Besides the kidney, the lungs, liver and skin also excrete waste products.
The lungs excrete carbon dioxide and water , the waste products of cellular respiration.
The liver cells make bile which is needed for the digestion of fats. The bile contains bilirubin, a greenish yellow pigment which is a break down product of haemoglobin. Bilirubin is harmful if allowed to accumulate in the body. When bile is secreted into the small intestine during digestion of food, the bilirubin is also got rid of . It undergoes changes in the intestine and leaves the body in the faeces, which owes its yellowish brown colour to this pigment. Thus, by excreting bilirubin via bile, the liver acts as an excretory organ.
The skin of humans contains sweet glands. We control our body temperature through producing sweat. Sweat contains water with sodium chloride and traces of urea dissolved in it. The substances in sweat can be considered as excretory products. However, the only purpose of sweating is to cool the body. Excretion of substances in sweat is thus incidental.
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