Preload And Afterload

Cardiac muscle contractility

1) Preload: the load that is given to the muscle prior to its contraction. 2) Afterload: the load that is given to the muscle after the beginning of the contraction.

1) Effects of preload

Frank-starling law:

Energy of heart contraction is proportional to the initial length of cardiac muscle fiber.

The Frank-Starling law means that the greater the heart muscle is stretched during filling, the greater the force of contraction and the greater the quantity of blood pumped into the aorta.

What is the explanation of the FrankStarling mechanism? When an extra amount of blood flows into the ventricles, the cardiac muscle itself is stretched to greater length. The ventricle, because of its increase pumping, automatically pumps the extra blood into the arteries.

Under most conditions the amount of blood pumped by the heart each minute is determined almost entirely by the rate of blood flow into the heart from the veins, which is called venous return.

That is, each peripheral tissue of the body controls its own local blood flow, and the total of all the local blood through all the peripheral tissues flowing by way of the veins to the right atrium. The heart in turn automatically pumps this incoming blood into the systemic arteries, so that it can flow around the circuit again. Therefore, preload is proportional to the venous return.

Importance of Heterometric Autoregulation 1. It is responsible for the precise, beat-to-beat balance between the outputs of the right and left ventricles. 2. It thus insures that the total blood volume remains distributed in the proper way between the systemic circulation and pulmonary circulation. 3. Without this precise balance, blood would pool in the lungs causing pulmonary edema.  Just remember: "What goes in must come out."

2) Effects of afterload

Afterload can be viewed as the "load" that the heart must eject blood against 　 In simple terms, the afterload is closely related to the aortic pressure.

Briefly, an increase in afterload decreases the velocity of fiber shortening, resulting a decrease in SV.

With the rising of blood pressure, the afterloads become to increase, which make the heart to beat with more energy and more difficulty. That is to say, cardiac output is inverse proportion to blood pressure.

BP ↑ → Afterload↑→The ventricle will have to generate increased pressure→ decreases the velocity of cardiac fibers shortening→The ejection velocity after the valve opens will be reduced→less blood will be ejected→SV↓ →ESV↑+venous return→EDV↑→Adjust By Frank-Starling law.

SUMMARY  Preload of the ventricles is provided by venous pressure (VP)  Afterload is provided by arterial blood pressure.  SV (or SW) is proportional to the initial fiber length ( Starling’s law).  The mechanism underlying Starling’s law is the degree of overlap of muscle filaments.  Operation of Starling’s law matches CO from both sides of the heart.

Cardiac Output

Ask Yourself: 1.

Were the ranking of your reclining, sitting, standing, and active cardiac output measurements the same as your predictions? Explain. 2. What factors influence your cardiac output? 3. Compare your resting and active cardiac output. What is your cardiac reserve? See background notes.

4. Compare your cardiac output measurements with four other classmates . Are they the same-why or why not? Explain:   5. Compare the cardiac output measurements of males and females. Can you make a statement based on your observations about gender and cardiac output?    

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6. Cardiac functions such as cardiac output are regularly recorded for the astronauts during their space flight. Is it important to have each astronaut's "earth normal" or pre-flight measurements? Why?