Blood 2 14 4 Hemostasis Hemostasis refers to

Blood #2

14. 4: Hemostasis • Hemostasis refers to the stoppage of bleeding • Actions that limit or prevent blood loss include: • Blood vessel spasm • Platelet plug formation • Blood coagulation 2

Blood Vessel Spasm • Blood vessel spasm • Triggered by pain receptors, platelet release, or serotonin • Smooth muscle in blood vessel contracts 3

Platelet Plug Formation • Platelet plug formation • Triggered by exposure of platelets to collagen • Platelets adhere to rough surface to form a plug Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Endothelial lining Collagen fiber 1 Break in vessel wall Platelet Red blood cell 2 Blood escaping through break 3 Platelets adhere to each other , to end of broken vessel, and to exposed collagen 4 Platelet plug helps control blood loss 4

Blood Coagulation • Blood coagulation • Triggered by cellular damage and blood contact with foreign surfaces • A blood clot forms • This is a: • Hemostatic mechanism • Causes the formation of a blot clot via a series of reactions which activates the next in a cascade • Occurs extrinsically or intrinsically 5

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Extrinsic Clotting Mechanism • Extrinsic clotting mechanism • Chemical outside of blood vessel triggers blood coagulation • Triggered by tissue thromboplastin (factor III) (not found in blood) • A number of events occur that includes factor VII, factor X, factor V, factor IV, and factor II (prothrombin) • Triggered when blood contacts damaged blood vessel walls or tissues • This is an example of a positive feedback mechanism 7

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Intrinsic Clotting Mechanism • Intrinsic clotting mechanism • Chemical inside blood triggers blood coagulation • Triggered by Hageman factor XII (found inside blood) • Factor XII activates factor XI which activates IX which joins with factor VIII to activate factor X • Triggered when blood contacts a foreign surface 9

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Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Extrinsic Clotting Mechanism Tissue damage Releases Tissue thromboplastin (Factor lll) Activates (Ca+2) Intrinsic Clotting Mechanism Blood contacts foreign surface Activates Hageman Factor Xll Activates Factor Vll Factor Xl Activates (Ca+2) Activates Factor X Factor l. X Activates Factor V (Ca+2) Factor Vlll platelet phospholipids Factor X Activates (Ca+2) Factor V Prothrombin activator Converts Thrombin (Factor lla) Prothrombin (Factor ll) Converts Fibrinogen (Factor l) Fibrin Stabilizes Fibrin clot Factor Xlll 11

Fate of Blood Clots • After a blood clot forms it retracts and pulls the edges of a broken blood vessel together while squeezing the fluid serum from the clot • Platelet-derived growth factor stimulates smooth muscle cells and fibroblasts to repair damaged blood vessel walls • Plasmin digests the blood clots • A thrombus is an abnormal blood clot • An embolus is a blood clot moving through the blood vessels 12

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Lumen (a) Artery wall Lumen Plaque Artery wall (b) © The Mc. Graw-Hill Companies, Inc. /Al Telser, photographer 13

Prevention of Coagulation • The smooth lining of blood vessels discourages the accumulation of platelets and clotting factors • As a clot forms fibrin absorbs thrombin and prevents the clotting reaction from spreading • Anti-thrombin inactivates additional thrombin by binding to it and blocking its action on fibrinogen • Some cells such as basophils and mast cells secrete heparin (an anticoagulant) 14

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14. 5: Blood Groups and Transfusions • In 1910, identification of the ABO blood antigen gene explained the observed blood type incompatibilities • Today there are 31 different genes known to contribute to the surface features of RBCs determining compatibility between blood types 16

Antigens and Antibodies • Terms to become familiar with: • Agglutination – clumping of red blood cells in response to a reaction between an antibody and an antigen • Antigens – a chemical that stimulates cells to produce antibodies • Antibodies – a protein that reacts against a specific antigen 17

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Red blood cell Anti-B antibody Antigen A Anti-A antibody Antigen B Type B blood Type A blood Red blood cell Anti-A antibody Anti-B antibody Antigen A Antigen B Red blood cell Type AB blood Type O blood 18

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Antigen A Anti-B antibody (a) (c) Agglutinated red blood cells Red blood cell Anti-A antibody (b) (d) c: © G. W. Willis/Visuals Unlimited; figure d: © George W. Wilder/Visuals Unlimited 19

ABO Blood Group • Based on the presence or absence of two major antigens on red blood cell membranes • Antigen A • Antigen B 20

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Rh Blood Group • The Rh blood group was named for the rhesus monkey • The group includes several Rh antigens or factors • Rh positive – presence of antigen D or other Rh antigens on the red blood cell membranes • Rh negative – lack of these antigens • The seriousness of the Rh blood group is evident in a fetus that develops the condition erythroblastosis fetalis or hemolytic disease of the newborn 22

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. –– – – – – –– – – – + – – + + + – + – – – – –– – – – – –– – – – – ++ + – + – + + + – + – – Rh-negative woman with Rh-positive fetus Cells from Rh-positive fetus enter woman’s bloodstream ––– – – – – –– – –– – – – – + – – – – Woman becomes sensitized— antibodies ( + ) form to fight Rh-positive blood cells ––– – – – – –– – –– – – – – – In the next Rh-positive pregnancy, maternal antibodies attack fetal red blood cells 23