Cardiovascular Core Physiology

Perfusion is the delivery of oxygenated blood to tissue at a rate the tissue can use. It depends on both flow and pressure; a normal blood pressure does not guarantee adequate cardiac output, and a reduced ejection fraction does not by itself prove that tissue perfusion is failing. Cardiac output is the product of heart rate and stroke volume: CO = HR × SV Heart rate can increase cardiac output only while ventricular filling and contraction remain effective. A very rapid rate shortens diastole, reduces filling time, and may lower stroke volume enough that total output falls. Stroke volume is shaped by three variables: - Preload is the ventricular filling and myocardial fibre stretch before contraction. Increasing preload can improve contraction within a physiologic range, but excessive filling raises venous or pulmonary pressures and produces congestion. - Afterload is the pressure and resistance the ventricle must overcome to eject blood. Increased systemic vascular resistance or arterial pressure makes left ventricular ejection more difficult and can reduce stroke volume. - Contractility is the strength of myocardial contraction independent of preload and...