Question

according to what you have learned about the circulatory and respiratory systems, explain how and why your heart changed its rate.

Answer 1

Your heart rate can change due to a number of things including even breathing a certian speed, people who exercise has a slower heart beat then someone who doesnt exercise. 
Or even if something is wrong with your heart, your heart rate can change too.

Answer 2

Heart rate is an indicator of the number of times the heart beats per minute (bpm). In adulthood, the average value ranges between 60 and 100 bpm. However, there are several factors that can change their number, such as age, the practice or not of physical exercise or a disease that affects the region.

Thus, the harder the heart needs to work to pump blood to the body, the greater the effort. Consequently, a person's heart rate tends to increase.

But the increase in these beats is not always caused by a worrying situation. Usually, the pulse becomes more intense and faster at times of our routine as the practice of exercise that makes the heart pump blood more quickly.

In summary, the heart rate is modified relative to the blood pumping speed.