During diastole the left ventricle fills with blood, and during systole this blood is squeezed out of the ventricle into the aorta. At the end of systole and beginning of the diastole, there is still blood left in the ventricle. Ventricles are thus not drained entirely.
The blood volume of the left ventricle immediately before contraction is called end diastolic volume (EDV). This will normally be somewhere between 120 to 130 ml. When the heart has contracted at its maximum we are left with a blood volume in the ventricles we call end systolic volume (ESV). In a normal resting heart this blood volume is about 60 to70 ml.
The difference between EDV and ESV is the heart’s stroke volume (SV), i.e. the actual volume the heart pumps per heartbeat. We write:
SV = EDV – ESV
Both EDV and ESV can be changed. If the sympathetic nervous system is stimulated more blood is ejected (as described above). This means that the ESV is lower, and consequently increases the SV (given that EDV is constant). Similarly, EDV increases during sympathetic stimulation of the veins by returning more blood to the heart, for example during physical activity. More blood returning to the heart leads to increased filling of the heart and provides increased EDV. Increased EDV provides increased SV.
The amount of blood the heart pumps per minute is called the cardiac output (CO). CO is the SV multiplied by the number of times the heart beats per minute, the heart rate (HR). We write:
CO = HR x SV
By sympathetic stimulation (as described above) the heart will thus increase both SV and HR, which increases the CO.
|Exercise: Complete the sentence|