Mammals are complex organism that produce a lot of excretory materials as a result of the metabolic  activities of their body. Some of the substances excreted include CO2 , uric acid, urea, excess mineral salts, etc.  it is important that these waste/excretory products be removed from the body of living organisms or else they become harmful and poisonous to the living cells.
Many organs are involved in the excretion of toxic wastes in mammals. These include skin, kidney, lungs, etc, all these organs are involved in the excretion of different materials in man.
Skin: helps in excretion of nitrogenous substances which are released through the skin pore to the outside; particularly, when the animal is active or during exercise. It contains excess water, salt, uric acid, ammonia and urea. Sweat is produced in the sweat gland and passes through sweat pores via the sweat duct out of the body.
Lungs: a quantity of water and carbon dioxide (which is very important in keeping the body pH constant) are expelled from the lungs daily. These two substances are transported from tissues by the blood and diffused into the moist air sacs (alveoli) of the lungs and finally exhaled in gaseous forms.
Liver: helps in the detoxification of toxic wastes and sends the wates product produced to the kidney for it to be passed out of the body.
Nephron: is the basic excretory unit of the kidney. Each kidney consists of millions of nephrons. The removal of wast products from the body by nephron takes place in three stages:
1. Ultrafiltration:  products carried by blood from the liver and other parts of the body are filtered through the pores of the bowman’s capsule of the nephron by high pressure of the blood. Molecules of smaller sizes such as glucose, mineral salts, amino acids, urea, vitamins pass through and fibrin will not pass through into the proximal convoluted tubule.
2. Selective reabsorption: this occurs in the loop of henle. Molecules such as water, urea, glucose, etc are selectively reabsorbed on the basis of importance as regulated by hormones controlling tubule permeability.
1. Removal of unwanted materials remaining after some of the water, glucose and mineral salts are finally reabsorbed in the second or distal convoluted tubule. The remaining waste products are passed to the collecting duct from where it passed to the urinary bladder and finally out of the body through the urethra.