An excellent companion to the study of these concepts can be found at the CCC, courtesy of LITFL: their Cardiovascular Physiology Overview benefits enormously from flash-animated diagrams. This is merely a brief point-form summary to help the time-poor exam candidate. In fact, those candidates who are poorest can completely omit this vanity topic, as it is has never appeared in the past papers.
Systolic and diastolic blood pressure
- Varies according to site of measurement
- The more distal from the aortic root, the higher the systolic and the lower the diastolic
- Mean arterial pressure is less site-dependent
Mean Arterial Pressure (MAP)
the average arterial pressure during a single cardiac cycle
MAP = (Systemic vascular resistance) × (cardiac output)
(more on this elsewhere)
Systemic Vascular Resistance (SVR)
The resistance to blood flow offered by all of the systemic vasculature
SVR = ( 80 x (MAP - mean RAP) / cardiac output)
(where RAP is right atrial pressure)
Mean Circulatory Filling Pressure (Pmcf or MCFP)
The mean vascular pressure that exists after a stop in cardiac output and redistribution of blood, so that all pressures are the same throughout the system (Rothe).
It represents the average filling pressure of the entire circulatory system.
Its major determinant is the tone of veins and venules thoughout the body, as these act the dominant capacitance vessel.
Mean Systemic Filling Pressure (MSFP)
In contrast to the MCFP, this is the mean vascular pressure of the systemic circuit only.
MSFP excludes the right heart and pulmonary circulation.
MSFP – RAP = CO × venous resistance
However, venous "resistance" is usually a minor contributor to venous flow mechanics (there is so little resistance to speak of).
Venous capacitance is regulated by the sympathetic nervous system; noradrenaline is a great venoconstrictor, and most of the time its effect is due to the increased cardiac output resulting from decreased venous capacitance.
Under ideal (apneic) conditions, venous return and cardiac output must be equal.
Thus, venous return = cardiac output.
Stroke volume of the heart increases in response to an increase in the end-diastolic volume, when all other factors remain constant.
Preload, Afterload and Contractility
- Preload = initial myocardial fibre length prior to contraction
- Afterload = left ventricular wall tension required to overcome resistance to ejection (impedance to ejection of blood from the heart into the arterial circulation).
- Contractility = the change in force generated independent of preload; synonymous with inotropy
(more on this elsewhere)