The correct answer is C because:

Urea is one of the substances forced out of the blood in the kidneys; this occurs in the glomerulus (capillary network) where urea passes into the Bowman’s capsule at the start of a kidney tubule (nephron) and is not reabsorbed back into the bloodstream
Excess amino acids are broken down primarily by the liver, the nitrogen part (called an amine group) is converted into urea and then secreted back into the bloodstream. The remaining part of the amino acid can be converted into a number of substances that can be used in respiration or stored
The lungs are responsible for excreting carbon dioxide; there is a diffusion gradient between blood passing through the lungs from the heart which contains carbon dioxide that has diffused into the blood from respiring tissues, and the air in the lungs

A is incorrect because the liver does not excrete amino acids; it breaks them down into a number of substances of which urea is a waste product which is then secreted into the bloodstream to be removed by the kidneys.

B is incorrect because the lungs do not excrete urea and the kidneys do not excrete carbon dioxide.

D is incorrect because the kidneys are not involved in the excretion of urine; they produce urine by filtering the blood which is excreted by the urinary system of which the kidneys are a part. Excretion is defined as ‘the removal from organisms of toxic materials and substances in excess of requirements’. It is better to say that the kidneys excrete urea, excess salts and water than urine.

	kidneys	liver	lungs
A	excretes urea	excretes amino acids	removes carbon dioxide
B	removes carbon dioxide	deamination of amino acids	excretes urea
C	removes urea from blood	deamination of amino acids	removes carbon dioxide
D	excretes urine	deamination of urea	removes carbon dioxide

Excretion (Cambridge O Level Biology)

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