-The kidney carry out two essential functions namely:
-The excretion of nitrogenous materials, excess water and salts
-An exchange of materials  between the blood and the urinary system take place in kidneys
-This occurs in the Bowman’s capsule
-Urine is made up of approximately 95% water, 1% sodium chloride and smaller amounts of other nitrogenous compounds and mineral salts
-And salts are actively secreted from capillaries in the skin
-By the tubules of the sweat glands
-Sweat is secreted onto the skin
-Carbon dioxide and water vapour
-Diffuse from the moist alveolar surfaces of the lungs
-Which in mammals are the sole excretory organs for carbon dioxide
-Formation of nitrogenous waste products by the deamination of excess amino acid
-The nitrogenous product of deamination is ammonia
-Urea formation
-Ammonia plus carbon dioxide give urea and water
-Bile pigments from the breakdown of the haemoglobin of old red blood cells