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The role
of the cardiovascular system, taken in its broadest sense, is
to deliver oxygenated blood to the tissue where the oxygen is
utilized to support aerobic metabolism, and return the blood with
byproducts of metabolism to the liver and lungs. This process
can be analyzed in terms of oxygen content, cardiac function (the
pump), and oxygen use (conditioning).
Oxygen delivery to
the tissues is compromised when systolic cardiac function is diminished,
when anemia is present thereby reducing oxygen content of the
blood, or when there are focal obstructions in the peripheral
vasculature. The mechanism by which reduced oxygen delivery is
perceived and processed to produce breathing discomfort remains
unclear since there is no apparent apparatus within the body that
monitors oxygen content (as opposed to partial pressure of oxygen)
of the blood or delivery of oxygen to the tissue.
To the extent that
insufficient oxygen delivery to metabolically active tissue may
result in local anaerobic metabolism and production of lactic
acid, there may be stimulation of "metaboreceptors"
in the tissue that send signals to the brain that are perceived
as dyspnea.
Altered cardiac function,
whether systolic or diastolic, is typically associated with elevated
pulmonary capillary wedge pressure. This may lead to stimulation
of pulmonary vascular receptors and, to the extent that fluid
passes into the pulmonary interstitium, C-fibers may also be stimulated.
Furthermore, the production of interstitial fluid reduces pulmonary
compliance and may increase airway resistance, thereby imposing
an increased load on the ventilatory muscles and the ventilatory
pump. Hypoxia and hypercapnia may follow in severe cases. Cardiac
valvular disease clearly can contribute to these physiological
derangements.
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