Anatomy and Physiology II Practice Test 2025 - Free Anatomy Practice Questions and Study Guide.

The strength of heart contraction is significantly influenced by heart rate and blood volume.

Heart rate is important because it determines how often the heart contracts, and an increase in heart rate can lead to more forceful contractions through a mechanism known as the Frank-Starling law of the heart. Essentially, this law states that the force of contraction is dependent on the initial stretch of the cardiac muscle fibers; therefore, a higher heart rate can enhance the cardiac output if the heart is capable of managing the increased workload.

Blood volume plays a crucial role as well because it affects the preload, or the initial stretch of the heart's ventricular walls. A greater volume of blood returning to the heart (increased preload) stretches the cardiac muscle, leading to stronger contractions. This enhanced contraction results in increased stroke volume, which is the amount of blood ejected from the heart with each beat.

In summary, both heart rate and blood volume are vital in modulating the strength of heart contraction, making this choice a comprehensive representation of the factors influencing cardiac output.