<P> Excretion is the process by which metabolic waste is eliminated from an organism . 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 . Excretion is an essential process in all forms of life . For example, in mammals urine is expelled through the urethra, which is part of the excretory system . In unicellular organisms, waste products are discharged directly through the surface of the cell . </P> <P> Green plants produce carbon dioxide and water as respiratory products . In green plants, the carbon dioxide released during respiration gets utilized during photosynthesis . Oxygen is a by product generated during photosynthesis, and exits through stomata, root cell walls, and other routes . Plants can get rid of excess water by transpiration and guttation . It has been shown that the leaf acts as an' excretophore' and, in addition to being a primary organ of photosynthesis, is also used as a method of excreting toxic wastes via diffusion . Other waste materials that are exuded by some plants--resin, saps, latex, etc. are forced from the interior of the plant by hydrostatic pressures inside the plant and by absorptive forces of plant cells . These latter processes do not need added energy, they act passively . However, during the pre-abscission phase, the metabolic levels of a leaf are high . Plants also excrete some waste substances into the soil around them . </P> <P> 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 . </P> <P> 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 as there is less water in the environment and ammonia itself is toxic . </P>

Which is the main excretory product in human
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