Which of the following describes a positive feedback loop?
when blood sugar is low, the body sends hormones that increase blood sugar.
when the body senses that the heart rate is very high, it secretes hormones that decrease the heart rate.
when a pregnant woman begins to go into labor, the body sends hormones that increase the intensity of the labor contractions.
when someone is exercising intensely, the body sends hormones that increase oxygen supply for cellular respiration.