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Activated platelet phospholipid surface

Pathway · Phosphatidylserine Calcium (IV) PAR-1

Platelet activation flips anionic phosphatidylserine to the outer leaflet and sheds procoagulant microparticles, creating the membrane on which the vitamin K-dependent factors assemble through calcium bridges. Alpha granules deliver factor V, fibrinogen and vWF to the same spot. Calcium is factor IV and is indispensable, which is why citrate anticoagulates a blood tube.

Traced from the start

  1. Subendothelial collagen and von Willebrand factor
  2. Activated platelet phospholipid surface

Detail

Provides
A phosphatidylserine-rich surface for intrinsic tenase (IXa-VIIIa) and prothrombinase (Xa-Va), confining thrombin generation to the site of injury
Activated by
Thrombin via PAR-1 and PAR-4, collagen via GPVI, ADP via P2Y12, and thromboxane A2
Granule contents
Alpha granules: factor V, fibrinogen, vWF, platelet factor 4. Dense granules: ADP, ATP, serotonin, calcium and polyphosphate
Calcium
Factor IV; citrate and EDTA chelate it, so coagulation samples are taken into 3.2% sodium citrate at a 9:1 blood-to-citrate ratio

When it goes wrong

Scott syndrome — defective phosphatidylserine scrambling by TMEM16F

Bleeding despite a normal platelet count, normal PT, APTT and vWF: the platelet plug forms but thrombin generation on its surface is inadequate

Massive transfusion of citrated blood, or a citrate load during apheresis or liver transplantation

Citrate toxicity with ionised hypocalcaemia: perioral paraesthesia, tetany with a positive Trousseau sign, and a prolonged QT interval; treat with intravenous calcium

An underfilled citrate tube, or a haematocrit above about 55%

Relative citrate excess chelates the added calcium and falsely prolongs both the PT and the APTT

Practise this structure

2 questions in the bank tagged Ionised Calcium.

A 22-year-old woman is brought to the emergency department 30 minutes after an argument at home, breathing rapidly and deeply. She complains of tingling around…

Foundations · middle twist

A 22-year-old woman is brought to the emergency department after 30 minutes of rapid, deep breathing that began during an argument. She reports tingling around…

Foundations · middle twist