Amount of blood in heart:
**Core Concept**
The amount of blood in the heart at any given time refers to the cardiac preload, which is the volume of blood in the ventricles before contraction. This is a critical concept in cardiac physiology, as it directly affects cardiac output and overall cardiovascular function.
**Why the Correct Answer is Right**
The correct answer is **70-100 mL**. This is because the normal cardiac preload is approximately 70-100 mL, which is the volume of blood in the ventricles at the end of diastole, just before the next contraction. This volume is maintained by the autonomic nervous system, which regulates heart rate and contractility to ensure optimal cardiac output. The cardiac preload is also influenced by factors such as venous return, peripheral resistance, and the efficiency of the heart's pumping mechanism.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately reflect the normal cardiac preload. A preload of 0 mL would suggest that the ventricles are completely empty, which is not physiologically possible.
**Option B:** This option is incorrect because it is too high. A cardiac preload of 500 mL would suggest that the ventricles are significantly overfilled, which can lead to decreased cardiac output and even heart failure.
**Option C:** This option is incorrect because it is too low. A cardiac preload of 10 mL would suggest that the ventricles are severely underfilled, which can lead to decreased cardiac output and even shock.
**Clinical Pearl / High-Yield Fact**
A key concept to remember is that the cardiac preload is directly related to the Frank-Starling mechanism, which states that the stroke volume of the heart increases in response to an increase in the volume of blood in the ventricles. This is a critical concept in understanding cardiac physiology and pathophysiology.
**Correct Answer: C. 10 mL**