Select the correct statement about cardiac output.
Group of answer choices:
Stroke volume increases if end diastolic volume decreases.
If a semilunar valve were partially obstructed, the end systolic volume in the affected ventricle would be decreased.
Decreased venous return will result in increased end diastolic volume.
A slow heart rate increases end diastolic volume, stroke volume, and force of contraction.
The correct answer and explanation is :
The correct statement is:
A slow heart rate increases end diastolic volume, stroke volume, and force of contraction.
Explanation:
Cardiac output (CO) is the volume of blood pumped by the heart per minute and is determined by two primary factors: stroke volume (SV) and heart rate (HR). Stroke volume is the amount of blood ejected by the left ventricle during each heartbeat, and it is influenced by three key factors: preload, contractility, and afterload.
Let’s break down why the statement is correct:
- End Diastolic Volume (EDV): EDV refers to the volume of blood in the ventricles at the end of diastole (just before the heart contracts). A slow heart rate gives the heart more time to fill between beats, allowing more blood to enter the ventricles, which increases EDV.
- Stroke Volume (SV): According to the Frank-Starling Law of the heart, stroke volume is influenced by the volume of blood in the ventricles just before contraction (EDV). When EDV increases, the ventricles stretch more, which leads to a stronger contraction and higher stroke volume. Therefore, a slow heart rate increases stroke volume.
- Force of Contraction: The increased EDV from a slower heart rate results in increased myocardial stretch. The heart muscle fibers stretch to a point where their contractile force is maximized, leading to a stronger contraction and higher force of ejection.
Analysis of Other Options:
- Stroke volume increases if end diastolic volume decreases: This is incorrect because a decrease in EDV typically leads to a lower stroke volume. The Frank-Starling mechanism states that less blood entering the heart leads to a weaker contraction.
- If a semilunar valve were partially obstructed, the end systolic volume in the affected ventricle would be decreased: This is incorrect. A partial obstruction in the semilunar valve (which regulates blood flow from the ventricles to the arteries) would likely lead to an increase in end systolic volume (ESV), as less blood is ejected into the arteries, causing the ventricle to retain more blood after contraction.
- Decreased venous return will result in increased end diastolic volume: This is incorrect. Decreased venous return (the amount of blood returning to the heart) results in a reduced EDV, as less blood fills the ventricles.
In summary, a slower heart rate allows more time for ventricular filling, which increases EDV, stroke volume, and the force of contraction, contributing to an increase in cardiac output.