PROBLEM #3 INTRODUCTION: The contractile performance of heart muscle depends on

Question

PROBLEM #3 INTRODUCTION: The contractile performance of heart muscle depends on its inotropic state, preload and afterload. These factors also determine the stroke output of the heart. In vivo, the aortic pressure, the pressure against which the left ventricle must pump, is the main afterload. If the outflow resistance of the aortic valve should increase, however, it too can impose a significant afterload on the ventricle. Use the following websites to understand what preload and afterload are and how they affect the force of contraction in the heart.

http://www.cvphysiology.com/Cardiac%20Function/CF007.htm

http://www.cvphysiology.com/Cardiac%20Function/CF004.htm

http://www.cvphysiology.com/Cardiac%20Function/CF008.htm

PROBLEM #3: Predict the consequences (increase, decrease, no change) of an increase in afterload on heart muscle and the effect of a sudden stenosis of the aortic valve (reduction in the cross-sectional area of the valve) on the heart. Explain your predictions.

            CARDIAC MUSCLE                                   

            1. velocity of shortening

            2. shortening

            HEART

            3. end systolic volume

            4. end diastolic volume

            5. stroke volume

Explanation / Answer

Answer 1and 2--An increase in afterload means there is increase of resistance against which the left ventricle has to contract to pump the blood in aorta.Thus if afterload increases there is lengthening of ventricular muscles and more tension will be created in the muscle and left ventricular muscle contract rapidly so there is increase in velocity of shortening of ventricular muscle and decrease in shortening.

Answer 3,4and 5---Stenosis of aortic valve causes increase pressure against which left ventricle has to pump blood thus increase in afterload. Thus there will be increase in end systolic volume because less blood will be pumped against more pressure. There will be decrease in end diastolic volume because of less empty of left ventricle and there will be decrease in stroke volume because of increase in afterload due to aortic stenosis.

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