Cardiac Output and Vascular Resistance
The vascular beds are a dynamic and connected part of the circulatory system against which the heart must pump to transport the blood. Q is influenced by the resistance of the vascular bed against which the heart is pumping. For the right heart this is the pulmonary vascular bed, creating Pulmonary Vascular Resistance (PVR), while for the systemic circulation this is the systemic vascular bed, creating Systemic Vascular Resistance (SVR). The vessels actively change diameter under the influence of physiology or therapy, vasoconstrictors decrease vessel diameter and increase resistance, while vasodilators increase vessel diameter and decrease resistance. Put simply, increasing resistance decreases Q; conversely, decreased resistance increases Q.
This can be explained mathematically:
By simplifying Darcy's Law, we get the equation that
- Flow = Pressure/Resistance
When applied to the circulatory system, we get:
- Q = (MAP – RAP)/TPR
Where MAP = Mean Aortic (or Arterial) Blood Pressure in mmHg,
- RAP = Mean Right Atrial Pressure in mmHg and
TPR = Total Peripheral Resistance in dynes-sec-cm-5.
However, as MAP>>RAP, and RAP is approximately 0, this can be simplified to:
- Q ≈ MAP/TPR
For the right heart Q ≈ MAP/PVR, while for the left heart Q ≈ MAP/SVR.
Physiologists will often re-arrange this equation, making MAP the subject, to study the body's responses.
As has already been stated, Q is also the product of the heart rate (HR) and the stroke volume (SV), which allows us to say:
- Q ≈ (HR × SV) ≈ MAP / TPR
Read more about this topic: Cardiac Output
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