Q.Define cardiac output and give its normal approximate value (in litres per minute) at rest.

Cardiac output (CO) is the volume of blood ejected by each ventricle per minute, calculated as CO = heart rate (HR) x stroke volume (SV). Heart rate, or pulse, is simply the number of beats per minute, while stroke volume is the amount of blood one ventricle pumps out with each individual beat, equal to the difference between the ventricle's fullest and emptiest points in the cycle: SV = EDV - ESV (end diastolic volume minus end systolic volume). According to the Frank-Starling law of the heart, the single biggest factor controlling stroke volume is how much the cardiac muscle fibres are stretched immediately before they contract, which in turn is driven mainly by venous return - the volume of blood flowing back into the heart and filling its ventricles; during vigorous exercise, increased venous return can double stroke volume, and this Frank-Starling mechanism also protects the heart against sudden, abnormal swings in blood volume. Cardiac output feeds directly into blood pressure: mean arterial pressure is a function of cardiac output and arteriolar resistance together, and it is kep …