Mednotes

L06 Blood Introduction to Haemostasis - Platelets

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tagsblood, lecture
aliasesL06 Blood Introduction to Haemostasis - Platelets

14 min read

Overview

This lecture introduces haemostasis (blood stopping, distinct from homeostasis) as the combined action of vessel constriction, platelet plug formation and the coagulation cascade. It then focuses on platelets: their normal adhesion/activation biology, receptors and granule contents, production from megakaryocytes, the drugs that target them, and the clinical consequences of too few or dysfunctional platelets (thrombocytopenia and platelet function defects), including how these are assessed. An optional set of self-learning cases (Essential Blood, Moodle) on platelet count changes over time is also flagged for further practice.

Haemostasis: the three components

  • Haemostasis = “blood stop”; not the same as homeostasis (any self-regulating process maintaining stability while adjusting to optimal conditions for survival).
  • Bleeding is stopped by three components acting together: 1) vessel constriction, 2) platelet activation and platelet plug formation, 3) coagulation cascade activation producing fibrin. All three converge to achieve haemostasis.
  • Platelets: normally stick to damaged vessels; important for cuts and for repairing constant low-level injury in the mouth, gut, uterus, skin, etc. Too much platelet stickiness → arterial thrombosis. Too little (low platelets or dysfunctional platelets) → bleeding from mucosal surfaces, gut bleeds, menorrhagia, bruising.
  • Coagulation is the “jelly” component: a fibrin jelly. Normal: haemostasis/repair after vascular injury. Too much: clot (jelly) forms in veins, e.g. deep vein thrombosis. Too little: bleeding disorder, e.g. haemophilia, or anticoagulant use.
  • Haemostasis overall = a combination of fibrin clot and platelet aggregates (both fibrin fibres and platelet aggregates are seen together in a clot).

Platelet adhesion, activation and plug formation

  • Healthy arterial endothelium is like “teflon” (non-stick).
  • Damaged endothelium promotes platelet adhesion, in four sequential steps:
    1. Injury: normal endothelium either side of the injury releases PGI2 and NO (endothelial CD39 also present).
    2. Initiation: platelets begin adhering at the injury site.
    3. Extension: platelets accumulate at the site and release ADP and TxA2; thrombin appears at the vessel wall.
    4. Stabilization: a dense mass of aggregated platelets forms, still releasing ADP and TxA2, with thrombin present and fibrin strands reinforcing the platelet mass.
  • Summary of this process:
    • A. Normal endothelium secretes anticlotting agents: PGI2 (prostacyclin, inhibits platelet activation) and NO (nitric oxide, dilates blood vessels).
    • B. When the endothelium is damaged/activated, platelets stick to the damaged endothelium and to underlying collagen and von Willebrand Factor.
    • C. Platelets secrete ADP and thromboxane to recruit more platelets.
    • D. Platelets stick together, and activate the coagulation system to reinforce the plug with fibrin.
  • At the same time, the coagulation cascade is activated. Only the tissue factor pathway is shown: tissue damage exposes tissue factor, which together with factor VIIa converts factor X to Xa; Xa converts prothrombin to thrombin; thrombin converts fibrinogen to a fibrin clot. Platelet activation/plug formation and coagulation cascade activation both converge to produce the combined fibrin/platelet clot.
  • Normal platelets in a blood film are about 1-2 µm in diameter. Platelet lifespan in blood is about 10 days.

Platelet granules and receptors

  • Upon activation, platelet microtubules contract and granules are released. Key granule components:
    1. ADP - activates adjacent platelets.
    2. Serotonin - causes vasoconstriction and activates adjacent platelets.
    3. Von Willebrand Factor.
    4. Fibrinogen and factor V - enhance coagulation.
    5. Platelet-derived growth factor (PDGF) - recruits fibroblasts for healing.

Warning

  • Key platelet receptors:
    1. vWF receptor - initial adhesion and activation.
    2. Collagen receptor - initial adhesion and activation.
    3. ADP receptor - responds to ADP secreted by adjacent platelets.
    4. Thrombin receptor - thrombin is a product of coagulation.
    5. Thromboxane (TXA2) receptor - responds to TXA2 secreted by adjacent platelets.
    6. Fibrinogen receptor - forms bridges between adjacent platelets.
  • Platelet adhesion to von Willebrand factor triggers platelet activation, resulting in:
    • an increase in intracellular calcium
    • increased phosphatidylserine exposure, creating a negatively charged surface for coagulation
    • cytoskeletal rearrangement
    • alpha and dense granule release
    • conversion of the αIIbβ3 (GPIIb/IIIa) receptor to a high-affinity state
    • thromboxane A2 generation

Platelet production (thrombopoiesis)

  • Platelets are produced by megakaryocytes in the bone marrow. A megakaryocyte has a giant nucleus that divides within the cell; granules and organelles are formed; platelet strings (proplatelets) are then budded off.
  • An average adult produces approximately 1 x 10^11 platelets per day.
  • Platelet production is stimulated by thrombopoietin (TPO).
  • Approximately 25% of platelets are sequestered in the spleen at any one time.
  • Megakaryocytes extend long, branching proplatelet processes from the cell body over time (observed extending progressively over a 0-10 hour time-lapse).
  • Within the bone marrow niche: haematopoietic stem cells (HSCs) differentiate into immature then mature megakaryocytes, which extend proplatelet processes (via microtubules, with actin podosomes) into the sinusoid blood flow; these proplatelets then fragment into individual platelets that are released into the circulating blood.

Antiplatelet drugs and their targets

Why this matters clinically: about a quarter of patients die from thrombosis, mostly platelet-associated; a large proportion of the population takes antiplatelet drugs (especially low-dose aspirin or NSAIDs); platelets exposed to aspirin are “paralysed” for their entire lifespan, which matters when a patient on aspirin needs urgent surgery; many cardiac patients take other antiplatelet drugs including ADP receptor blockers; many patients take SSRIs (serotonin reuptake inhibitors); lack of von Willebrand Factor (von Willebrand disease) leads to poor platelet function; and coagulation (thrombin activation) is necessary for the platelet plug to form. Leading global causes of death in 2019 (age-standardised rate per 100,000) were ischaemic heart disease (110.9) and stroke (89.3), both largely platelet-associated (GBD 2021 Causes of Death Collaborators).

  • Platelet receptor/signalling targets mapped to drugs:
    • Thromboxane A2/TPα-R receptor (linked to Gq, G12/G13, Gs signalling).
    • Thrombin/PAR-4 receptor.
    • P2Y1 and P2Y12 ADP receptors - P2Y12 is the target of P2Y12 inhibitors: clopidogrel, prasugrel, ticagrelor.
    • 5-HT2A serotonin G-protein coupled receptor.
    • COX-1, which converts arachidonic acid pathway product to thromboxane A2 via TXS - COX-1 is the target of COX-1 inhibitors: aspirin.
    • Surface adhesion receptors: GP Ia/IIa and GP VI, and GP Ib/IIa-GP IX-GP V, which bind endothelial collagen and vWF; GP IIb/IIIa, which binds fibrinogen to another platelet.
  • Aspirin is an irreversible inhibitor of COX-1, preventing production of thromboxane A2, which is necessary for stimulation of adjacent platelets. Because platelets cannot make new COX-1, a single dose of aspirin affects a platelet for its entire (about 10-day) lifespan. Non-steroidal anti-inflammatory drugs (e.g. voltaren, brufen) are reversible inhibitors of COX-1.
  • Clopidogrel is a commonly used antiplatelet drug that blocks the ADP receptor P2Y12. It is often used in patients who have had a heart attack, to prevent extension of the clot in their coronary arteries.
  • Acquired platelet function defects, by duration of action:
    • Long-acting agents: aspirin (cyclooxygenase inhibitor; blocks TXA2 synthesis; reduces platelet activation, producing a slower, smaller platelet plug); clopidogrel (ADP receptor blocker).
    • Short-acting agents/foods: NSAIDs (a large group used for musculoskeletal pain and inflammatory disorders, but not paracetamol, opiates or tramadol); many other drugs mildly impair platelet function.

Thrombocytopenia (low platelets)

  • Low platelets cause petechiae (small pinpoint red-purple spots) and purpura. Large areas of purpura are called ecchymoses; small pinpoint bleeds are called petechiae.

Warning

  • Causes of low platelets:
    • Reduced marrow production (see below).
    • Immune destruction - immune thrombocytopenia (ITP) or drug-associated immune destruction.
    • Consumption - disseminated intravascular coagulation (DIC).
    • Genetic.
    • Splenomegaly - normally about 25% of platelets live in the spleen, but this can rise to about 90% if the spleen is massively enlarged.
  • Causes of reduced platelet production:
    • Marrow depression: chemotherapy, radiotherapy, some drugs and chemicals.
    • Marrow failure due to marrow replacement: cancer of bone marrow cells (leukaemias, lymphomas, fibrosis); secondary marrow infiltration by metastatic cancer.
    • Selective depression of megakaryocytes: some viral infections (e.g. chicken pox, EBV infection); many drugs and chemicals.
    • Genetic.
  • Platelet count thresholds and clinical significance:
    • 150-400 x 10^9/L: normal.
    • 100-150 x 10^9/L: no worries.
    • 50-100 x 10^9/L: caution, especially with surgery/trauma.
    • <50 x 10^9/L: bruising is common; stop all anticoagulants/antiplatelet agents.
    • 10-20 x 10^9/L: extra caution; platelet transfusion sometimes given.
    • <10 x 10^9/L: platelet transfusion.
    • High platelet counts are noted as a separate topic not covered in this lecture (“another day”).
  • Case scenario: a 68-year-old man noticed spontaneous bruising and a nosebleed lasting 6 hours; he was taking aspirin. Platelets were 1 x 10^9/L (reference 150-400 x 10^9/L); haemoglobin was 159 g/L (reference 130-175 g/L). He was treated with high-dose steroids and IV immunoglobulin; latest platelets rose to 303 x 10
  • A related case example: a patient with a platelet count that was “too many” (elevated) had had a transient ischaemic attack the week before, illustrating a clinical association between an elevated platelet count and thrombotic events such as TIA.

Inherited and acquired platelet function defects

  • Von Willebrand disease: prevalence 1:500-1:1000, autosomal dominant; caused by low levels of von Willebrand factor (covered in more detail in another lecture).
  • Inherited platelet defects: moderately common, ~1:500-1:1000. Mild platelet defects cause increased bruising, dental/surgical bleeding, heavy (often recurrent) periods, and often recurrent nosebleeds; iron deficiency can occur due to recurrent bleeding. They are caused by a wide range of genetic conditions. Diagnosis is by history from the patient and family studies, normal von Willebrand and coagulation factor levels, and genetic testing.
  • Questions to ask a patient before surgery to screen for a bleeding tendency:
    • Do you bleed from small cuts for longer than other people? (significant if >5 minutes)
    • Do you get spontaneous nosebleeds, and how often/how long do they last? (significant if usually >5 minutes)
    • Do you bruise more easily than others, and how often do you get a bruise? (significant if often monthly or more)
    • How large are the bruises? (significant if often >2.5 cm)
    • For women: menstruation (assessed using the PBAC score, which is noted as tricky).
  • A formal bleeding assessment tool exists (the ISTH-SSC Bleeding Assessment Tool, noted as interest only): it scores symptoms including epistaxis, cutaneous bleeding, bleeding from minor wounds, oral cavity bleeding, GI bleeding, haematuria and tooth extraction, each on a 0-4 severity scale.
  • Acquired platelet function defects are covered above under antiplatelet drugs (aspirin, clopidogrel, NSAIDs and other drugs).

Assessing platelet function

  • The PFA (Platelet Function Analyser) test is a laboratory, in vitro method for measuring platelet function.
  • The cartridge contains a capillary coated with activators: either collagen-adrenaline (normal range 82-160 s) or collagen-ADP (normal range 62-120 s).
  • Method: fresh citrated blood is drawn through the capillary, and the time for the capillary to be occluded by a forming platelet plug is measured (progressive occlusion is seen over time, e.g. more open at 80 s versus more occluded at 110 s).

Further practice

Optional self-learning cases (Essential Blood, on Moodle) extend this lecture’s material: Case 2 focuses on changes in platelet count in one person over time, including a reactive rise in count; Case 4 covers changes in platelet count after a large blood loss and the subsequent reactive changes, noting the time scale of the changes.

Self-test

  1. Define haemostasis and explain how it differs from homeostasis.
  2. List the three components that together achieve haemostasis.
  3. Describe the four steps by which platelets adhere and form a stabilised plug at a site of endothelial injury.
  4. What two anticlotting agents does normal, undamaged endothelium secrete, and what does each do?
  5. Describe the tissue factor coagulation pathway shown in the lecture, from tissue damage to fibrin clot.
  6. List five components released from platelet granules on activation, with the role of each.
  7. List six key platelet receptors and state what each responds to or does.
  8. Describe the events that occur inside/on a platelet after it adheres to von Willebrand factor.
  9. Describe how platelets are produced, from megakaryocyte to circulating platelet, including the role of thrombopoietin.
  10. What proportion of platelets is normally sequestered in the spleen, and what happens to this proportion in massive splenomegaly?
  11. Explain the mechanism of action of aspirin on platelets, and explain why a single dose affects a platelet for its whole lifespan.
  12. Distinguish the mechanism of action of clopidogrel from that of aspirin, and state a clinical situation in which clopidogrel is used.
  13. Distinguish petechiae from ecchymoses.
  14. List the five categories of causes of low platelets.
  15. List the categories of causes of reduced platelet production, with one example of each.
  16. A patient has a platelet count of 30 x 10^9/L before planned surgery. Using the platelet count thresholds given in the lecture, what level of caution is warranted and would anticoagulants/antiplatelets need to be stopped?
  17. A 68-year-old man on aspirin presents with spontaneous bruising and a 6-hour nosebleed; his platelet count is 1 x 10^9/L. Describe the treatment given in the case and its effect on the platelet count.
  18. Distinguish inherited platelet defects from von Willebrand disease in terms of prevalence and underlying cause.
  19. List four questions used to screen a patient for a bleeding tendency before surgery, with the threshold that makes each significant.
  20. Describe how the PFA (Platelet Function Analyser) test measures platelet function, including the two activator types used and their normal ranges.

Answers

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