Platelets and Hemostasis

Platelets and Hemostasis

• Platelets are formed elements in blood, being small, colorless, and non-nucleated fragments of cytoplasm.
• Platelet size: diameter of 2.5 μm and volume of 7.5 cu μm
• Platelet shape: normally without processes (filopodia), but activated platelets develop filopodia.
• Platelet Structure: composed of cell membrane, microtubules, and cytoplasm.

Platelet Structure and Composition

• Cell membrane: Contains lipids, carbohydrates, and proteins. Glycoproteins and phospholipids are functionally important components.
• Glycoproteins: Prevent normal platelet adherence to endothelium but accelerate adherence to damaged endothelium/collagen in ruptured vessels.
• Phospholipids: Accelerate clotting reactions and form precursors of thromboxane A2.
• Cytoplasm: Contains cellular organelles and chemical substances (proteins, enzymes, hormones, etc.).

Platelet Proteins

• Contractile proteins:
  • Actin and myosin: responsible for platelet contraction.
  • Thrombosthenin: responsible for clot retraction.
• Von Willebrand factor: responsible for platelet adherence and regulating plasma factor VIII levels.
• Fibrin-stabilizing factor: a key clotting factor.
• Platelet-derived growth factor (PDGF): involved in repairing damaged blood vessels and healing wounds.

Hormonal and Other Chemical Substances

• Platelet-activating factor (PAF): causes platelet aggregation during blood vessel injury.
• Hormonal substances:
  • Adrenaline
  • Serotonin
  • Histamine
• Other chemical substances:
  • Glycogen
  • Blood group antigens
  • Calcium, copper, magnesium, and iron

Normal Platelet Count and Variations

• Normal platelet count: 250,000 to 400,000/cu mm of blood.
• Age: Platelet counts are lower in infants (150,000-200,000/cu mm) and reach normal levels by the third month after birth.
• Sex: No significant difference between males and females, except during menstruation in females when they may be reduced.
• High altitude: platelet counts increase.
• After meals: platelet counts increase.

Platelet Properties

• Adhesiveness: the property of sticking to a rough surface.
• Aggregation: the grouping of platelets.
• Agglutination: the clumping together of platelets.

Platelet Functions

• Role in blood clotting
• Role in clot retraction
• Role in preventing blood loss (hemostasis)
• Role in repairing ruptured blood vessels
• Role in defense mechanism

Platelet Lifespan

• Average lifespan: 8-11 days
• Destruction occurs via tissue macrophage system in the spleen.
• Splenomegaly (enlarged spleen) decreases platelet count, while splenectomy increases it.

Hemostasis

• Definition: Hemostasis is the arrest or stoppage of bleeding.
• Stages of hemostasis:
  • 1. Vasoconstriction: blood vessel constricts after injury to reduce blood loss.
  • 2. Platelet plug formation: platelets adhere to the exposed collagen and release contents.
  • 3. Coagulation of blood: fibrin threads form a network that traps platelets, RBCs, and WBCs to form a blood clot.

Coagulation of Blood

• Definition: Coagulation is the process where blood loses fluidity and forms a jelly-like mass.
• Stages of blood clotting:
  • 1. Formation of prothrombin activator
  • 2. Conversion of prothrombin to thrombin
  • 3. Conversion of fibrinogen to fibrin.

Clotting Factors

• Thirteen clotting factors are identified and play a role in the process.

Blood Clot

• Definition: a mass of coagulated blood containing RBCs, WBCs, and platelets entrapped in a fibrin meshwork. RBCs give it the red color.
• RBCs and WBCs are not crucial for clotting but trapped as the clot forms.

Clot Retraction

• Blood clot shrinks, which expels straw-colored serum after 30-45 minutes post-formation, a process called clot retraction.
• Contractile proteins (actin, myosin, thrombosthenin) are responsible.

Fibrinolysis

• Definition: Lysis, or breakdown of blood clots inside blood vessels is known as fibrinolysis.
• Role: Removes clots to restore blood vessel function.
• Mechanism: Plasmin or fibrinolysin is the substance necessary for this process.

Explore the fascinating world of platelets and their crucial role in hemostasis. This quiz covers their structure, composition, and function in the clotting process. Test your knowledge on how these small yet vital components of blood operate during injury and repair.