Frank-Starling’s Law: what is it and what does it explain about the heart?

Let's see what Frank-Starling's Law is, and what it tells us about the heart's Blood pumping.

The heart, together with the brain and the lungs, forms the triangle of physiological essentiality in living beings. This small organ (equivalent to 0.4% of the body weight of an adult person) pumps about 70 milliliters of blood with each heartbeat, or approximately 5 liters of fluid per minute.

Bearing in mind that a human being has 4.5 to 6 liters of blood in his whole body, we can say that the heart pumps about 70 milliliters of blood with each heartbeat.The heart pumps practically all of this fluid in a 60-second interval.

This work does not come for free: a heart can burn between 0.9 and 1.2 kilocalories per kilogram of the individual's weight per hour, which translates into 400-600 calories per day. A large part of our basal metabolism (energy needed to live at rest) is explained by the action of this organ and the brain, since they are in continuous operation and represent a real factory of resource consumption.

We could spend hours and hours compiling curious facts about the human heart, since it really gives us the possibility to exist and defines us to a large extent as a species. Today, however, we want to go into a little more detail, into more complex and specific terms: stay with us if you want to know all about the Frank-Starling law.

The functioning of the heart

First of all, we have to establish a number of basal mechanisms as far as blood flow is concerned. The human heart is a hollow muscular organ with 4 chambers (2 atria and 2 ventricles) that are septate, i.e. completely separated from each other. Making this distinction is essential, because other non-human vertebrates have hearts with or without partial septa, so that there is a certain degree of mixing between oxygenated and deoxygenated blood. In our species, this is not the case.

The heart pumps blood to all parts of pumps blood to all parts of the body, but there is a clear distinction between blood that carries oxygen after passing through the lungs (oxygenated) and blood that returns to the lungs to pick up O2 (deoxygenated).. The Centers for Disease Control and Prevention (CDC) gives us a general idea of blood pumping in the following list:

• The superior vena cava (SVC) and inferior vena cava (IVC) are the two main conduits that allow deoxygenated blood to return to the heart.
• This deoxygenated blood enters the heart through the right atrium (RA), which then communicates the blood to the right ventricle (RV).
• The right ventricle pumps the blood to the pulmonary arteries, which branch to small capillaries located in the alveoli of the lung.
• Human respiration makes it possible, at this point, to exchange the carbon dioxide in the blood for oxygen at the capillary level.
• In summary, blood returns to the heart through the left atrium (LA), flows into the left ventricle (LV), and the LV pumps the blood into the aorta, which distributes the oxygenated blood throughout the body.

This cycle describes only the oxygenation and deoxygenation of the blood, because you should not forget that the blood passes through the liver, kidneys and other organs to purify itself and deposit substances.. Undoubtedly, describing the circulatory system is a mammoth task worthy of several encyclopedia volumes.

How does the Frank-Starling law apply to all of the above?

The Frank-Starling law was described from the names of 2 researchers specialized in physiology: Otto Frank and Ernest Henry Starlingboth professionals in the field of anatomy in the 20th century. However, they were not the first to postulate and suspect certain of the correlations we show you below.

Simply stated, the Frank-Starling law states that the heart possesses an intrinsic ability to respond to increasing volumes of blood flow. Based on this premise, cardiac output (volume of blood expelled by the ventricle in one minute) is expected to increase or decrease in response to changes in heart rate and stroke volume.

To give an example: when a person gets up from his or her seat, cardiac output decreases, since the decrease in central venous pressure (CVP) translates into a decrease in systolic volume (remember, this is the volume of blood that the heart expels into the aorta or pulmonary artery during contraction).

In summary, central venous pressure is important in this case, since it defines the filling pressure of the right ventricle and, therefore, directly determines the systolic volume of blood ejection.. We know that this terminology may seem rather confusing, but the formulas will certainly help you to understand the law described here a little better.

The basics of the Frank-Sterling law

Cardiac work (D): systolic volume (SV) x heart rate (HR)

We recall that cardiac work or cardiac output (D) refers to the amount of blood expelled from a ventricle of the heart in 60 seconds. On the other hand, systolic volume (SV) exemplifies the volume of blood expelled by the heart into the aorta or pulmonary artery. Finally, heart rate (HR) is a parameter that reflects the number of beats per unit of time.

If we take into account that (in a normal situation) a person presents a systolic volume of 60 milliliters per beat at a heart rate of 75 beats per minutewe obtain that the total cardiac work per minute is 4.5 liters, the figure we have shown you at the beginning of this space.

Based on this premise, the Frank-Sterling law explains that, as the heart is filled with more blood volume, the force of contraction will increase significantly. In other words, if a person makes a muscular effort at a given moment, the volume of blood returned through the venous system will increase, so the systolic volume (the force of contraction of the heart) will be greater. This way, this complex mechanism is understood a little better; isn't it?

The law and the anatomy of the heart

This theory is not only founded mathematically, but has to present a physiological explanation that justifies the postulated. The Frank-Sterling law is based on the following premise: there is a relationship between the initial length of the myocardial fibers (which form the cardiac muscle) and the force generated by the contraction of the heart.

The increase in blood flow in the venous return translates into greater filling of the ventricle, since this is responsible for collecting blood in the heart. This promotes the stretching of the myocardial fibers of the organ, which results in an increase in the length of the sarcomeres (muscle units resulting from the fiber bundle). With an increase in sarcomeric length, a greater generation of force during contraction is made possible, so the heart is able to eject more blood into the arteries (systolic volume).

In general, all this can be summarized in an easy-to-understand idea: if more blood fills the ventricular chamber, the muscle fibers lengthen and tighten more, which promotes the release of a more drastic force to eject the excess blood that has reached the heart through the veins into the arteries. Perhaps erring on the side of reductionism, it could be summarized as a "rubber effect": the more something is stretched by external pressure, the greater the force with which it returns to its natural shape.

Summary

In summary, the normal ventricle of a human being with a "healthy" heart is able to increase systolic volume when more blood reaches it, in order to expel excess fluid in the chamber. Unfortunately, this does not necessarily apply to people with cardiovascular problems, so various clinical events can be generated in response to "non-compliance" with this law.

In any case, it should be noted that there is no Frank-Sterling "curve" (which can be generated from what is presented) applicable in each and every case. The ventricle adopts different shapes on the curve, depending on the state of the heart and the nature of the afterload period. If anything is clear to us after going through these lines, it is that the heart is a much more intricate organ than it might appear.

Bibliographic references:

• How does the heart work? Centers for Disease Control and Prevention (CDC). Recogido a 11 de marzo en https://www.cdc.gov/ncbddd/spanish/heartdefects/howtheheartworks.html#:~:text=El%20flujo%20de%20sangre%20a%20trav%C3%A9s%20del%20coraz%C3%B3n&text=La%20sangre%20suministra%20ox%C3%ADgeno%20y,sangre%20se%20convierte%20en%20desoxigenada.
• Frank-Sterling Mechanism. Cardiovascular Physiology Concepts. Recogido a 11 de marzo en https://www.cvphysiology.com/Cardiac%20Function/CF003
• Saks, V., Dzeja, P., Schlattner, U., Vendelin, M., Terzic, A., & Wallimann, T. (2006). Cardiac system bioenergetics: metabolic basis of the Frank‐Starling law. The Journal of physiology, 571(2), 253-273.
• Sequeira, V., & van der Velden, J. (2015). Historical perspective on heart function: the Frank–Starling Law. Biophysical reviews, 7(4), 421-447.
• Solaro, R. J. (2007). Mechanisms of the Frank-Starling law of the heart: the beat goes on. Biophysical journal, 93(12), 4095.