One important organ in the body to help maintain homeostasis is the liver. It assists many of the body systems, including the excretory system. It does this by removing nitrogen from excess amino acids which cannot be stored and forming urea--a nitrogenous waste product. Urea is then added to the circulating blood and is transported to the kidneys to be excreted.

Mammals have a pair of well-developed kidneys which have the purpose of controlling water balance in the body. This is an adaptation that allows mammals to exist in many different habitats on Earth with different climates. Kidneys also filter nitrogenous wastes in the bloodstream. These wastes, known as urea, are then combined with water and are excreted as urine. The blood that enters that kidneys contains wastes and when it leaves, it is purified and can then circulate through the body again. 

Nephrons are the functional units of the kidneys. They are microscopic filters that remove wastes from the blood and return useful substances back into circulation.  Filtration occurs in the glomerulus--which is enclosed inside the Bowman's capsule. There is a close association between blood capillaries and kidney filters that allow wastes to enter the kidneys and for useful substances to be reabsorbed, including food molecules and some of the water.

The remaining filtrate includes water, excess salt and urea. This becomes urine and passes from each kidney to a tube connected to each one (called a ureter), to the urinary bladder. This organ can temporarily store urine until it is released through the urethra, which is a tube that allows urine to exit the body. 

Body temperature must be regulated in mammals in order to maintain homeostasis. Muscles in the skin of many mammals are attached to hair, which can cause the hair to stand up. This conserves heat by allowing air to fill in the spaces between the hairs and insulates the body. Humans, which evolved in hot tropical climates, lost the thick body-hair covering that many other mammals have. When temperatures drop, the muscles in the skin contract causing goose bumps, which help to retain heat. Sweat glands in the skin perspire when temperatures increase. This allows the body to cool down as the sweat evaporates. It is also an additional way to excrete small amounts of urea present in perspiration along with excess water and salt. Fatty tissue below the skin acts as an insulator, preventing heat loss to the environment.

I have included a link showing kidney filters. I have a second link with a cross section of skin to help visualize its components.

A.) Urea is a product of the urea cycle (also called the ornithine cycle) which takes place primarily in the cells of the liver. The urea cycle is a series of five biochemical, enzyme-catalyzed reactions which convert toxic ammonia and carbon dioxide to urea. 

B.) Urea is produced in the liver where it becomes dissolved in the bloodstream. The blood travels from the liver to the kidneys through the circulatory system. The kidneys filter the urea from the blood and into the kidney filtrate, which then passes from the kidney into the ureter to the bladder. Urea is eliminated as a component of urine through the urethra from the bladder.

C.) Mammals regulate their body temperature in a number of ways. The secretion and evaporation of sweat from the skin surface is used to cool the body. Skin capillary dilation is also used to shed heat through radiation. When heat must be conserved, sweating does not take place and skin capillaries are constricted to prevent heat loss. Shivering can generate heat through muscle contraction.