Blood Coagulation and Hemostasis

Blood Coagulation

• Blood clotting, or coagulation, prevents excessive bleeding when a blood vessel is injured.
• Platelets and proteins in plasma work together to stop the bleeding by forming a clot over the injury.
• Hemostasis, or haemostasis, is a process that causes bleeding to stop, meaning to keep blood within a damaged blood vessel.

Steps in Hemostasis

• Hemostasis has three major steps:
  • Vasoconstriction (vascular spasm)
  • Temporary blockage of a break by a platelet plug
  • Blood coagulation, or formation of a fibrin clot
• These processes seal the hole until tissue is repaired.

Basic Events in Blood Clotting

• Vascular injury leads to vascular spasm, platelet plug formation, and blood coagulation.
• Vascular spasm (vasoconstriction) reduces blood flow through the area and limits blood loss.

Platelet Plug Formation

• Platelets adhere to damaged endothelium to form a platelet plug (primary hemostasis) and then degranulate.
• Plug formation is activated by a glycoprotein called Von Willebrand factor (vWF).
• Platelets express certain receptors, which are used for adhesion to collagen.
• When platelets are activated, they express glycoprotein receptors that interact with other platelets, producing aggregation and adhesion.

Clot Formation

• Once the platelet plug has been formed, clotting factors are activated in a sequence of events known as the 'coagulation cascade'.
• This leads to the formation of fibrin from inactive fibrinogen plasma protein.
• A fibrin mesh is produced around the platelet plug to hold it in place, a step called "Secondary Hemostasis".

Platelets

• Platelets are not cells, but small, irregularly shaped, non-nucleated fragments produced from large cells called megakaryocytes.
• Function: Platelets play an important role in stopping bleeding, so they are important in blood clotting.
• Shape: Platelets are spherical or rod-shaped and become oval or disc-shaped when inactivated.
• Size: The average size of platelets is 2-4 μm (microns).
• Life Span: The average lifespan of platelets is 3-10 days.
• Normal Range: About 150,000-450,000 cells are present per cubic millimeter (cmm) of blood.

Platelet Count

• The calculated number of platelets in a volume of blood, usually expressed as platelets per cubic millimeter (cmm) of whole blood.
• Platelet count is of great importance in helping to diagnose bleeding disorders.
• Methods: Automated method, blood smear, and manual method are used to count platelets.

Manual Method of Platelet Count

• Materials and instruments: whole fresh blood, RBC pipettes, haemocytometer, diluting fluid (ammonium oxalate 1%), microscope, petridish, filter paper, and lancet.
• Procedure: 1. Prick the finger and draw blood, then mix with diluting fluid. 2. Wait for 10-15 minutes for RBC hemolysis. 3. Discard the first 3-4 drops and fill the counting chamber. 4. Leave the chamber for 20 minutes to allow platelets to settle. 5. Count platelets under a microscope using lens power 10x and 40x.
• Calculations: Total platelet count = Total no. of platelets counted x 1000