Emergency care is an emergent process requiring input from various healthcare professionals within the hospital. To avoid crowding and efficiently flow patients through the ED, collaboration between professionals across multiple sub-units is crucial. Yet, the multidisciplinary nature of emergency care presents a challenge to the optimization of patient flow, as specialization and functional differentiation restrict integration efforts. To deliver efficient and effective emergency care, professionals must integrate rapidly, working across functional, spatial, and professional boundaries. To provide a more holistic understanding of the inherent challenges to professional integration in this setting, we carried out an in-depth case study at a busy, level one trauma center in the Netherlands that recently implemented a reorganization of acute care delivery to improve emergency department flow. Speaking directly with medical professionals at the interfaces of emergency care across two models of care delivery we provide insights into key relational and cultural factors that come to effect supply chain functioning. We find the existence of structural, relational and cultural barriers across both models of care, and demonstrate how each boundary uniquely influences collaboration, and ultimately, patient flow.