Mechanical forces such as blood flow play a key role in regulating vascular remodelling and angiogenesis. Vessel diameter must be tightly controlled to establish correct hierarchical vascular architecture, but how this is achieved is unclear. Now, on p. 4041, Mary Dickinson and colleagues report that vessel diameter in the mouse embryonic yolk sac is regulated by blood flow via two distinct mechanisms: vessel fusion and endothelial cell migration. Using live confocal imaging of dual Flk1-reporting whole embryos, the authors observe that vessel fusion only occurs in areas of relatively high flow, and that this leads to a rapid increase in vessel diameter. In addition, high flow also facilitates the preferential recruitment of endothelial cells from smaller capillaries, which further increases vessel diameter. Analyses using the Mlc2a mutant, in which blood flow is compromised, supports these findings. This study highlights the importance of mechanical forces in regulating individual cell behaviour as well as more complex processes such as the establishment of correct vascular architecture.