End diastolic voIume increases in:
**Core Concept**
The end-diastolic volume (EDV) is the amount of blood present in the ventricles at the end of diastole, just before the start of systole. It is a key component of the Frank-Starling mechanism, which describes the relationship between the preload (initial stretching of the cardiac myocytes prior to contraction) and the resulting stroke volume.
**Why the Correct Answer is Right**
The Frank-Starling mechanism states that the stroke volume of the heart increases in response to an increase in the end-diastolic volume, as long as the contractility of the heart remains constant. This is because an increase in EDV results in a greater stretch of the cardiac myocytes, which activates more contractile units and leads to a greater force of contraction. This is mediated by the troponin-tropomyosin complex, which is activated by the binding of calcium ions to troponin. As a result, the ventricles are able to pump more blood with each beat.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a condition that would increase EDV. While certain conditions such as heart failure or cardiomyopathy may increase EDV, this option is too vague to be a correct answer.
**Option B:** This option is incorrect because it does not provide a clear mechanism by which EDV would increase. While certain drugs such as beta blockers may increase EDV by reducing the force of contraction, this option is too vague to be a correct answer.
**Option C:** This option is incorrect because it does not provide a clear mechanism by which EDV would increase. While certain conditions such as anemia may increase cardiac output, this option is too vague to be a correct answer.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that the Frank-Starling mechanism is a critical component of the regulation of cardiac output. An understanding of this mechanism is essential for understanding the pathophysiology of conditions such as heart failure, where the contractility of the heart is impaired.
**Correct Answer: C. Anemia.**