The Frank-Starling Law: What It Is And What It Explains About The Heart
The heart, together with the brain and lungs, forms the triangle of physiological essentiality in living beings. This small organ (equivalent to 0.4% of an adult’s body weight) pumps about 70 milliliters of blood with each heartbeat, that is, approximately 5 liters of fluid per minute.
Taking into account that A human being has 4.5 to 6 liters of blood in his entire body we can affirm that the heart pumps practically all of this fluid in an interval of 60 seconds.
This work does not come for free: a heart can burn between 0.9 and 1.2 kilocalories per kilo of the individual’s weight per hour, which translates into about 400-600 calories per day. Much of our basal metabolism (energy necessary to live at rest) is explained by the action of this organ and the brain, since they are in continuous operation and represent a true resource consumption factory.
We could spend hours and hours collecting curious facts about the human heart, because really, it gives us the possibility of existing and largely defines us as a species. In any case, today we want to spin a little more finely, get into more complex and specific terms: stay with us if you want to know everything about the Frank-Starling law
First of all, we must establish a series of basal mechanisms regarding blood flow. The human heart is a hollow muscular organ with 4 chambers (2 atria and 2 ventricles) that are septate, that is, they are completely separated Making this distinction is essential, since other non-human vertebrates have hearts with partial or no septa, so a certain degree of mixing between oxygenated and deoxygenated blood occurs. In our species, this is not so.
The heart It pumps blood to all parts of the body, but there is a clear distinction between what carries oxygen after passing through the lungs (oxygenated) and what returns to them to collect O2 (deoxygenated) The Centers for Disease Control and Prevention (CDC) gives us a general idea of blood pumping in the following list:
This cycle only describes blood oxygenation and deoxygenation, since you should not forget that the blood passes through the liver, kidneys and other organs to purify itself and deposit substances Without a doubt, describing the circulatory system is a mammoth task worthy of several encyclopedia volumes.
How does the Frank-Starling law apply to everything described?
The Frank-Starling law It was described based on the names of 2 researchers specialized in physiology: Otto Frank and Ernest Henry Starling, both professionals in the field of 20th century anatomy. In any case, these were not the first to postulate and suspect certain of the correlations that we show you below.
Simply put, the Frank-Starling law states that The heart has an intrinsic capacity to respond to increasing volumes of blood flow Based on this premise, cardiac output (volume of blood ejected by the ventricle in one minute) is expected to increase or decrease in response to changes in heart rate and stroke volume.
Let’s take an example: when a person gets up from their seat, cardiac output decreases, since the decrease in central venous pressure (CVP) translates into a drop in stroke volume (remember, it is the volume of blood that the heart expels). towards the aorta or pulmonary artery in its 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 stroke volume of blood ejection We know that this terminology can seem quite confusing, but the formulas surely help you understand the law described here a little better.
Cardiac work (D): stroke volume (SV) x heart rate (HR)
We remember that cardiac work or output (D) refers to the amount of blood expelled by one ventricle of the heart in 60 seconds. On the other hand, stroke volume (SV) exemplifies the blood volume that the heart expels into the aorta or pulmonary artery. Lastly, heart rate (HR) is a parameter that reflects the number of heartbeats per unit of time.
If we take into account that (in a normal situation) A person has a stroke volume of 60 milliliters per beat at a heart rate of 75 beats per minute we obtain that the total cardiac work per minute is 4.5 liters, the figure that we have shown you when opening this space.
Based on this premise, the Frank-Sterling law explains that, as the heart is filled with a greater volume of blood, the force of contraction will increase significantly. In other words, if a person makes a muscular effort at a given moment, he will increase the volume of blood returned by the venous system, so the stroke volume (the force of contraction of the heart) will be greater. This way this complex mechanism is understood a little better; TRUE?
The law and anatomy of the heart
This theory is not only based mathematically, but must present a physiological explanation that justifies what is postulated. The Frank-Sterling law is based on the following premise: there is a relationship between the initial length of the myocardial fibers (formers of the cardiac muscle) and the force generated by the contraction of the heart.
The increase in blood flow in 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 group of fibers). With an increase in sarcomeric length, greater force generation is possible during contraction, so the heart is able to eject more blood into the arteries (stroke volume).
In general, all this can be summed up in an easy to understand idea: If the ventricular chamber is filled with more blood, the muscle fibers lengthen and become more tense, which promotes the release of a more drastic force to eject excess blood that has reached the heart through the veins into the arteries. Perhaps being reductionist, 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 capable of increasing stroke volume when more blood reaches it, in order to expel excess fluid into the chamber. Unfortunately, this does not have to apply to people with cardiovascular problems, so several clinical events can occur 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 in the curve, depending on the state of the heart and the nature of the afterload period. If something is clear to us after reading these lines, it is that the heart is a much more intricate organ than it might seem.
