LET'S GET PUMPED UP WITH KNOWLEDGE
Stroke Volume (SV, mL) is the amount of blood pumped out of the left ventricle and into the aorta every time the heart contracts (every heartbeat). The importance of stroke volume is that it is the next bout of blood supply, and in turn, the next bout of supply of Oxygen and nutrients for the cell.
It also indirectly determines the next bout of Oxygen available to be extracted into the cells, and the next bout of blood that will serve part of the buffering and disposal processes. Stroke volume has a very strong relationship with heart rate, as the two main determinants of cardiac output. Cardiac Output is the amount of blood pumped out of the left ventricle and into the aorta every minute.
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The amount of blood that can potentially be pumped out depends first and foremost on the amount of blood that filled up the ventricle to begin with. Venous Return is the amount of blood returning to the heart from the tissues and organs of the body. The greater venous return is, the greater stroke volume will end up being. According to the Frank Starling Law, the more blood that fills the ventricle, the greater ventricular contraction (force production), and the more blood is pushed out of the ventricle and into the aorta.
Fluids cannot be condensed, and thus, the more blood in the ventricle (greater ventricular filling, EDV, End Diastolic Volume) the greater the force they apply to the ventricle walls, causing them to stretch. As the walls of the ventricle stretch, the gain is two fold. 1) Tension is stored in the fascia and are released when most needed; 2) The length of the sarcomeres within the hearts fibers (myocardiocytes) increased producing more contraction force.
The greater the force applied to the blood within the ventricle, the more the blood is going to move, and the more blood is going to end up being pumped out (greater stroke volume). The amount of force produced by the heart is called cardiac contractile force; One of the greatest determinants of stroke volume. As in any muscle made up of muscle fibers and fascia, the total force produced is the result of the combined force production of both muscle fibers and fascia together. 90-95% of the total force produced and applied to the blood within the ventricle originates from the work of the sarcomeres, while the rest originates from fascia releasing their stored tension.
Better synchronization of the contraction of the ventricle's walls, assists with the efficiency of the contraction, as more blood is directed towards the aortic valve, and into the aorta, increasing stroke volume. This need to accurately direct blood to a certain direction and valve, is partially governed by the neural activation of the muscular walls of the ventricle. As neural impulse travels throughout the heart from atria to ventricles, it moves in a specific pattern that causes the heart to contract in a sack-like fashion, determining blood flow to the aortic valve.
As the aortic valve opens, buckling under the pressure from within the ventricle (represented by diastolic blood pressure; DBP, mmHg), the blood being pushed into the aorta by the ventricle is met with the resistance to flow generated by the aorta itself. Being the largest artery in the body, is presents the ventricle with great pushback and resistance to having blood flow into it. This is known as aortic resistance to flow, or arterial resistance to flow. In order to blood to actually flow into the aorta, the extent of force applied to the blood being pumped out must be greater than that of the resistance presented by the aorta.
This force required to overcome the resistance of the aorta is represented by systolic blood pressure (SBP, mmHg).As the ventricle overcomes aortic resistance, the amount of blood within the ventricle decreases, until no more blood can successfully be pumped out. Unfortunately, no human actually pumps out all of the blood that filled the ventricle, represented by end diastolic volume (EDV, mL), thus blood remains in the ventricle without being pumped out. The volume of blood that remains within the ventricle at the end of the mechanical cycle of the heart is called end systolic volume (ESV, mL).
Stroke volume is also the result of EDV and ESV, as it is calculated as one minus the other, as (EDV - ESV). In order for stroke volume to be maximal, EDV must be maximal, while ESV must be minimal. The fact that blood always remains within the ventricle means the EDV and ESV are never equal to each other. The lesser ESV is in percentage of EDV, the more effective and efficient the heart is as it relates to pumping blood, and the better off the person is.
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