How is the excretory system like your respiratory system?
When you inhale, you take in oxygen. Your body then uses this oxygen in aerobic respiration and in this process, carbon dioxide (the waste product) is produced. Thus this carbon dioxide is released by your body when you exhale, and we say that it is excreted as a waste/excretory product. This is similar to the excretory system whereby the waste products are removed from the body through sweat, urine etc.