Temporal chaos in the microcirculation.
In vivo, spontaneous rhythmic contraction and dilatation of the vessel wall and the nonlinear rheological properties of blood both contribute to irregular fluctuations in microcirculatory blood flow over time. In isolated vessels, vasomotion exhibits patterns of behaviour that are highly characteristic of many non-biological nonlinear systems, including exactly integral periodicity and well-characterized routes for the transition from periodic to chaotic dynamics such as period-doubling. Nonlinear mathematical analysis of the effects of specific pharmacological interventions suggests the participation of a minimum of 4 key control variables which, at the cellular level, appear to involve Ca(2+)-induced Ca2+ release from intracellular stores and inward Ca2+ and outward K+ fluxes at the cell membrane. Four dominant variables may thus be [Ca2+] in the cytosol, [Ca2+] in the sarcoplasmic reticulum, membrane potential and the open state probability of certain K+ channel subtypes. Nonlinear analysis also indicates that nitric oxide synthesis by the vascular endothelium and changes in intraluminal flow and pressure are not significant determinants of the complexity of the underlying dynamics. Chaotic systems exhibit extreme sensitivity to initial conditions and their behaviour may appear highly unpredictable. This potentially accounts for variability in response to both pharmacological interventions and altered conditions of perfusion. The sensitivity of chaotic systems to perturbation can nevertheless be exploited to bring about large changes in state with minimum expenditure of energy, so that chaotic dynamics may confer a high degree of flexibility in overall control. Indeed, relatively simple control techniques based on negative feedback can readily stabilize the irregular responses of isolated arteries as periodic or steady-state behaviour but, alternatively, may also lead to an increase in overall dynamical complexity. At the present time, however, it remains unknown whether the chaotic nature of vasomotion confers specific benefits over and above those obtainable with sinusoidal control of vascular calibre (e.g.) in the regulation of vascular resistance, mass transport and tissue pressure.
Griffith
T M
TM
Department of Diagnostic Radiology, University of Wales College of Medicine, Cardiff, UK.
D006801:Humans; D008833:Microcirculation; D017711:Nonlinear Dynamics; D012039:Regional Blood Flow; D013997:Time Factors; D014666:Vasomotor System / Q000502:physiology*