system
Given the following post, respond as if you're commenting on Reddit:
human
How does exercise reduce your resting heart rate?

Does it make your cells physically need less oxygen?  Does it make your heart more efficient, how?
gpt
The ventricular walls will also thicken, increasing the contractility of the heart. This increased power in turn leads to a greater stroke volume (amount of blood pumped per contraction).

The cardiac output is further defined as heart rate x SV: hence functional cardiac output can be maintained at a lower heart rate with increased stroke volume.