Centro de Ciencias de la Complejidad (C3)
Instituto de Ciencias Nucleares (ICN)
Universidad Nacional Autónoma de México (UNAM)
Although homeostasis is a key concept of physiology and the basis to understand chronic degenerative disease and human ageing, it is difficult to quantify in clinical practice. Time series resulting from continuous and non-invasive physiological monitoring are conjectured to reflect the underlying physiological regulatory processes, but it is not clear why the variability of some variables such as heart rate gives a favourable health prognosis whereas the variability of other variables such as blood pressure implies an increased risk factor. A systems perspective suggests that different physiological variables may play distinct roles in their respective regulatory mechanisms and one can distinguish between regulated variables, such as blood pressure or core temperature, and physiological responses, such as heart rate and skin temperature. We give evidence that in optimal conditions of youth and health the former are characterized by Gaussian statistics, low variability and represent the stability of the internal environment, whereas the latter are characterized by non-Gaussian distributions, large variability and reflect the adaptive capacity of the human body; in the adverse conditions of ageing and/or disease, adaptive capacity is lost and the variability of physiological responses is diminished, and as a consequence the stability of the internal environment is compromised and its variability increases. We conclude that time-series analysis allows to quantify homeostasis in the optimal conditions of youth and health and the degradation of homeostasis or homeostenosis in the adverse conditions of ageing and/or disease and may offer an alternative approach to diagnosis in clinical practice.