SN1
SHORT NOTE
5M
★ High-yield
Describe the formation and functions of platelets.
Definition and Normal Value
Platelets (thrombocytes) = small, non-nucleated, disc-shaped cell fragments concerned with haemostasis.Count 1.5–4.5 lakh/µL; diameter 2–4 µm; lifespan 8–12 days.
Formation
Stem cell → Megakaryoblast → Promegakaryocyte → Megakaryocyte → Cytoplasmic fragmentation → Platelets
- Regulated by thrombopoietin from the liver
- One megakaryocyte yields 2000–4000 platelets
- One-third are stored in the spleen
Contents
| Granule | Contents |
|---|---|
| Alpha granules | Fibrinogen, factor V, vWF, PDGF, platelet factor 4 |
| Dense granules | ADP, ATP, serotonin, Ca2+ |
| Cytoplasm | Actin, myosin, thromboxane A2 synthase |
Functions
- Haemostatic plug formation — adhesion → activation → aggregation
- Provide platelet factor 3 (phospholipid surface) for coagulation
- Clot retraction — by actin and myosin (thrombasthenin)
- Vasoconstriction — release serotonin and TXA2
- Repair — PDGF stimulates smooth muscle and fibroblast growth
- Store and transport serotonin and histamine
Mechanism of Plug Formation
Endothelial injury exposes collagen → Adhesion via von Willebrand factor to GpIb receptor → Activation — shape change, release of ADP and TXA2 → Aggregation — fibrinogen bridges GpIIb/IIIa receptors → Temporary platelet plug
Applied Aspects
- Thrombocytopenia (< 1 lakh) → petechiae, purpura; spontaneous bleeding below 20,000. Causes — ITP, dengue, aplastic anaemia, hypersplenism
- Aspirin irreversibly inhibits COX-1 → ↓ TXA2 for the platelet's whole 8–12 day lifespan
- Clopidogrel blocks the ADP (P2Y12) receptor; abciximab blocks GpIIb/IIIa
- Glanzmann thrombasthenia — GpIIb/IIIa defect; Bernard–Soulier — GpIb defect
All questions in Blood (haematology)
- Q1Define erythropoiesis. Describe its sites, stages, and the factors regulating it. Add a note on the fate of red blood cells. ★
- Q2Describe the mechanism of blood coagulation. Explain the intrinsic and extrinsic pathways and the tests used to assess them. Add a note on natural anticoagulants. ★
- Q3Classify blood groups. Describe the ABO and Rh systems and discuss mismatched transfusion reaction. Add a note on erythroblastosis fetalis. ★
- Q4Classify leucocytes and give their normal values. Describe the functions of neutrophils and discuss the types of immunity. ★
- Q5Describe the structure and types of haemoglobin. Outline its synthesis and describe its fate after red cell destruction. ★
- SN1Describe the formation and functions of platelets. ★
- SN2Classify anticoagulants. Describe the mechanism of action of heparin and warfarin. ★
- SN3Define ESR. Give its normal values and enumerate the factors affecting it. ★
- SN4Define anaemia. Classify it morphologically and aetiologically. ★
- SN5Describe bilirubin metabolism. Classify jaundice and give the findings in each type. ★
- SN6Enumerate the plasma proteins with their normal values and describe their functions.
- SN7Write a short note on haemophilia.