1. Endothelium constantly releases VWF, from the Weibel-Palade bodies, into the blood
2. When damage occurs to the endothelium the subendothelial collagen becomes exposed and any VWF circulating in the blood will bind to it
3. Platelets carry receptors for VWF and collagen and so are activated- this activation expresses fibrinogen receptors which are necessary for aggregation
4. Activated platelets can aggregate by binding to fibrinogen, and they release ADP and TXA2