The heart cannot pump blood that it does not receive

New indices from microneurography to investigate the arterial baroreflex

“…; Wieling et al. ), some debate remains regarding the relative contribution of the cardiac and vascular responses to orthostasis (Convertino ). This may be perpetuated in part by the relative ease of examination of cardiac compared to vascular responses.…”

Cardiovascular regulation in women with vasovagal syncope : With special reference to the venous system

“…Vagally mediated tachycardia has been suggested to protect against reduction in CO and subsequent hypotension during the initial seconds of an orthostatic transition (19). However, the importance of reflex tachycardia on hemodynamic stability for extended durations of orthostasis is less clear (20). One reason could be that the effect of increased HR only seems to have an initial transient effect on CO since right atrial pressure falls (RAP) towards 0 mmHg upon standing.…”

“…Wiessler showed that administration of atropine in the upright position increased HR but had marginal effects on BP as well as CO, and was unable to prevent impeding VVS (21,22). Further, patients with cardiac transplants have intact BP control during orthostasis without any increase in HR, while patients with sympathetic vasomotor lesions display pronounced orthostatic hypotension despite intact vagal reflex tachycardia (20,23).…”

“…This implies that peripheral vasoconstriction is crucial for maintaining BP during orthostatic stress and that the venous blood reservoir is an important determinant because the heart cannot pump blood that it does not receive (12,20). Arterial vasoconstriction reduce blood flow to the venous section and decrease peripheral venous pressure, leading to passive elastic recoil of pooled venous blood from the lower limbs and splanchnic vasculature to the central circulation (24).…”

Neuro-humoral control during orthostasis in health and disease

“…Studies in healthy young individuals indicate that in addition to sympathetic vasoconstriction, the heart rate response may also contribute importantly to orthostatic tolerance (Convertino, 2014 ). On the other hand, it is argued that cardiac effector mechanisms appear not to be important for the adjustment of arterial pressure to the upright posture in humans (Wieling et al, 2014 ). Indeed, investigations into the role of the sympathetic nervous system in orthostatic intolerance have yielded mixed results.…”