I think that this is an example of excretion of metabolic waste. Metabolic waste are substances that have been left after metabolism that the body cannot make use of and have to removed from the body. Body excretes are in form of water solute except for carbon dioxide which is in form of gas together with water vapor from the lungs.
In the process of diffusion, a substance tends to move from an area of high concentration to an area of low concentration until its concentration becomes equal throughout a space.
