extracellular calcium and contractility of the heart

[Akam ahz]

Hello,
I want to understand the relation between extracellular Ca ion and the contractility of the heart.

My book states that increasing extracellular Ca ion increases the forces of contractility of the heart, and vice versa, but at high concentration the heart stops in systole, and when removing the Ca ion from the extracellular fluid the heart stops in diastole."

I don't understand the mechanism by which the calcium ion works to increase the force of contractility.

Also, I don't understand the part of stopping heart in systole if the concentration of calcium is high, and stopping the heart in diastole if the concentration of calcium is low.

Could anyone explain?

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[PhilzCoffeeAddict]

Okay I'll take a stab at it.

Physiology of cardiac conduction and contractility | McMaster Pathophysiology Review
^ Take a look at the cardiac action potential

First, you know that calcium helps depolarize cardiac cells into an action potential correct? Na+ starts the initial depolarization, Ca2+ then finishes the rest of it. Therefore, intracellular Ca2+ is responsible for the contractility/action potential of the heart. If you have a lot of extracellular calcium, this should put the heart in a prolonged QT contraction state. Once the calcium is removed, the heart is filled but doesn't contract which is why it stops in diastole.

I hope this helps.