A patient with a defective tricuspid valve undergoes valve replacement surgery. After surgery, the patient should be monitored for proper opening of the valve during which part of the cardiac cycle?

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. Myoglobin and lipid inclusions: Both skeletal and cardiac muscle fibers contain myoglobin, which stores oxygen for aerobic metabolism, and lipid inclusions as energy reserves. These features support high metabolic demands in both muscle types and are not unique to skeletal muscle.

B. A single nucleus per cell: Cardiac muscle cells are typically uninucleated, whereas skeletal muscle fibers are multinucleated. However, the presence of a single nucleus is characteristic of cardiac myocytes, not a distinguishing feature of skeletal muscle.

C. Triads formed by long T tubules and cisternae of the sarcoplasmic reticulum: Skeletal muscle fibers have well-organized triads, where a T tubule is flanked by two terminal cisternae of the sarcoplasmic reticulum, allowing rapid calcium release for synchronous contraction. In contrast, cardiac contractile cells have diads (one T tubule with one adjacent cisterna) and smaller T tubules, reflecting slower calcium handling.

D. Sarcomeres along myofibrils: Both skeletal and cardiac muscle fibers contain sarcomeres arranged along myofibrils, giving them striated appearance under microscopy. Sarcomeres are essential for contraction in both types of striated muscle and are not a distinguishing structural feature.