Blood flow through the developing vasculature regulates vessel formation - both via the distribution of endocrine factors, and via mechanical force-induced responses. Several signalling pathways are known to be involved in this process, including signalling via the TGFb receptor Alk1, whose activity promotes quiescence in newly formed arteries and whose expression is itself dependent upon blood flow. On p. 3403, Beth Roman and colleagues demonstrate that not only Alk1 expression but also its activity are dependent upon blood flow in developing zebrafish. They identify Bmp10 as the endogenous ligand for Alk1 in this context, and find that Bmp10 is exclusively expressed in the heart, and not in the vascular tissue. Through elegant experiments using embryos in which the heart has been stopped but alk1 expression restored, they show that Bmp10 injection can locally rescue Alk1 pathway activity and downstream transcriptional responses. Thus, their data suggest that blood flow is required to distribute cardiac-derived Bmp10 into the vasculature, where it activates Alk1 to promote quiescence in endothelial cells.