During life activities such ascellular respiration, several chemical reactions take place in the body. These are known asmetabolism. These chemical reactions produce waste products such ascarbon dioxide, water,salts,urea anduric 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.
Greenplants excrete carbon dioxide and water as respiratory products. In green plants, thecarbon dioxide released during respiration gets used during photosynthesis.Oxygen is a byproduct generated duringphotosynthesis, and exits throughstomata, root cell walls, and other routes. Plants can get rid of excess water by transpiration andguttation. 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.[3][4] Plants also excrete some waste substances into the soil around them.[5]
Aquatic animals usually excreteammonia directly into the external environment, as this compound has highsolubility and there is ample water available for dilution. Interrestrial 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 istoxic. This process is called detoxification.[7]
White cast of uric acid defecated along with the dark feces by alizard. Insects, birds and some other reptiles also use a similar mechanism.
Birds excrete their nitrogenous wastes asuric acid in the form of a paste. Although this process is metabolically more expensive, it allows more efficientwater retention and it can be stored more easily in theegg. Many avian species, especiallyseabirds, can also excrete salt via specialized nasal salt glands, the saline solution leaving through nostrils in thebeak.[citation needed]
Ininsects, a system involvingMalpighian tubules is used to excretemetabolic waste. 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.[citation needed]
The excreted material may be calledejecta.[8] Inpathology the word ejecta is more commonly used.[9]
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