the fibrin thread close to each other. • Retraction the clot makes it firm & pulls the vessel wall close to each other. • The clot start to lysis after formation of fibrous tissue which grow into the blood clot and permanently seals the opening in the blood vessel. • The clot is removed by protolytic enzymes plasminogen which is converted to plasmin. • Plasmin dissolve the clot, which in turn removed by reticulo endothelial system.

Factors affecting clotting • Factors preventing clotting – Low temperature. – Contact with non water wettable surface (silicnized) – Use of anticoagulant which remove Ca++ ions from blood (potassium oxlate, sodium citrate) ﺯ – Use substances of biological origin: • • Ptotamin. Peptones. Hirudin. Dicumarol. • Factors increasing clotting – Warm. – Contact with water wettable surface. – Injection of vit. K (for promotion of factors II, VII, IX and X)

Coagulation disorders • Bleeding disorders – Decrease fibrinogen – Decrease prothrombin – Decrease platelet purpura - Decrease clotting factors hemophilia • Intravascular clotting – Thrombosis; it is a blood clot which is formed within blood vessel. – Embolus: it is a part detached from the clot and move through the circulatory system and block blood vessel.

Anticoagulants These are substances which are used to prevent blood clotting inside the blood vessel or in test tubes: Anticoagulant in vitro These are substances used to prevent blood clotting outside the body: • 1 - Precipitation of Ca++ ions (inactivation of calcium) by addition of sodium oxalate, ammonium oxalate or sodium citrate. • 2 - Collection of blood in a siliconized non-wettable electropositive charged container, which prevent activation of factor XII and platelets. • 3 - Addition of heparin.

Anticoagulant in vivo • These are substances used to prevent blood coagulation inside the body e. g. treatment of intravascular thrombosis: – 1 - Heparin. – 2 - Oral anticoagulant likes dicumarol.

1 - Heparin: • • It is a sulfate mucopoly-saccharide with strong acidic properties. • Mast cells and basophil leukocytes form it. • It is released into the general circulation in minute amounts to prevent any tendency of the blood to clot inside the blood vessels. • It is given by intravenous route.

Action of heparin: • 1 - It activates antithrombin II • 2 - It inactivates factor IXa, XIa, XIIIa and thrombin. • 3 - It acts as a co-factor for lipoprotein lipase, thus it clears blood from lipids after meals. • 4 - It’s antidote is protamine sulphate which is given to counteract the high doses of heparin, where it combine with heparin to neutralize its action.

2 - Dicumarol • It is of plant origin, it inhibits vitamin K. • its action is through the prevention of the formation of prothrombin, factors VII, IX and X. • - It has a slow onset and long duration of action. • - Its antidote is vitamin k.

Abnormalities of Hemostasis Conditions that cause excessive bleeding in man: A- hemophilia B- purpura C- Vitamin K deficiency 11) Thromboembolic conditions in human being. I)

A) Hemophilia It is a congenital sex-linked recessive disease transmitted by females to their sons who manifest signs of the disease. The disease is characterized by bleeding tendency and prolonged clotting time (1 -12 hours). Types of Hemophilia 1 - Hemophilia A: 85% of cases it is due to lack of FVIII. 2 - Hemophilia B: 10% of cases it is due to lack of FIX. 3 - Hemophilia C: it is due to lack of FXI.

B- Purpura • It is a disease characterized by spontaneous small hemorrhages in the skin and mucous membrane (petichae). • In purpura there are defect in vasoconstriction of the cut vessel (decreased serotonin), deficient clot retraction (retractozyme) and poor platelet aggregation. • The formed clot is soft and poorly retracts. • There is prolonged bleeding time. Types of purpura • 1 - Thrombocytopenic purpura: It is due to decrease in platelet count. • 2 - Thromboasthenic purpura: The count is normal but its function is abnormal.

C- Vitamin K deficiency • Vit. K is very important for the formation of clotting factors II, VII, IX and X in the liver. • In the absence of this vitamin, bleeding tendency from insufficiency of these clotting factors can occur. • This condition occurs when there is a liver disease or obstruction of bile duct, which causes poor absorption of vitamin k from the intestine due to absence or decrease in the bile secretion.

11) Thromboembolic conditions in human being. • It is the formation of blood clot inside blood vessel. • This condition usually occurs when there is a slow blood flow e. g. after delivery and operation.