Figure 18-12 A, Diagram depicting the components of cardiac excitation-contraction coupling. Calcium pools are noted in bold letters. B, Extracellular (arrows A, B1, B2) and intracellular calcium flux (arrows, C, D, E, F, G) is shown. The thickness of the arrows indicates the magnitude of the calcium flux, the vertical orientations describe their energetics: downward-pointing arrows represent passive calcium flux, and upward pointing arrows represent energy-dependent calcium transport. Calcium entering the cell from extracellular fluid through L-type calcium channels triggers calcium release from the sarcoplasmic reticulum (SR). Only a small portion directly activates the contractile proteins (A1). Arrow B1 depicts active transport of calcium into the extracellular fluid by means of the plasma membrane pump ATPase and the Na⁺ -Ca²⁺ exchanger. Sodium that enters the cell in exchange for calcium (dashed line) is pumped out of the cytosol by the sodium pump. SR regulates calcium efflux from the subsarcolemmal cisternae (arrow C) and calcium uptake into the sarcotubular network (arrow D). Arrow G represents calcium that diffuses within the SR. Calcium binding to (arrow E) and dissociation from (arrow F) high-affinity calcium binding sites of troponin C activate and inhibit interactions of the contractile proteins. Arrow H depicts calcium movement into and out of mitochondria to buffer the cytosolic calcium concentration. (From Katz AM: Calcium fluxes. In Physiology of the Heart, 3rd ed. Philadelphia, Lippincott-Raven, 2001, pp 232–233.)