Complex sentence: The kinin--kallikrein system or simply kinin system is a poorly understood system, not much research has been carried out on this, of blood proteins that play a role in inflammation, blood pressure control, coagulation and pain.
Split into simpler sentences: The kinin--kallikrein system or simply kinin system is a poorly understood system with limited available research. It consists of blood proteins that play a role in inflammation, blood pressure control, coagulation and pain.