12.6.f^Section Ending^429^430^,,^19241^19491%
Page 429
SECTION F SUMMARY
Plasma
  1. Plasma is the liquid component of blood; it contains proteins (albumins, globulins, and fibrinogen), nutrients, metabolic end products, hormones, and inorganic electrolytes.

  2. Plasma proteins, synthesized by the liver, play many roles within the bloodstream, such as exerting osmotic pressure for absorption of interstitial fluid and participating in the clotting reaction.

The Blood Cells
  1. The blood cells, which are suspended in plasma, include erythrocytes, leukocytes, and platelets.

  2. Erythrocytes, which make up more than 99 percent of blood cells, contain hemoglobin, an oxygen-binding protein. Oxygen binds to the iron in hemoglobin.

    1. Erythrocytes are produced in the bone marrow and destroyed in the spleen and liver.

    2. Iron, folic acid, and vitamin B12 are essential for erythrocyte formation.

    3. The hormone erythropoietin, which is produced by the kidneys in response to low oxygen supply, stimulates erythrocyte differentiation and production by the bone marrow.

  3. The leukocytes include three classes of polymorphonuclear granulocytes (neutrophils, eosinophils, and basophils), monocytes, and lymphocytes.

  4. Platelets are cell fragments essential for blood clotting.

  5. Blood cells are descended from stem cells in the bone marrow. Hematopoietic growth factors control their production.

Hemostasis: The Prevention of Blood Loss
  1. The initial response to blood vessel damage is vasoconstriction and the sticking together of the opposed endothelial surfaces.

  2. The next events are formation of a platelet plug followed by blood coagulation (clotting).

  3. Platelets adhere to exposed collagen in a damaged vessel and release the contents of their secretory vesicles.

    1. These substances help cause platelet activation and aggregation.

    2. This process is also enhanced by von Willebrand factor, secreted by the endothelial cells, and by thromboxane A2, produced by the platelets.

    3. Fibrin forms the bridges between aggregating platelets.

    4. Contractile elements in the platelets compress and strengthen the plug.

  4. The platelet plug does not spread along normal endothelium because the latter secretes prostacyclin and nitric oxide, both of which inhibit platelet aggregation.

  5. Blood is transformed into a solid gel when, at the site of vessel damage, plasma fibrinogen is converted into fibrin molecules, which then bind to each other to form a mesh.

  6. This reaction is catalyzed by the enzyme thrombin, which also activates factor XIII, a plasma protein that stabilizes the fibrin meshwork.

  7. The formation of thrombin from the plasma protein prothrombin is the end result of a cascade of reactions in which an inactive plasma protein is activated and then enzymatically activates the next protein in the series.

    1. Thrombin exerts a positive feedback stimulation of the cascade by activating platelets and several clotting factors.

    2. Activated platelets, which display platelet factor and binding sites for several activated plasma factors, are essential for the cascade.

  8. In the body, the cascade usually begins via the extrinsic clotting pathway when tissue factor forms a complex with factor VIIa. This complex activates factor X, which then catalyzes the conversion of small amounts of prothrombin to thrombin. This thrombin then recruits the intrinsic pathway by activating factor XI and factor VIII, as well as platelets, and this pathway generates large amounts of thrombin.

  9. The liver requires vitamin K for the normal production of prothrombin and other clotting factors.

  10. Clotting is limited by three events: (a) Tissue factor pathway inhibitor inhibits the tissue factor–factor VIIa complex; (b) protein C, activated by thrombin, inactivates factors VIIIa and Va; and (c) antithrombin III inactivates thrombin and several other clotting factors.

  11. Clots are dissolved by the fibrinolytic system.

    1. A plasma proenzyme, plasminogen, is activated by plasminogen activators to plasmin, which digests fibrin.

    2. Tissue plasminogen activator is secreted by endothelial cells and is activated by fibrin in a clot.

SECTION F KEY TERMS

albumin 417

antithrombin III 426

basophil 420

bilirubin 418

blood coagulation 423

bone marrow 418

clot 423

clotting 423

eosinophil 420

erythropoiesis 419

erythropoietin 419

extrinsic pathway 424

ferritin 418

fibrin 423

fibrinogen 417

fibrinolytic system 426

globulin 417

hematopoietic growth factor (HGF) 421

hemoglobin 417

hemostasis 422

heparin 426

intrinsic factor 419

intrinsic pathway 424

iron 418

lymphocyte 420

megakaryocyte 421

monocyte 420

neutrophil 420

plasma protein 417

plasmin 426

plasminogen 426

plasminogen activator 426

platelet activation 422

platelet aggregation 422

platelet factor (PF) 424

platelet plug 422

pluripotent hematopoietic stem cell 421

polymorphonuclear granulocyte 420

protein C 426

prothrombin 423

reticulocyte 418

serum 417

thrombin 423

thrombomodulin 426

thromboxane A2 422

thrombus 423

tissue factor 424

tissue factor pathway inhibitor (TFPI) 426

tissue plasminogen activator (t-PA) 426

transferrin 418

vitamin B12 418

vitamin K 425

von Willebrand factor (vWF) 422

anemia 419

Page 430
SECTION F CLINICAL TERMS

aspirin 427

Desmodus rotundus salivary plasminogen activator (DSPA) 427

hematoma 422

hemochromatosis 418

hemophilia 424

hypercoagulability 426

iron deficiency 418

iron-deficiency anemia 420

malaria 420

oral anticoagulants 427

pernicious anemia 419

polycythemia 420

recombinant t-PA 427

sickle-cell disease 419

thrombolytic therapy 427

SECTION F REVIEW QUESTIONS
  1. Give average values for total blood volume, erythrocyte volume, plasma volume, and hematocrit.

  2. Which is the most abundant class of plasma protein?

  3. Which solute is found in the highest concentration in plasma?

  4. Summarize the production, life span, and destruction of erythrocytes.

  5. What are the routes of iron gain, loss, and distribution? How is iron recycled when erythrocytes are destroyed?

  6. Describe the control of erythropoietin secretion and the effect of this hormone.

  7. State the relative proportions of erythrocytes and leukocytes in blood.

  8. Diagram the derivation of the different blood cell lines.

  9. Describe the sequence of events leading to platelet activation and aggregation and the formation of a platelet plug. What helps keep this process localized?

  10. Diagram the clotting pathway beginning with prothrombin.

  11. What is the role of platelets in clotting?

  12. List all the procoagulant effects of thrombin.

  13. How is the clotting cascade initiated? How does the extrinsic pathway recruit the intrinsic pathway?

  14. Describe the roles of the liver and vitamin K in clotting.

  15. List three ways in which clotting is limited.

  16. Diagram the fibrinolytic system.

  17. How does fibrin help initiate the fibrinolytic system?