Why the lungs are considered part of the excretory system?

The Lungs. The lungs play a mandatory role in the excretory system. Your lungs remove the carbon dioxide built up in your blood using specialized cells known as alveoli. Your lungs process the molecules into gas and when you breathe, the gas is ejected from your body.

Because, by exhaling, they release (or excrete) the carbon dioxide that builds up in the blood, as a waste product of cellular respiration. It's a very important role, even if we often forget the lungs as part of the excretory system.