Why is the concentration of calcium so high in the sarcoplasmic reticulum during rest?

Explanation:

The sarcoplasmic reticulum is a system found within muscle cells and involved in the storage of intracellular calcium, this function is crucial for the cells because calcium is used to trigger the muscle contraction through excitation and depolarization of the cell, shifting the membrane potential.

Since the presence of too much calcium in the cells leads to calcification (hardening of structures), the sarcoplasmic reticulum is also an important way to control the calcium ion levels in the cell absorbing it from the cell to go back to rest.

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