An echocardiogram records an EDV of 150 mL and an ESV of 90 mL. What is the stroke volume?

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  Based on normal values, is this patient's stroke volume within the expected range?

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A. It binds and stores oxygen for aerobic metabolism: Myoglobin is an oxygen-binding protein located within the cytoplasm of cardiac and skeletal muscle cells. It serves as an intracellular oxygen reservoir, facilitating rapid oxygen delivery to mitochondria during periods of high metabolic demand. This supports sustained aerobic metabolism and continuous ATP production necessary for cardiac contraction.

B. It transports glucose into heart cells: Glucose transport into cardiomyocytes is mediated by glucose transporters (GLUT1 and GLUT4), not by myoglobin. Myoglobin’s role is specifically related to oxygen handling, not nutrient transport.

C. It generates electrical impulses for contraction: Electrical impulses in the heart are generated by pacemaker cells in the sinoatrial node and conducted through the cardiac conduction system. Myoglobin has no role in depolarization or action potential propagation.

D. It breaks down fatty acids into ATP: Fatty acid oxidation occurs in mitochondria through beta-oxidation, producing ATP. Myoglobin does not catalyze this process; its primary function is oxygen storage and delivery to support mitochondrial metabolism.

A. The sodium ion channels open, allowing Na+ to enter the cell: Opening of sodium channels occurs during the depolarization phase of cardiac action potentials, not repolarization. The influx of Na+ rapidly raises the membrane potential, initiating the action potential and triggering subsequent calcium influx for contraction.

B. The calcium ion channels open, allowing Ca2+ to enter the cell: Calcium channels open primarily during the plateau phase (phase 2) of the cardiac action potential, allowing Ca2+ entry to sustain contraction. This occurs before repolarization and contributes to excitation-contraction coupling rather than the return to resting membrane potential.

C. Sodium ion channels close, preventing Na+ from entering: Sodium channel closure occurs immediately after depolarization and contributes to the refractory period, but it does not itself drive repolarization. The cell requires potassium efflux to restore the resting membrane potential.

D. Potassium ion channels open, allowing K+ to leave the cell: During repolarization (phase 3) of cardiac conducting cells, voltage-gated potassium channels open, permitting K+ to exit the cytoplasm. The outward movement of positively charged K+ restores the negative resting membrane potential, terminating the action potential and preparing the cell for the next depolarization.