Platelets

Platelets & Formed Elements

• Platelets: anucleated cell fragments; technically not true cells.

• Red blood cells (erythrocytes) also anucleated ➔ both classified, with leukocytes, as "formed elements" rather than blood cells.

Overview of Hemostasis (cessation of bleeding)

• Trigger: vessel wall laceration exposes sub-endothelial collagen to blood.

• Three tightly linked phases:

  1. Vasoconstriction

  2. Platelet plug formation

  3. Coagulation (fibrin clot) and subsequent clot removal/repair

Phase 1 – Vasoconstriction

• Smooth muscle around injured site contracts ➔ reduces lumen diameter & blood flow.

• Maintains some flow to deliver clotting factors/platelets while minimizing loss.

Phase 2 – Platelet Plug Formation

• Collagen exposure = adhesion site; endothelium normally prevents adhesion via nitric oxide.

• Platelets bind collagen & each other ➔ initial plug.

• Secrete Platelet-Activating Factor (PAF): positive feedback, recruits more platelets.

• Release serotonin & thromboxane A2 ➔ sustain/enhance vasoconstriction.

Phase 3 – Coagulation (Stabilizing the Plug)

• Fibrin needed to cement platelets.

• Circulating fibrinogen (inactive) converted to fibrin (active) by enzyme thrombin.
  \text{fibrinogen} \xrightarrow{\text{thrombin}} \text{fibrin}

• Fibrin mesh + platelet mass = stable clot sealing vessel.

Clot Dissolution & Vessel Restoration

• Clot narrows lumen; must be removed after tissue repair.

• Plasminogen (inactive) converted to plasmin by tissue plasminogen activator (tPA):
  \text{plasminogen} \xrightarrow{\text{tPA}} \text{plasmin}

• Plasmin digests fibrin ➔ clot dissolution, lumen returns to normal.

Additional Notes

• Growth factors (not detailed here) rebuild endothelium & collagen layer post-injury.

• Positive feedback loop: PAF ↑ platelets ↑ PAF, accelerating plug formation.