Frank Starling Mechanism of the Heart
The frank starling mechanism, or the frank starling law of the heart is the relationship between stroke volume (SV) and end-diastolic volume (EDV). The frank starling mechanism can be illustrated as a curve, where you can see how an increase or decrease in the curve will cause a shift in both SV and EDV. However, before we get into that, let’s define some terms here.
What is stroke volume? Stroke volume is the amount of blood pumped out of either ventricle of the heart (left or right) per beat. End-diastolic volume is the amount of blood in either ventricle at the height of diastole (relaxation). So, EDV is the measurement of the amount of blood in either ventricle just before systole (contraction).
The relevance of the frank starling mechanism as it relates to the relationship between SV and EDV is that the more blood the ventricles can hold, the greater your SV will be. The frank starling mechanism is a great tool that cardiologists, exercise physiologists, and other cardiac rehab professionals can use to assess and analyze the status quo of their patient’s heart. What the frank starling mechanism claims is that as EDV increases, so will SV. Also, as EDV decreases, SV will decrease as well.
The frank starling mechanism effect
If you have a healthy heart, then you can expect the ventricles in your heart to fill up with blood efficiently and effectively. So much so that the pressure put upon your ventricles due to the blood filling it up will cause a more forceful contraction. Thus, improving your SV. Let me break down how this works.
You will see an increase in the frank starling mechanism curve if there is an increase in EDV. An increase in EDV (blood filling the ventricles) will cause an increase in preload. This will then cause an increase in myocardial stretch. This stretch in the heart muscle causes an increase in myocardial contraction. This, inevitably causes an increase in SV.
So what do these terms mean? Well, preload is the pressure put upon the ventricles during diastole. So, as EDV increases due to a surge of blood in the ventricles, the pressure (or preload) in the ventricles will also increase. Picture filling a balloon up with water. This is a basic way to think of this concept. As you fill the balloon with more water, you increase the pressure the balloon is under as it stretches. The same can be said for the ventricles as they fill up with blood.
Myocardial stretch is merely a stretching of the heart muscle tissue in the ventricle when it fills with blood. This, in turn, increases the myocardial contraction in the ventricles due to the myosin heads of the heart’s muscle tissue being in perfect alignment. Thus, encouraging a very forceful contraction. It is with this forceful contraction of the left ventricle where we get our SV.
An increase or decrease in the frank starling mechanism effect can be due to many different factors, such as whether or not the individual is engaging in exercise, whether they have cardiovascular disease, are on specific medications, etc.
A decrease in the frank starling mechanism curve would be due to a decrease in EDV (less blood in the ventricles). This would then cause a decrease in preload, which would cause myocardial stretch and myocardial contraction to decrease as well due to a lower amount of pressure in the ventricles. Thus, limiting the “recoil affect” of the ventricles. This would result in a decrease in SV and ultimately less blood in systemic circulation.
How myocardial thickness affects the frank starling mechanism curve
Physiological Hypertrophy and the frank starling mechanism
This is when there is a thickening of the ventricles and atria of the heart. This thickening occurs in the myocardium. This increased size can be due to resistance exercise, pregnancy, or other reasons. This physiological hypertrophy will increase the frank starling mechanism curve by allowing optimal EDV. And as you now know, optimal or increased EDV leads to an increase in SV.
Pathological Hypertrophy and the frank starling mechanism
This is when there is an over-thickening of the myocardium. This over-thickening “shrinks” the size of the ventricles. Smaller ventricles mean that less blood will be able to occupy them during diastole. Pathological hypertrophy can occur possibly due to cardiac dysfunction. The frank starling mechanism curve will experience a dip, as less blood in the ventricles means less EDV, which will decrease SV.
Cardiac Dilation and the frank starling mechanism
This is when there is a significant decrease in the size of the myocardium. It can be caused by a myocardial infarction. This decrease in girth causes the size of the ventricles to increase greatly. There is a significant decrease in EDV due to a lack of myocardial stretch. This lack of myocardial stretch results in a decrease in myocardial contraction. Thus, decreasing SV. So, cardiac dilation would also cause the frank starling mechanism curve to dip as well.