The Seidel-Herzel model of the autonomic-cardiorespiratory system is used in this work to derive three observers for the estimation of the concentrations of the neurotransmitters and external excitations of the sympathetic and parasympathetic systems. The observers require noninvasive measurements of respiration and blood pressure which simplifies the development of patient-specific applications. The errors stemming from model approximations and uncertain parameters are analyzed. The applicability of the approach is discussed in a simulation study.