Biology (5090) Topic 13 · Excretion in humans 13.1 Excretion Practise the meaning of excretion, carbon dioxide as a respiratory waste, and urea production in the liver from excess amino acids. Key TheoryLearn FirstLoading… Common MistakesAvoid TheseLoading… StrategyApply ThisLoading… Practice 1EasyLoading… Practice 2MediumLoading… Practice 3AdvancedLoading…