CO2 produced by aerobic respiration. CO2 dissolves in H2O to produce a weak
acid, so if too much builds up in body fluids the pH drops, which can damage cells and
disrupt metabolism. CO2 is transported to the lungs dissolved in blood plasma and
excreted in expired air.
Nitrogenous excretory products, in particular urea. Excess amino acids cannot
be stored in the body. In the liver, they are converted to urea, CO(NH2)2, and a keto
acid. The keto acid can be respired to provide energy, or converted to fat for storage.
The urea dissolves in the blood plasma and is removed and excreted by the kidneys.
