Biomedical Engineering 403

Coordinated Cardiovascular Responses

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13
Coordinated Cardiovascular Responses



Cardiovascular physiology does not occur in a vaccuum.

Each major adaptation is a combination of several smaller responses.

Adaptation to a change in posture:

Gravity has an enormous effect on the distribution of venous blood.
Moving from lying to standing causes a 20% fall in intrathoracis blood
volume over about 15 sec, reducing cardiac filling pressures and therefore
energy of myocardial contraction (via Starling's law).  Stroke volume
declines 30-40%.  Mean BP falls only transiently (because of correction
discussed below), but even so, transient hypertension can be severe enough
to impair cerebral perfusion, causing dizziness and visual fading for a
few seconds.  Does not usually progress to postural syncope unless
compensatory mechanisms are deficient.

Carotid baroreceptor traffic is reduced by the fall in BP (caused by
gravity and reduced cardiac output), which informs the brain of the
gravity of the situation, eliciting a reflex reduction in vagal outflow to
the heart and an increase in sympathetic outflow to both heart and
vasculature.  Heart rate increases 15-20 beats/min.  Combined with the
sympathetically mediated increase in contractility, this limits the fall
in cardiac output.  Sympathetically mediated vasoconstriction in skeletal,
splanchnic and renal vascular beds raises arterial pressure.

Valsalva Maneuver:

Bowel movements, coughing, lifting heavy weights, etc.  Initially BP
increases because of the high intrathoracic pressure.  Mean BP than begins
to fall because the high intrathoracic pressure impedes venous return,
impairing stroke volume via Starling's law.   Elicits reflex tachycardia
and vasoconstriction.

Exercise:
Challenges:
        1.      Pulmonary blood flow must be increased to enhance gas
exchange (achieved by increasing right            ventricular output)
        2.      Blood flow through active muscles must be raised
(achieved by locally mediated vasodilation and                an increase
in left ventricular output to make more blood available)
        3.      Reasonably stable BP must be maintained (achieved by
controlled vasoconstriction of non-active          tissues)
Cardiac output can show a 4-fold increase in response to exercise.

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