Answer:
The best answer to your question: What should the word in italics be replaced with to make this sentence more precise, would be:
Your cardiovascular system gets "stronger and more efficient" (in replacement for better) when you increase the time in intensity of your workout.
Step-by-step explanation:
The cardiovascular system also requires training to become better and more efficient. As you become engaged in cardiovascular exercize, like jogging, your heart, lungs and vessels need to bring up their efforts in order to feed the growing demands of your muscular and skeletal system. More oxygen needs to be taken to tissues, more CO2 needs to be removed, and energy-providing molecules like lipids and glucose have to be taken faster so that tissues can have what is needed to perform. If this is not met, then fatigue and then exhaustion takes place. Not to mention the dangerous build-up of toxins. Thus, as cardiovascular training happens, the cardiovascular system responds to the increasing needs by adjusting its parameters and becoming much more efficient in its processes. Thus, we can say that our system does not just become better, but rather strengthens itself and becomes more efficient.