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Homeostasis is the maintenance of a steady state within the body. Excretion is the removal of waste products and this system must function properly in order to maintain homeostasis. The kidneys filter urea from the blood, which forms after proteins are metabolized and combine it with water to become urine, which is propelled by the ureters to the bladder. Eventually, it is excreted through the urethra. High levels of urea can be toxic and deadly. If a person's kidneys function poorly due to kidney disease, dialysis is necessary to remove the urea, or else the person may die as these substances build up and become toxic. This is an example of homeostasis being maintained by the kidneys. The skin is part of the excretory system as it contains sweat glands. Urea is also removed whenever someone sweats. Sweat glands not only excrete urea, but also excess salts and water. Sweat glands are located in many areas all over the body. The lungs are part of the excretory system when carbon dioxide and water vapor are excreted when a person exhales. These gases are the wastes that form during respiration. They are constantly removed from the blood when the circulating blood enters the capillaries in the lungs and these gases diffuse into the alveoli, and then, are ultimately exhaled. The liver is part of the excretory system as it detoxifies many substances including alcohol. It is where nitrogenous wastes from protein metabolism are converted into the less toxic urea, to be later excreted by the kidneys. Fun facts: the kidneys are bean shaped and each has about one million tiny filters called nephrons that filter the circulating blood. If one kidney fails, the other can deal with functions of water balance and excretion!
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