NREMT Paramedic Practice

What determines the preload in the heart?

1. The amount of blood in the arterial system

2. The volume of blood returning to the ventricles

3. The resistance in the vascular system

4. The heart rate of the patient

The correct answer is: The volume of blood returning to the ventricles

Preload in the heart is determined primarily by the volume of blood returning to the ventricles, which significantly influences the stretch of the cardiac muscle fibers before contraction. This concept is tightly linked to the Frank-Starling law, which states that the greater the volume of blood returning to the heart during diastole, the greater the stretch of the ventricles and, consequently, the stronger the subsequent contraction will be. When there is an increased venous return, the ventricles fill to a greater extent, resulting in a higher preload. This is critical during physical activity or in conditions where the circulatory system needs to be more efficient in pumping blood. The other choices, while they relate to aspects of cardiac function, do not directly dictate preload. Blood volume in the arterial system pertains more to afterload and systemic vascular resistance rather than venous return. The resistance in the vascular system primarily affects afterload rather than preload, and while the heart rate can influence overall cardiac output and performance, it does not determine the preload level itself. Understanding this distinction is essential for managing various clinical scenarios where preload might be altered, such as in cases of hypovolemia or heart failure.