Heart failure is described as arising from a malfunction of which structure in cardiac muscle cells?

1. Heart failure does not mean the heart has stopped, but that it cannot pump with sufficient force. 2. Contractile force in a muscle cell depends on the supply of calcium ions. 3. Calcium ions are stored by the endoplasmic reticulum of the muscle cell. 4. Heart failure occurs when the endoplasmic reticula of cardiac muscle cells do not function properly. 5. The result is an insufficient number of calcium ions to trigger a sufficient contractile force. 6. Cristae serve respiration and gap junction pores carry the electrical signal, so neither is the structure named as failing. 7. The malfunctioning structure is the endoplasmic reticulum. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 4 "Cell Structure", section 4.4 The Endomembrane System and Proteins_