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16 Cards in this Set

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  • Back
What is the difference between isometric and isotonic contraction?
A muscle undergoes isometric contraction when it maintains a fixed length during the course of contraction.
In isotonic, the muscle shortens as it tightens, maintaining the same tension during the course of movement.(biceps muscle alone).
Do either of these contraction concepts pertain to the heart?
part of cardiac contraction is isometric, and part is isotonic. In diastole, the increased volume of the ventricles expands the cardiac wall.
At the outset of systole there is an initial isometric contraction against this increased volume.
What is preload?
It is the increase in tesion in ventricular cardiac muscle fibers when the ventricle expands in diastole.
What is afterload?
It is the increase in tension in the ventricular cardiac muscle fibers when they actively contract against the expanded ventricular volume to expel the blood through the aortic valve.
What is the laPlace equation?
The Laplace equation is written as:
T=Pr, where in the case of the heart, T= tension of cardiac muscle, P= intraventricular pressure, and r= radius of the ventricle.
Elaborate via an example regarding the LaPlace equation.
Imagine that toward the end of ventricular diastole, the intraventricular pressure is P, the muscle tension is T, and the ventricular radius is r.
Imagine that aortic pressure is 25P. In order for the ventricle to expel the blood into the aorta, it must generate a pressure of 25P.
Since 25T=25Pr, the tension in the ventricular musculature must increase 25 fold to 25T to expel the blood.
Illustrate a clinical picture of laplace principle.
Imagine a patient has an abnormally increased systemic blood volume and the ventricular radius is 2r at the end of diastole, but the aortic pressure is still 25P. The equation is now 50T=25P(2r). In order for the ventricle to expel the blood against the aortic pressure of 25P, it has to generate a tension of 50T, a significant increase.Increasing the radius of the ventricle(commonly through increasing systemic blood volume) will increase not only the preload, but also the afterload and the energy demand on the heart.
Most myocardial O2 consumption occurs during the isometric phase of cardiac contraction. TRUE/FALSE
TRUE. This is expected since the ventricular radius decreases as contraction proceeds; the smaller radius, according to the LaPlace equation, decreases the tension that the heart muscle has to generate to expel the blood against the aortic pressure.
One might conclude, in T=Pr, an increase in P increases the afterload, and an increase in r increases the preload. An increase in either will increase T, thereby increasing O2 demand. TRUE/FALSE
Are increased stroke volumes always beneficial?
Increased blood pressure increases afterload and decreases cardiac stroke volume because the ventricle encounters resistance in contracting against an increased systemic blood pressure.
Increased blood volume increases preload and increases cardiac stroke volume due to increased cardiac filling. One can now see that increased stroke volume can contribute to hypertension in addition to increased O2 demands.
Diuretics which decrease blood volume, may be useful in congestive heart failure by decreasing preload.Drugs that reduce systemic blood pressure reduce afterload. TRUE/FALSE
How can digitalis make the heart beat with more efficiency?
It is more energy efficient for a heart to beat at a relatively low rate with higher stroke volume than for the heart to exhibit the same cardiac output, but with high rate and low stroke volume.
Digitalis increases stroke volume and decreases heart rate, promotes more effecient cardiac O2 utilization.
What is meant by positive inotrope, chronotrope and dromotrope?
Positive inotrope is a drug that increases the strength of the hearts contraction.
Chronotropes will increase heart rate.
Dromotropes will increase conduction velocity.
What is the cardiac index?
Normal cardiac output varies with an individuals weight.
This index measures cardiac output per square meter of body surface area.
What is ejection fraction?
This equals stroke volume/end diastolic volume. It is a truer reflection of cardiac function. In failing hearts, the EDV may increase as the SV decreases. The ejection fraction is thus low, indicating poor cardiac pumping action. Imaging techniques are usually employed for these assessments.
List three measuring techniques used for assessing cardiac function.
1. cardiac angiography
2. echocardiography
3. radionucleotide ventriculography.