PROBLEM #2 INTRODUCTION: The contractile performance of heart muscle is determin

Question

PROBLEM #2 INTRODUCTION: The contractile performance of heart muscle is determined by its inotropic state, preload and afterload. These factors also determine the stroke output of the heart. The inotropic state is influenced by a variety of factors, including the extracellular Ca2+ concentration.

PROBLEM #2: Predict the consequences (increase, decrease, no change) of the acute onset of hypocalcemia on the following:

            CARDIAC MUSCLE                                   

            1. velocity of shortening

            2. inotropic state (contractility)

            3. peak force developed

            HEART

            4. stroke volume

            5. end systolic volume

            6. end diastolic volume

Explanation / Answer

ANSWER:

Increase , Decrease or constant amount of calcium ions affects the cardiac muscle"s

1.velocity of shortening- calcium plays an important role in initiation of action potential and change in membrane potential.

2.Ionotropic state(contractility)-because the calcium acts as a coupling agent in muscle contraction

3.Peak force developed-force of contraction depends on the amount of calcium ions released.

Increase in calcium ions HYPERCALCEMIA causes constriction and spasticity of the myocardium,this is caused due to the increased cardiac contractile process.

It also creates tachycardia, due to increased conduction of cardiac impulses from atria to ventricles through the A-V bundle.

Decrease in calcium levels HYPOCALCEMIA causes cardiac flaccidity, dilation of the heart.

It also slows the heart rate causing bradycardia and also can block the conduction of impulses from the atria to the ventricles through the A-V bundle.

Such weakness of the heart and abnormal rhythm can cause death.Low calcium concentration decreases the resting membrane potential in the cardiac muscle fibers . As the membrane potential decreases , the intensity of the action potential also decreases, which makes contraction of the heart progressively weaker.

When an extra amount of blood flows in to the ventricles ,the cardiac muscle itself is stretched to greater length.This inturn causes the muscle to contract with increased force because the actin and myosin filaments are brought to a more nearly optimal degree of interdigitation for force generation..Therefore the ventricles because of its increased pumping ,automatically pumps the extra blood in to the arteries.

HEART:

4.STROKE VOLUME:

The stroke volume is the amount of blood pumped from the left ventricle for each beat to all parts of the body.The stroke volume is 70 ml.

When the heart contracts strongly,the end systolic volume can be decreased to as little as 10 to 20 milliliters.

Conversely when large amounts of blood flow in to the ventricles during diastole ,the ventricular end diastolic volumes can become as great as 150 to 180 milliliters in the healthy heart.

5.END SYSTOLIC VOLUME:

As the ventricles empty during the systole ,the volume decreases to about 70 ml ,which is called the stroke volume output.The remaining volume in each ventricle about 40-50 ml is called the END SYSTOLIC VOLUME.It is the amount of blood during the contracted phase that is present in the ventricle.

6.END DIASTOLIC VOLUME:

During the diastole the filling of the ventricles normally increases the volume of each ventricle to about 110 to 120 ml.This volume is known as END DIASTOLIC VOLUME,it is the amount of blood existing in the left ventricle before the beginning of the systolic phase.

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