At the beginning of the cardiac cycle, which of the following occurs?

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.

Correct answer: F.

A. Trabeculae carneae: These are irregular, muscular ridges lining the inner walls of the ventricles. They prevent suction during contraction, aid in ventricular contraction efficiency, and contribute to overall cardiac structural integrity.

B. Pulmonary semilunar valve: This valve is located between the right ventricle and pulmonary artery. It prevents backflow of blood into the ventricle during diastole and ensures unidirectional pulmonary circulation toward the lungs.

C. Papillary muscles: Papillary muscles are conical projections of ventricular myocardium that anchor chordae tendineae. During ventricular contraction, they contract to prevent inversion or prolapse of atrioventricular valves, ensuring proper unidirectional blood flow.

D. Pectinate muscles: Pectinate muscles are comb-like muscular ridges in the atrial walls, particularly prominent in the right atrium. They enhance atrial contraction, increasing blood flow into the ventricles efficiently during systole.

E. Chordae tendineae: These are thin, fibrous cords connecting atrioventricular valve leaflets to papillary muscles. They prevent valve prolapse during ventricular contraction, maintaining proper closure and unidirectional blood flow from atria to ventricles.

F. Right atrium: It is located on the superior right side of the heart and receives deoxygenated blood from the superior vena cava, inferior vena cava, and coronary sinus. It forms the right border of the heart. Physiologically, the right atrium functions as a receiving chamber that collects systemic venous blood and delivers it through the tricuspid valve into the right ventricle during atrial contraction.

G. Bicuspid valve: Also called the mitral valve, it is located between the left atrium and left ventricle. It prevents backflow into the atrium during ventricular contraction, ensuring efficient systemic circulation.

H. Fossa ovalis: This is a depression in the interatrial septum, the remnant of the fetal foramen ovale. It allowed blood to bypass the fetal lungs and normally closes after birth.

I. Left ventricle: The left ventricle pumps oxygenated blood into the aorta under high pressure. Its thick muscular wall enables strong contractions necessary to sustain systemic circulation throughout the body.

J. Interventricular septum: This thick muscular wall separates the left and right ventricles. It prevents mixing of oxygenated and deoxygenated blood and contributes to the contractile force of ventricular systole.