Platelets are small anucleate blood cells1,2 with critical roles in haemostasis, thrombosis, inflammation, vascularization, innate immunity and tissue regeneration3,4. In vitro-derived platelets, as an alternative to native platelets, are attractive for fundamental research because of their rapid genetic tractability, as vectors for drug and genetic component delivery5 and in clinical platelet transfusion. At present, however, their very low production rate, and poor agonist responsiveness, are major obstacles. Platelets are formed by fragmentation from mature polyploid megakaryocytes (MKs), their precursor cells6, although the process of their generation remains incompletely understood7,8. Bone marrow is proposed to be the main site of platelet production, however indirect evidence since the 1930s9,10 and recent direct observation11 has shown that the lung can also be a primary site of platelet biogenesis. Here we established an ex vivo mouse heart-lung model (Fig. 1a) through which we were able to perfuse murine MKs. Remarkably, we could show for the first time that MKs, despite their large size, can pass multiple times through the lung vasculature, and that this leads to the generation of very large numbers of fully functional platelets (up to 3,000 per megakaryocyte7,12). Using this system and a novel in vitro microfluidic chamber we show roles for ventilation, oxygenation and healthy pulmonary endothelial cells in platelet generation. We show that MKs undergo enucleation upon repeated passage through pulmonary vasculature before fragmentation to generate platelets, with this final process dependent on the actin regulator TPM4. This advances our understanding of platelet formation in the body and establishes a novel approach to generate large numbers of them outside the body.