Blood Components: Focus on Platelets

Study Notes

The presentation is about blood components and their functions, specifically focusing on platelets.
Platelets (Thrombocytes):
- Shape: Minute round or oval discs (2-4 μm), non-nucleated, size can increase to 6 μm after activation.
- Number: 150,000 – 400,000/mm³.
- Formation: Formed in the bone marrow as buds on the surface of megakaryocytes, released into the bloodstream under the influence of thrombopoietin from the kidney.
- Life span: 8-12 days, then removed by tissue macrophages.
Platelet Structure:
- Outer coat: Mucopolysaccharide surface coat, crucial for adhesion and aggregation. Glycoproteins bind to Von Willebrand Factor (vWF), necessary for adhesion to damaged endothelial cells; also acts as fibrinogen receptors.
- Plasma membrane: Basis structure of platelet factor 3 (PF3).
- Cytoskeleton (under the membrane):
  - Microfilaments: Composed of actin, myosin, and thrombasthenin.
  - Microtubules: Maintain the disc shape of the platelets.
- Granules: Three types:
  - Dense granules: Small, contain nucleotides (ADP), serotonin, and calcium.
  - Specific (Alpha) granules: Large, contain platelet factors responsible for hemostasis.
  - Dense tubular system: Contains calcium; site for prostaglandin and thromboxane A2 synthesis; regulates platelet activation by sequestering or releasing calcium.
- Other components: Mitochondria, glycogen, and lysosomes containing hydrolytic enzymes.

Form temporary hemostatic plugs (platelet adhesion, activation, and aggregation).
Release serotonin and thromboxane A2.
Crucial for blood coagulation (supplying platelet factor III and factor XIII).
Cause clot retraction.
Release platelet-derived growth factor (PDGF) for healing injured endothelium.

Stoppage of bleeding by forming blood clots and maintaining blood fluidity in the vascular system.
Three main components:
- Vasoconstriction: Contraction of smooth muscle in vessel walls (vascular spasm).
- Platelet aggregation: Platelets adhere to exposed collagen fibers in damaged blood vessels, releasing chemicals to cause aggregation (temporary platelet plug).
- Coagulation (blood clotting): Occurs via clotting factors (proteins, mostly synthesized in the liver).

Essential process because aggregation and vasoconstriction alone are not enough to stop bleeding entirely.
Involves clotting factors that work together to transform fibrinogen into fibrin, forming a clot.
Two pathways to blood clot formation: Intrinsic and extrinsic pathways.

Circulate briefly in the bloodstream, predominantly migrating through body tissues.
Migrate from blood vessels through extravasation.
Some WBCs (neutrophils, eosinophils, monocytes) exhibit phagocytosis.

Classified into granulocytes (include neutrophils, eosinophils, basophils) and agranulocytes (include lymphocytes—B and T lymphocytes and natural killer cells, and monocytes).