At a time when knowledge about microvascular physiology and subcellular myocardial and vascular biochemistry has accumulated at such a tremendous rate, I perceive that a realistic global understanding of the cardiovascular system has been partially lost in that voluminous accumulation of minutiae. In order to be able to see "the forest, and not just individual leaves on the trees," one must first have a clear understanding of the gross mechanical function of the cardiovascular system as a whole.
In determining whether to replace older "sacred cows" with newer concepts, your litmus test should be the extent to which each explains observations in both normal and pathological states. Thus, a valid concept of cardiovascular physiology must be compatible with the following frequently overlooked facts:
- A blood volume equilibrium persists between the systemic and pulmonary circuits even when there are massive shunts between the two, and remains even after closure of those shunts.
- During cardiopulmonary bypass, the empty heart may continue to beat strongly even in the absence of any diastolic filling or stretching of the ventricles.
- Booster pumping does not increase circulation rate in the absence of heart failure.
- Increasing pacemaker rate above that necessary to prevent failure does not increase cardiac output.
- Ventricular pressures measured during heart catheterization are always above zero (in relation to the intrathoracic pressure).
- After heart transplantation, without nerve supply to the heart or artificial pacing, the cardiac output and pulmonary/systemic blood volume balance remain normal.
- In the absence of heart failure, an increase in arterial resistance does not reduce cardiac output.
An overall concept of cardiovascular physiology should accommodate these facts and all other available data. The concept presented in the following chapters explains and accommodates all of these findings.