All living organisms maintain a boundary between their internal environment and the external environment. Which of the following systems functions only within the internal environment of the human organism?

a. urinary system

b. circulatory system

c. respiratory system

d. digestive system

e. integumentary system

b. circulatory system

Explanation:

the urinary system will excrete urine, respiratory system undergoes gas exchange with the environment. The digestive system takes nutrients from the outside an excretes feces. The integumentary system (skin, nails, hair) protects the body from the environment.

The circulatory system is the only one that keeps moving substances but only inside the body