Contraction of the cardiac muscles starts with an action potential acting as a signal, which depolarizes myocytes, making them to contract. Ca 2+ is key in both processes, signaling and contraction; in this work we are going to focus in the role its concentration plays in contraction of cardiomyocytes, and therefore beating of the heart. A cardiac cell can be modelled as an array of Calcium Release Units (CaRUs), each of which present five different compartments and calcium concentration; the response of this concentrations to the external action potential determines the cardiac contraction. 2016/2017