Coordinating the body's responses to changes to the external environment is primarily the function of the system.

The function of coordinating the body's responses to changes in the external environment is primarily the responsibility of the nervous system.

Here's a step-by-step explanation:

1. The nervous system is made up of the brain, spinal cord, and nerves. These components work together to transmit signals throughout the body.

2. When there is a change in the external environment, sensory receptors in the body detect this change. These receptors could be in the skin, eyes, ears, nose, or mouth.

3. These sensory receptors then send signals to the brain via the nerves.

4. The brain processes this information and determines the appropriate response.

5. The brain then sends signals back down the nerves to the relevant parts of the body to coordinate a response. This could involve moving muscles, releasing hormones, or other actions.

6. This entire process happens very quickly, allowing the body to respond to changes in the external environment in real time.