What are the biological functions of sarcoplasmic reticulum?

The primary function of sarcoplasmic reticulum (SR) is regulating Ca2+ concentrations during and after muscle contractions. SR is the storehouse of calcium, which is essential for muscle contractions. Special proteins embedded in the membrane of the sarcoplasmic reticulum allow Ca2+ to move back and forth from inside the cell to the SR, keeping the concentration of calcium ions inside and outside the cell constant.