Real-time MRI emerges as a feasible modality for guiding interventions and surgeries on the beating heart. One challenge is the path planning task for intra-cardiac procedures, which requires tracking the inner contour of the left endocardium on the beating heart. We propose a bottom-up approach to solve this tracking problem. On the bottom level, a coarse contour is generated based on a collaborative network of trackers at every time step. On the top level, PCA based shape prior is used to refine the coarse contours, and eventually obtain a smoothed contour of the left endocardium over time. The approach is validated on 14 MRI sequences of a beating heart, with comparisons to the state-of-the-art approaches. Experimental results show that our approach outperforms other approaches, providing a robust, accurate and fast solution to track the left endocardium of the beating heart.