Cardiovascular System
Cardiac Output
Cardiac output (CO) is the volume of blood ejected by the heart in 1 minute.
CO = stroke volume (SV) x heart rate (HR)
Approximately, 70ml (SV) x 70 bpm (HR) = 5L of blood!
CO may increase three to fourfold in strenous exercises.
Cardiac Index (CI) is the CO per square metre of body surface area (BSA), on average 3.2L
Starling's Law of the Heart
Frank-Starling mechanism describes the ability of the heart to change its force of contraction and therefore stroke volume in response to changes in venous return.
In other words, the energy of contraction of a myocyte is a function of the initial length of the muscle fibre. The greater the stretch of the ventricle during diastole, the greater the stroke volume.
If there are more blood in the heart i.e end-diastolic volume, the more sacromeres are stretched. This means an increase in venous return will in turn increase the force the heart muscle can exert. This is only up to a certain point – beyond the critical point, any further increase in the amount of blood will decrease the force the myocyte can exert. Pretty much, they tire out!