Platelets, Viscosity, and Hemostasis

Where do platelets originate from?

• They originate from megakaryoblasts from the myeloid lineage

• Megakaryoblasts break into hundreds of fragments, which are the platelets

What are the platelets function?

• They are involved in the clotting of blood

What is viscosity? What is the viscosity of blood determined by?

• Viscosity is the resistance to flow

• The viscosity of blood is determined by:

  • The number of erythrocytes (RBCs)

  • The amount of albumins (proteins)

What is Hemostasis? (not homeostasis)

• The reduction and stoppage of blood loss from a damaged blood vessel

What are the three mechanisms of Hemostasis?

• Three Mechanisms of Hemostasis:

  • Vascular spasm

  • Platelet plug formation

  • Blood clotting (coagulation)

Describe the Vascular Spasm mechanism

• The first response to a damaged blood vessel

• The smooth muscle contracts in response to a cut in the blood vessel, which reduces blood flow loss (short term)

  • This helps prevent excessive blood loss until the other longer term mechanism takes effect

Describe the Platelet Plug Formation mechanism

1. Broken tissue releases chemicals that attracts platelets

2. Platelets change shape and extend projections to stick onto the broken tissue, forming a platelet plug

   1. This mechanism is also short term, stopping blood for a few hours

Describe the Blood Clotting Mechanism Pathways

• Blood clotting has two pathways:

  • Extrinsic Pathway: the tissue is damaged externally, with blood leaving the blood vessel (faster process)

  • Intrinsic Pathway: the tissue is damaged internally, with blood not leaving the blood vessel

Describe the Extrinsic Pathway Steps

1. Tissue damage with bleeding has occurred

2. Damaged tissues & platelets collect at the wound site and releases “tissue factor” (thromboplastin)

3. Thromboplastin converts a plasma protein into factor X (clotting factor)

4. Factor X becomes Prothrombinase

Describe the Intrinsic Pathway Steps

1. Tissue is damaged internally

2. This damage exposes the connective tissue of blood vessel

3. Platelets collect at the wound site then break down, releasing Platelet Factor 3

4. Platelet Factor 3 activates Factor XII, which converts Factor X into prothrombinase

What is the Common pathway and describe the steps

• Past prothrombinase, the pathway is the same

  • 5. Prothrombinase converts Prothrombin into thrombin

  • 6. Thrombin converts fibrinogen into fibrin

  • 7. Fibrin forms a net like structure across the wound, which traps platelets and red blood cells, forming a clot

What is Clot Retraction?

• As the clot retracts, it pulls the edges of the damaged blood vessel closer together

The Disorders of Hemostasis (Haemostasis)

• Thrombus: a blood clot forms in an unbroken blood vessel, which can potentially obstruct blood flow

• Embolus: a detached mass that flows through the blood (ex: thrombus)

• Hemophilia (haemophilia): a genetic condition where blood does not clot

How does our body prevent blood clotting?

• Our body has chemical inhibitors to avoid blood clots called Anticoagulants

  • Antithrombin & Heparin (examples)

What type of feedback is blood clotting?

• Positive Feedback

Why doesn’t blood clotting spread?

• Once the clot forms, almost all thrombin is bound to the fibrin it forms, preventing it from being released into the blood stream

How do we get rid of blood clots?

• Through fibrinolysis

  • The enzyme plasmin degrades fibrin, which breaks down the blood clot

Why is Vitamin K important for blood clotting?

• It stimulates the liver to produce prothrombin and other clotting factors

What are Anticoagulent drugs?

• Used to delay or prevent undesirable blood clots

• Examples include: Heparin & Aspirin

What are Thrombolytic Agents?

• Substances that dissolve formed blood clots

• Works by turning plasminogen into plasmin

  • Plasmin then breaks down the blood clot through fibrinolysis