A critical assessment of sympathetic restraint in submaximal exercise: Implications for integrated cardiovascular circuit control in exercise

Figures

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  FIGURE 1. The cardiovascular circuit identifying the site of two key regulated variables shaded in light blue: arterial blood pressure and oxygen delivery–demand matching. Autonomic neural and local vasodilatory mechanisms are activated with exercise. Their interaction determines oxygen delivery–demand matching and arterial blood pressure regulation. Autonomic neural control (sympathetic neural activity ‐ SNA; parasympathetic neural activity ‐ PSNA) is indicated by solid arrows, with ‘+’ and ‘−’ indicating excitatory/activating/increasing effects and inhibitory/deactivating/decreasing effects, respectively. Dashed lines indicate feedback and feedforward influences on the cardiovascular control centre (CVCC), including central command (CC), cardiopulmonary baroreflex (CPBR), carotid baroreflex (CBR), mechano and metaboreflex. Taps represent resistance to blood flow between compartments: muscle arteriole vascular resistance (Rma), splanchnic arteriole vascular resistance (Rsa), muscle venular resistance (Rmv), splanchnic venular resistance (Rsv), pulmonary arteriolar resistance (Rpa) and pulmonary venular resistance (Rpv). Compartments are as follows: arterial compartment (Ca), muscle venous…

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  FIGURE 2. (a) Cardiocentric determination of circuit flow magnitude and balance in submaximal exercise. Activation of the heart determines the rate and magnitude of increase in CO, which is the ‘bottleneck’ for circuit flow increase. Exercising muscle vasodilatation ‘excess’ must be restrained such that CO–PBF balance and target arterial blood pressure are achieved. (b) Peripherocentric determination of circuit flow magnitude and balance in submaximal exercise. Vasodilatation in exercising muscle creates the increase in venous return that allows CO to increase. Autonomic activation of the heart creates a given HR:SV ratio, and the heart pumps whatever venous return it receives. Abbreviations: CO, cardiac output; HR, heart rate; PBF, peripheral blood flow; SV, stroke volume.

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  FIGURE 3. Comparison of femoral artery vasodilator (ATP) infusion versus knee‐extension exercise. Leg blood flow increases on the x‐axis with increasing exercise intensity and vasodilator infusion. With dilator infusion, a much lower heart rate response occurred, and stroke volume continued to increase well above the limit observed in exercise. Continued proportional increases in leg blood flow were driven by vasodilatation. In exercise, a pressor response and noticeable restraint of exercising leg vasodilatation combined to increase leg blood flow at higher exercise intensities. *Significantly different from exercise at the same time point. Reproduced from Gonzalez‐Alonso et al. (2008) with permission.