Q: Which of the following formulas correctly calculates cardiac output ($$CO$$)?

$$CO = \text{Stroke Volume} \times \text{Heart Rate}$$

Question 1

Which of the following best explains why cardiac muscle cells (cardiomyocytes) cannot undergo summation of force like skeletal muscle cells?

Accepted Answer

The long refractory period due to the plateau phase of the cardiac action potential prevents summation.

Question 2

During the plateau phase of the cardiac action potential, which ion is primarily responsible for maintaining depolarization, and through which channel does it enter?

Accepted Answer

Calcium through L-type (long-lasting) voltage-gated calcium channels

Question 3

Which of the following sequences correctly describes the order of electrical conduction in the heart?

Accepted Answer

SA node → AV node → Bundle of His → Purkinje fibers

Question 4

A patient’s ECG shows an absence of P waves for several beats, followed by a return to normal rhythm. What is the most likely explanation?

Accepted Answer

Sinus arrest (SA node stopped firing temporarily)

Question 5

Which of the following best describes the role of the sodium-calcium exchanger (NCX) in cardiac muscle relaxation?

Accepted Answer

It uses the sodium gradient to export calcium from the cell in exchange for sodium influx.

Question 6

Which of the following changes would you expect in the pacemaker potential of SA node cells under increased parasympathetic stimulation?

Accepted Answer

The membrane potential becomes hyperpolarized, and the slope of depolarization decreases.

Question 7

Which of the following is a unique feature of cardiac muscle contraction compared to skeletal muscle contraction?

Accepted Answer

Calcium influx from the extracellular space is required to trigger further calcium release from the sarcoplasmic reticulum.

Question 8

A patient presents with an ECG showing rapid, regular QRS complexes at about 120 beats per minute, with normal P and T waves. Which condition is most likely present?

Accepted Answer

Sinus tachycardia

Question 9

Which of the following best explains the Frank-Starling mechanism in the heart?

Accepted Answer

Increased end-diastolic volume stretches the ventricle, resulting in a stronger contraction and increased stroke volume.

Question 10

Which of the following formulas correctly calculates cardiac output ($$CO$$)?

Accepted Answer

$$CO = 	ext{Stroke Volume} 	imes 	ext{Heart Rate}$$

Question 11

Which of the following is true regarding the effect of sympathetic stimulation on the SA node?

Accepted Answer

It increases sodium and calcium permeability, making the pacemaker potential steeper and increasing heart rate.

Question 12

Which of the following best describes the function of intercalated discs in cardiac muscle?

Accepted Answer

They contain gap junctions that allow rapid electrical communication between cells.

Question 13

A patient’s ECG shows regular P waves, but some are not followed by QRS complexes. What is the most likely diagnosis?

Accepted Answer

AV conduction block

Question 14

Which of the following statements about the autorhythmic cells of the heart is correct?

Accepted Answer

They spontaneously depolarize due to funny channels opening at around $$-60 mV.$$

Question 15

Which of the following best explains why the left ventricle is thicker than the right ventricle?

Accepted Answer

It pumps blood to the entire systemic circulation, requiring greater force.

Question 16

Which of the following is the primary neurotransmitter responsible for slowing the heart rate via the parasympathetic nervous system?

Accepted Answer

Acetylcholine

Question 17

Which of the following best describes the effect of increased venous return on cardiac output?

Accepted Answer

It increases end-diastolic volume, increasing stroke volume and cardiac output.

Question 18

Which of the following is true regarding the role of calcium in cardiac muscle contraction?

Accepted Answer

Calcium binds to troponin, allowing cross-bridge formation between actin and myosin.

Question 19

Which of the following best describes the effect of acetylcholine on the SA node?

Accepted Answer

It increases potassium permeability, causing hyperpolarization and slowing heart rate.

Question 20

Which of the following best describes the QRS complex on an ECG?

Accepted Answer

Ventricular depolarization

Cardiac Muscle Physiology, Conduction System, and ECG Interpretation

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