Select the correct statement about cardiac output. A. A slow heart rate reduces stroke volume and force of contraction. B. Decreased venous return will result in increased stroke volume. C. If a semilunar valve were partially obstructed, the stroke volume in the affected ventricle would be decreased. D. Stroke volume increases if the filling decreases. E. a and c
The Correct Answer and Explanation is:
The correct answer is:
C. If a semilunar valve were partially obstructed, the stroke volume in the affected ventricle would be decreased.
Detailed Explanation (300+ words):
Cardiac output (CO) refers to the volume of blood the heart pumps per minute. It is calculated as:
CO = Stroke Volume (SV) × Heart Rate (HR)
- Stroke volume is the amount of blood ejected by a ventricle with each heartbeat.
- Heart rate is the number of beats per minute.
Many factors influence stroke volume, including venous return (preload), myocardial contractility, and afterload (resistance against which the heart must pump).
Analysis of Each Option:
A. A slow heart rate reduces stroke volume and force of contraction.
❌ Incorrect. A slow heart rate typically allows more time for ventricular filling, which can increase stroke volume, not decrease it. This is especially true under normal conditions, due to the Frank-Starling mechanism (greater filling stretches the heart muscle more, leading to a stronger contraction). Unless the bradycardia is extreme and compromises cardiac output, this statement is inaccurate.
B. Decreased venous return will result in increased stroke volume.
❌ Incorrect. Decreased venous return reduces ventricular filling (preload), which decreases stroke volume, not increases it.
C. If a semilunar valve were partially obstructed, the stroke volume in the affected ventricle would be decreased.
✅ Correct. Semilunar valves (the aortic and pulmonary valves) open during ventricular systole to allow blood to exit the heart. If one of these valves is obstructed (due to stenosis, for instance), the ventricle faces greater resistance. This increased afterload reduces the amount of blood ejected, thereby decreasing stroke volume.
D. Stroke volume increases if the filling decreases.
❌ Incorrect. Stroke volume depends on ventricular filling. According to the Frank-Starling law, increased filling leads to increased contraction and higher stroke volume. Therefore, decreased filling leads to lower stroke volume.
E. a and c
❌ Incorrect. Statement A is incorrect, making E also incorrect.
Conclusion:
The only correct and physiologically accurate statement is C, which explains how an obstruction in a semilunar valve increases afterload and reduces stroke volume from the affected ventricle.