A number of studies in the literature have looked into the use of real-time biometric data to improve one's own physiological performance and wellbeing. However, there is limited research that looks into the effects that sharing biometric data with others could have on one's social network. Following a period of research on existing mobile applications and prototype testing, we developed a system, HeartLink, which collects real-time personal biometric data such as heart rate and broadcasts this data online. Insights gained on designing systems to broadcast real-time biometric data are presented. In this paper we also report emerging results from testing HeartLink in a pilot study and a user study that were conducted during sport events. The results showed that sharing heart rate data does influence the relationship of the persons involved and that the degree of influence seems related to the tie strength prior to visualizing the data.