What happens during repolarization of cardiac conducting cells?
The sodium ion channels open, allowing Na+ to enter the cell.
The calcium ion channels open, allowing Ca2+ to enter the cell.
Sodium ion channels close, preventing Na+ from entering.
Potassium ion channels open, allowing K+ to leave the cell.
The Correct Answer is D
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.
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Related Questions
Correct Answer is {"dropdown-group-1":"D"}
Explanation
A. Rate: The rate of contraction refers to the heart rate, which is determined by the frequency of action potentials generated by the sinoatrial node. While rate influences cardiac output, it does not describe the intrinsic strength of myocardial contraction.
B. Frequency: Frequency also relates to how often the heart beats over a period of time, essentially the number of cardiac cycles per minute. It does not reflect the force generated by the myocardial fibers during systole.
C. Speed: Speed could be interpreted as the velocity of contraction or conduction, but it does not measure the intrinsic strength of the ventricular contraction. Contractility is independent of how quickly the contraction occurs.
D. Force: Contractility refers to the inherent ability of cardiac muscle fibers to generate force during systole, independent of preload and afterload. It reflects the efficiency of actin-myosin cross-bridge formation and calcium handling within the myocardium, determining how strongly the heart can pump blood with each beat.
Correct Answer is D
Explanation
A. Blood is returned to the left side of the heart via the pulmonary veins.: The pulmonary veins are located posteriorly and return oxygenated blood from the lungs to the left atrium. From the left atrium, blood flows through the mitral (bicuspid) valve into the left ventricle during ventricular diastole. This describes normal anatomic blood flow into the left side of the heart.
B. The left ventricle contracts to push blood through the aortic semilunar valve.: The left ventricle forms the apex of the heart and has the thickest myocardial wall due to its role in systemic circulation. During left ventricular systole, it generates high pressure to open the aortic semilunar valve and eject blood into the ascending aorta.
C. The left ventricle fills after the blood passes through the bicuspid.: The bicuspid valve, also called the mitral valve, is located between the left atrium and left ventricle. During ventricular diastole, blood flows from the left atrium through this valve into the left ventricle. Proper opening of the mitral valve allows adequate ventricular preload before systole.
D. Blood in the left ventricle is deoxygenated.: The left ventricle receives oxygenated blood from the lungs via the left atrium and pulmonary veins. Its physiologic function is to pump oxygen-rich blood into the systemic circulation through the aorta. Deoxygenated blood is found on the right side of the heart, not within the left ventricular chamber.
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