Regulation of Stroke Volume
A healthy heart will pump out the blood that entered its chambers during previous diastole. The more blood that returns to the heart, results in more blood being ejected. At rest, SV is about 50-60% of the EDV because 40-50% of the blood remains in the ventricles after each contraction (ESV).
There are 3 factors that ensure the left and right ventricles pump equal volumes of blood:
1) Preload : the degree of stretch on the heart before it contracts
A greater preload (stretch) on cardiac muscle fibres prior to contraction increases their force of contraction. Ex: an elastic band – the longer it is stretched the greater the force of “snap-back”. Therefore, the more the heart fills with blood during diastole, the greater the force of contraction during systole. Frank-Starling Law of the Heart: preload is proportional to the EDV (the volume of blood that fills the ventricles at the end of diastole). Under normal conditions:
Increased EDV = Increased force of next contraction
2 Factors affect EDV:
The duration of ventricular diastole and venous return (the volume of blood returning to the heart). Less filling time means lower EDV. More venous return results in more ventricular filling and greater EDV. At heart rates around 160 beats/min, SV usually decreases due to shorted filling time. At such rapid heart rates, EDV is lowered and preload is lowered.
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