Cardiovascular and Respiratory Control Mechanisms During Exercise: an Integrated View

ABSTRACT Exercise can impose an immense stress upon many physiological systems throughout the body. In order that exercise performance may be optimally maintained, it is essential that a profound and complex series of responses is coordinated and controlled. The primary site for coordination is the central nervous system, whereas control mechanisms (both feedback loops and feedforward activation) involve complex sensory information, often in the form of neural coding but also in the form of blood-borne chemical signals, a number of levels of peripheral and central integration and, finally, the efferent branches of the nervous system coursing via sympathetic and parasympathetic nerves to target sites of action. The neurohumoral control of the cardiorespiratory responses to exercise has received intense attention for over two decades and some particularly important steps forward in its understanding have occurred within the last 10 years. The initial fast increase (phase 1) in cardiovascular and ventilatory flow parameters are brought about by neurally mediated muscle mechanoreceptor feedback reflexes and a feedforward ‘central motor command’. The blood pressure operating point is also raised by a combination of these two neural mechanisms. Fine control of the matching of cardiac output to ventilation may occur by means of a feedforward ventilatory control of cardiac origin. During the slower phase of adjustment (phase 2), the neurally mediated mechanisms are augmented by a cohort of humorally mediated feedback reflexes involving muscle and vascular chemoreceptors as well as being supported by central neural reverberation. A steady state of cardiorespiratory responses is achieved (phase 3) by an amalgamation of neural and humoral, i.e. ‘neurohumoral’, control mechanisms, which then must further modulate the cardiorespiratory responses to exercise in the face of increasing competition from other basic physiological requirements, such as thermoregulation and fluid homeostasis. The myriad of subtle modifications in the basic blueprint found throughout the vertebrates illustrates the flexibility of the principal design and also how it can be applied to an extraordinary number of specific ecophysiological niches.