Question

The force of contraction of the cardiac muscle is affected by:

1) the length of the muscle fiber at the beginning of contraction
2) the contractility of the heart
3) the end diastolic volume
4) the amount of blood entering the heart
5) all of these

Answer 1

The force of cardiac muscle contraction is affected by the length of muscle fiber at contraction onset, heart contractility, end diastolic volume, and the amount of blood entering the heart, according to the Frank-Starling mechanism and the concept of contractility.

Step-by-step explanation:

The force of contraction of cardiac muscle is influenced by several factors:

1. The length of the muscle fiber at the beginning of contraction: This is related to the Frank-Starling mechanism, where the force of heart contraction is directly proportional to the initial length of the muscle fiber.
2. The contractility of the heart: This refers to the inherent forcefulness of the heart's contraction, which controls the stroke volume (SV) and is impacted by positive and negative inotropic factors.
3. End diastolic volume (EDV) or preload: An increase in EDV leads to a greater stretch on ventricular muscle, allowing sarcomeres to reach their optimal length for a more forceful contraction.
4. The amount of blood entering the heart: This also affects EDV, and consequently, the strength of the subsequent contraction.

In summary, all these factors — muscle fiber length, heart contractility, EDV, and blood volume — collectively influence the force of cardiac contraction.