A portable adapter is provided that can include a closure system configured to control the flow of blood and/or dialysate between the adapter and a blood treatment apparatus. Modular systems are also provided that include the portable adapter engaged with various units such as a portable blood processing module, a non-portable base module, and/or a remote module. Methods of conducting blood treatments such as blood circulation, hemodialysis, and hemofiltration, hemodiafiltration, using the modular systems are also provided. The systems, units, and methods enable the engagement and disengagement of the adapter from the various units to conduct, interrupt, and resume blood treatments without disconnecting the adapter from the vasculature of a patient. Modular systems including interchangeable portable and base modules configured for various blood treatments are also provided that can be engaged and disengaged with each other without disconnecting the portable module from the vasculature of a patient.