The skin and lungs also have excretory functions: the skin eliminates metabolic wastes like urea and lactic acid through sweating,[2]p395 and the lungs expel carbon dioxide. Some terms - Mucociliary excretion is the excretion of mucus in the respiratory system. Cilia work to clear the small passages, and coughing removes phlegm. - Biliary excretion goes through the bile duct to the duodenum and removed in the faeces. - Perspiration is the excretion of salts and water from the body, although the main purpose is to cool the body. Other vertebrates Aquatic animals usually excrete ammonia directly into the water, as this compound is soluble and there is ample water available for dilution. In terrestrial animals ammonia-like compounds are converted into other nitrogenous materials as there is less water in the environment, and ammonia itself is toxic. Reptiles and birds excrete their nitrogenous wastes as uric acid in the form of a white paste. This allows efficient water retention and it can be stored more easily in the egg. Both uric acid and faeces are expelled through a common opening called the cloaca. Excretion is elimination of metabolic waste, which is an essential process in all organisms. In vertebrates, this is primarily carried out by the lungs, kidneys, and skin. This is in contrast with secretion, where the substance may have specific tasks after leaving the cell. For example, placental mammals expel urine from the bladder through the urethra, which is part of the excretory system. Unicellular organisms discharge waste products directly through the surface of the cell. Another example would be how mammals release solid waste (feces) through the anus during defecation. During activities such as cellular respiration, several chemical reactions take place in the body. These are known as metabolism. These chemical reactions produce waste products such as carbon dioxide, water, salts, urea and uric acid. Accumulation of these wastes beyond a level inside the body is harmful to the body. The excretory organs remove these wastes. This process of removal of metabolic waste from the body is known as excretion. In animals, the main excretory products are carbon dioxide, ammonia (in ammoniotelics), urea (in ureotelics), uric acid (in uricotelics), guanine (in Arachnida), and creatine. 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. Aquatic animals usually excrete ammonia directly into the external environment, as this compound has high solubility and there is ample water available for dilution. In terrestrial animals, ammonia-like compounds are converted into other nitrogenous materials, i.e. urea, that are less harmful as there is less water in the environment and ammonia itself is toxic. This process is called detoxification. Birds excrete their nitrogenous wastes as uric acid in the form of a paste. Although this process is metabolically more expensive, it allows more efficient water retention and it can be stored more easily in the egg. Many avian species, especially seabirds, can also excrete salt via specialized nasal salt glands, the saline solution leaving through nostrils in the beak.