sentences:  The exchanger is usually found in the plasma membranes and the mitochondria and endoplasmic reticulum of excitable cells.The sodium–calcium exchanger is only one of the systems by which the cytoplasmic concentration of calcium ions in the cell is kept low.The exchanger does not bind very tightly to Ca2+ (has a low affinity), but it can transport the ions rapidly (has a high capacity), transporting up to five thousand Ca2+ ions per second.