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Excretion is a process in which metabolic waste is eliminated from an organism. This is in contrast with secretion, where the substance may have specific tasks after leaving the cell. Excretion is an essential process in all forms of life. For example, the waste products of unicellular organisms are discharged directly through the surface of the cell by diffusion. Plants have simple excretion methods to dispose of oxygen and excess water. The disposal of waste products in animals differ across species, with the release of different chemicals through specialized organs. In vertebrates, this is primarily carried out by the lungs, kidneys, and skin.

During life activities such as cellular respiration, several chemical reactions take place in the body. These reactions are commonly known as metabolism. These chemical reactions produce waste products such as carbon dioxide, water, salts, urea and uric acid. The excretory organs will remove these wastes. Accumulation of these wastes beyond a level inside the body is harmful and can cause a variety of diseases within the excretory system. This process of removal of metabolic waste from the body is known as excretion. Excretion can also be quantified using what is known as fractional excretion.

Animals
In animals, the main excretory products are carbon dioxide, ammonia (in ammoniotelics), urea (in ureotelics), uric acid (in uricotelics), guanine (in Arachnida), and creatine. Ammonia is a waste product of most cells. Accumulation of ammonia in the body is toxic. Most terrestrial animals have removal strategies for ammonia to convert into less harmful compounds, such as urea or uric acid. This process is called detoxification. Many aquatic animals excrete ammonia directly into their aquatic environment because it is highly soluble in water. Many aquatic animals utilize Rhesus (Rh) glycoproteins for excretion of ammonia. Rh glycoprotein expression is up-regulated in the presence of increased ammonia levels to efficiently transport it and subsequently remove it from the body. The liver and kidneys clear many substances from the blood (for example, in renal excretion), and the cleared substances are then excreted from the body in the urine and feces. Kidneys can be classified into two categories within bilateral animals: protonephridia and metanephridia. Metanephridia are exclusive to coelomates and are considered homologous across species. Protonephridia vary in structure and often precede metanephridia in development.

Invertebrates
Most aquatic invertebrates excrete waste as ammonia directly into the environment. The most common external excretory organs in aquatic invertebrates are gills. Some aquatic invertebrates utilize nitrogenous waste for control in buoyancy. In insects, internal organs for excretion include Malpighian tubules and the hindgut. During insect excretion, metabolic waste diffuses or is actively transported into the tubule, which transports the wastes to the intestines. The metabolic waste is then released from the body along with fecal matter. Crustaceans utilize antennal and maxillary glands to excrete waste. Molluscs have kidneys that function in excretion. More primitive excretory systems, like nematodes, consist of laterally joined canals and an excretory pore.

Vertebrates
Birds excrete their nitrogenous wastes as uric acid in the form of a paste. Waste materials such as heavy metals can be shed in the eggs. Many birds, especially seabirds, can excrete salt via nasal salt glands in the beak.

Amphibians in the tadpole stage excrete ammonia in aquatic environments. As adults amphibians will convert the ammonia to urea and excrete it in terrestrial environments. Amphibian skin is used to excrete carbon dioxide. Gills in amphibians and fish are used to control ion excretion. Since fish are aquatic most of their nitrogenous waste is excreted in the form of ammonia.

Reptile species may live in aquatic or terrestrial environments. To conserve water, terrestrial reptiles release nitrogenous waste in the form of uric acid. The aquatic reptiles require less conservation of water and release nitrogenous waste as urea.

Mammals excrete their urinary waste in the form of urea. Urea acts as the end product of protein metabolism in mammals. Active secretion of urea occurs when quantities of urea are higher in urine than in the filtered fluid retained in the body. Active reabsorption of urea occurs when urea concentration is higher in blood plasma than in excreted urine. Mammals are the only animals with sweat glands on their skin surface. Water and salt are the main contents in sweat. The role of sweating is to cool the body and shed metabolic waste products, proteins, amino acids and toxicants.

Plants
Plants have less complex modes of excretion compared to animals. Respiration in green plants produce carbon dioxide and water. Plants can remove excess water by transpiration and guttation. Photosynthesis converts the carbon dioxide and water into sugars and oxygen. The oxygen is excreted from the plant through the stomata. The dropping of leaves is often a method of excretion to accumulate waste in the leaves and detach from the plant. Other waste materials in some plants serve as anti-herbivore defences or attractants, such as resin, saps, latex, etc. These waste materials are absorbed by the plant cells. Plant roots release waste and excess compounds into soil.

Excretory Disorders
There are many known excretory disorders that affect the removal of waste in organisms. There are also several diseases that occur from contact with excretory waste. Several disorders and diseases involving the excretory system are listed below:

Hypothyroidism
A disorder involving the thyroid hormone, known as hypothyroidism, can affect the renal function of some animals. Hypothyroidism refers to abnormally low levels of thyroid hormone in the body. This condition causes lower levels of water to be excreted, causing urea concentration to increase in urine.

Kidney Failure
Kidney failure occurs when partial or all function of the organ is lost. This may also be called renal failure. There are five different stages of kidney failure. Many different conditions can cause kidney disease, including diabetes, high blood pressure, and kidney stones. Renal failure can lead to further complications such as anemia, cardiovascular disease, and immunodeficiency.

Kidney Stones
Kidney stones, or renal calculi, occur when there is a buildup of salt and minerals in the kidneys. Diet, obesity, and certain medical conditions can play a role in kidney stone formation. These stones can completely block the flow of urine by becoming stuck in the ureters. Although painful, most people do pass these stones on their own, but some may require surgery.

Uremia
Uremia is a disorder that occurs when a person has abnormally high levels of waste products in the blood, uremia is often associated with kidney failure and is most likely to occur in the later stages. Doctors can usually slow kidney failure, but uremia is only treated by replacing kidney function. This is done primarily through dialysis and sometimes kidney replacement if available. Many people with uremia/renal failure are found to have a glucose intolerance, leading to insulin resistance.

Wilson's Disease
Wilson's disease occurs as a result of copper build up in the organs, primarily the liver. The disease was first described by Friedrich Theodor von Frerichs in 1854. The disease was later described by Samuel Wilson in 1912 and was eventually named after him. It is an autosomal recessive inherited disorder caused by the gene ATP7B. This gene encodes the copper transporting P-type ATPase, which is very important in transporting copper throughout the body. As such, people with Wilson’s disease have reduced copper excretion (abnormal copper metabolism).

Alport Syndrome
Alport syndrome is a rare syndrome, described and named after Arthur C. Alport in 1927. Although Alport was not the first to describe this type of nephritis, he was the first to link this form to blindness and ocular changes. Alport syndrome is caused by mutations in the COL4A3, COL4A4 and COL4A5 genes and is a hereditary congenital hemorrhagic nephritis. There are different phenotypes that this syndrome presents, such as microscopic amounts of blood in the urine, which is an end stage of renal disease.

Foot and Mouth Disease
Foot and Mouth Disease (FMD), a disease commonly found among cloven-hoofed animals, is a disease transferred by contact with the excretions of infected animals. This disease is caused by a particular aphthovirus, and is most commonly characterized by the appearance of blisters in the mouth and on the hooves of the animal.

Hantavirus
Hantavirus is a viral disease contracted from the inhalation of urine or excretory droppings of an infected rodent causing respiratory symptoms such as coughing and shortness of breath. There is no specific treatment or cure for this virus, but when detected early symptoms can be managed in a hospital setting by intubation and oxygen therapy. The early symptoms of this disease are often confused with influenza, causing the disease to be difficult to diagnose in some cases. To prevent infection of hantavirus, it is recommended to wear a respiratory mask when working in areas suspected of rodent infestation.

Excretion Quantification
Fractional Excretion: Fractional excretion (FE) is a method of measuring filtration of waste into urine by the kidney. FE is quantified by the amount filtered by the glomerulus in the kidney into urine for excretion. It is mathematically defined as the fractional amount of total solute excreted over the amount of solute filtered.