Blood

Part 2

Hemostasis

• Responses that stop bleeding• When blood vessels are damaged or ruptured, the

hemostatic response must be quick, localized to the region of damage and carefully controlled.

• Involves– Vascular spasm

– Platelet plug formation

– Blood coagulation (clotting)

Vascular Spasm

• Smooth muscle in the walls of arteries and arterioles contract immediately when damaged

• Reduces blood flow for minutes to hours

Platelet Plug FormationPlatelet Adhesion• Platelets contact and

stick to parts (collagen fibers) of damaged blood vessels.

Platelet Plug FormationPlatelet Release Reaction• Platelets become

activated, extend projections to contact each other and release the contents of their vesicles (clotting factors, ADP, ATP, Ca2+, enzymes that make thromboxane A2, prostaglandins, serotonin)

Platelet Plug FormationPlatelet Release

Reaction• ADP and

thromboxane A2 activate near by platelets

• Thromboxane A2 and serotonin act as vasoconstrictors

Platelet Plug FormationPlatelet Aggregation• Gathering platelets

• Eventually forms the platelet plug

• Platelet plug prevents blood loss in a small vessel and can stop blood loss completely if the vessel damage is not too large

• Platelet plug built in one minute

• Plug inhibited by prostacyclin released from intact endothelial cells

Blood Coagulation• Clot is a gel consisting of a network of insoluble

protein fibers (fibrin) in which the formed elements of blood are trapped.

• Involves clotting factors– Number I- XIII by order of discovery

– Most are plasma proteins made by liver that circulate in the inactive form

– Ca2+, inactive enzymes synthesized in liver and molecules associated with platelets or released by damaged tissues

Blood Coagulation

• Blood clotting involves a cascade of reactions that may be divided into three stages– 1. Formation of prothrombinase (prothrombin

activator)

• Either by the Extrinsic or Intrinsic Pathway

– 2. Conversion of prothrombin ( a plasma protein) into thrombin

– 3. Conversion of soluble fibrinogen into insoluble fibrin by thrombin

Extrinsic Pathway

• Occurs rapidly• Tissue protein called Tissue Factor

(TF)/thromboplastin leaks into blood from cells outside of blood vessels and starts the formation of prothrombinase

• TF released from surface of damaged cells, in the presence of calcium ions, after a series of reactions it activates clotting factor X, factor X combines with Factor V to form the active enzyme prothrombinase

Intrinsic Pathway

• Occurs slower• Its activators are either in blood or in direct

contact with blood• Contact with collagen fibers underlying

endothelium of blood vessels walls activates clotting factor XII, after a series of reactions Factor X is activated, it combines with Factor V and forms prothrombinase

Tissue trauma

Tissuefactor(TF)

Blood trauma

Damagedendothelial cellsexpose collagenfibers

(a) Extrinsic pathway (b) Intrinsic pathway

Activated XII

Ca2+

Damagedplatelets

Ca2+

Plateletphospholipids

Activated X

Activatedplatelets

Activated X

PROTHROMBINASECa2+

VCa2+

V

1

Tissue trauma

Tissuefactor(TF)

Blood trauma

Damagedendothelial cellsexpose collagenfibers

(a) Extrinsic pathway (b) Intrinsic pathway

Activated XII

Ca2+

Damagedplatelets

Ca2+

Plateletphospholipids

Activated X

Activatedplatelets

Activated X

PROTHROMBINASECa2+

VCa2+

Prothrombin(II)

Ca2+

THROMBIN

(c) Common pathway

V

1

2

+

+

Tissue trauma

Tissuefactor(TF)

Blood trauma

Damagedendothelial cellsexpose collagenfibers

(a) Extrinsic pathway (b) Intrinsic pathway

Activated XII

Ca2+

Damagedplatelets

Ca2+

Plateletphospholipids

Activated X

Activatedplatelets

Activated X

PROTHROMBINASECa2+

VCa2+

Prothrombin(II)

Ca2+

THROMBIN

Ca2+

Loose fibrinthreads

STRENGTHENEDFIBRIN THREADS

Activated XIIIFibrinogen(I)

XIII

(c) Common pathway

V

1

2

3

+

+

Coagulation

Detailed Events of Coagulation

Common Pathway

• The formation of prothrombinase starts the Common Pathway

• Prothrombinase and Ca2+ catalyze the conversion of prothrombin to thrombin

• Thrombin, in presence of Ca2+, converts fibrinogen (soluble) to loose fibrin threads (insoluble)

• Thrombin also activates Factor XIII/fibrin stabilizing factor which strengthens and stabilizes the fibrin threads into a sturdy clot

• The formed clot plugs the ruptured blood vessel and stops blood loss, usually within 3-6 minutes of damage

Clot Retraction

• The consolidation of the fibrin clot

• As the platelets pull on the fibrin, fibrin contracts and pulls the edges of the damaged blood vessel closer together

• In time the fibroblasts form CT in the ruptured area and new endothelial cells repair the vessel lining

Vitamin K

• Need adequate levels for clotting

• Not actively involved in clotting but required for synthesis of four of the clotting factors

Fibrolytic System

• Dissolves small inappropriate clots• Dissolves clots at site of damage once damage is repaired

(begins within two days and continues slowly over several days)

• Fibrinolysis is the dissolution of a clot• Inactive enzyme plasminogen is incorporated into the clot• Body tissues and blood contain substances that can

activate plasminogen and convert it to plasmin• Plasmin(fibronolysin) dissolves the clot by digesting fibrin

and inactivating fibrinogen, prothrombin, factor V and factor XII

Control of Clotting• Fibrolytic System• Prostacyclin

– A prostaglandin produced by WBC and endothelial cells, opposes the action of thromboxane A2 and inhibits platelet adhesion and release

• Anticoagulants in blood– Substances that delay, suppress or prevent blood

clotting• Antithrombin which blocks factor XII, X and II• Heparin produced by mast cells and basophils, combines with

antothrombin and increases its effectiveness• Activated protein C which inactivates 2 major clotting factors

and enhances activity of plasminogen activators

Clotting

• If blood takes too long to clot can have hemorrhage

• If blood clots too easily get thrombosis, clotting in undamaged blood vessel

Thrombosis• Intravascular clotting• Clot called a thrombus• Thrombus may dissolve spontaneously or may break away• A blood clot, bubble of air, fat from a broken bone or piece

of debris transported by blood stream called an embolus– In lung Pulmunary Embolus

• Virchow’s Triad– Damage to venous wall– Change in flow– Blood hypercoagulability– I AM CLOTTED

Blood Groups and Blood Types• The surface of RBCs contain a genetically

determined assortment of antigens composed of glycoproteins and glycolipids

• Antigens called agglutinogens occur in characteristic combinations

• Blood is categorized into different groups based on the presence or absence of various antigens

• Within a blood group there may be two or more types

• At least 24 groups and 100 antigens, ABO and Rh are two major blood groups

ABO Blood Group

• Based on two glycolipid antigens A and B

• Only antigen A displayed-Type A

• Only antigen B-Type B

• Both antigens-Type AB

• Neither A or B-Type O

ABO Blood Group

• Blood plasma contains antibodies that react with the A or B antigens if the two are mixed

• Anti-A antibody reacts with antigen A• Anti-B antibody reacts with antigen B• You do not have antibodies that react with

antigens of your own RBCs but you do have antibodies for antigens your RBCs lack

Transfusions

• Transfer of whole blood or blood components into bloodstream

• Incompatible blood transfusions the antibodies in the recipient’s plasma bind to antigens on donors RBC which causes agglutination or clumping of RBCs

• Type AB-universal recipients• Type O-universal donors, can only receive Type O• Blood is cross-matched or screened before

transfusions

Rh Blood Groups

• Antigen discovered in blood of rhesus monkey• Rh antigen on RBC Rh+• Lack antigen, Rh-• Normally blood plasma does not contain anti-Rh

antibodies– If an Rh- recieves an Rh+ transfusion the immune

system will start to make anti Rh antibodies

– Hemolytic Disease of Newborn

Hemolytic Disease of Newborn• Normally do direct contact

between maternal and fetal blood but leakage possible at delivery

• If Rh+ leaks into Rh- mother she will make anti Rh antibodies which will cause maternal and fetal incompatibility in 2nd pregnancy

• All Rh- women should receive Anti-Rh gamma globulin(RhoGAM) soon after every delivery, miscarriage or abortion

Typing Blood

– Single drops of blood are mixed with different antisera

– Agglutination with an antisera indicates the presence of that antigen on the RBC