Vital signs such as respiration and pulse and electrocardiography are crucial for monitoring disease and promoting health in the age of ubiquitous cloud computing. However, systems and devices that sense, process and analyse vital signs suffer from a lack of interoperability caused by differences in the platforms, devices and data formats used. Consequently, a particular vendor can provide a limited number of services, but users are not able to access more shared services. To counter these problems, this study proposes a service-oriented architecture for healthcare systems utilising vital signs. The proposed structure differs from the one used to handle business logics in general, because vital signs data are distinct from the types of data used in other enterprises. In this study, these data are expressed as the Health Level Seven standard schema, and the structure uses Object Constraint Language (OCL), a platform-independent specification language, to access the vital signs data and evaluate the state of health. In particular, the use of OCL enables a fast response to changes in service content and reduces maintenance costs.