Excretion is defined as the process of discharging waste matter from an organism. Every organism has metabolic waste generated in its body. The process of elimination of such waste from the organism is called excretion. In humans and animals, this is generally carried out by the skin, kidney, and lungs. This is in complete contrast to the function of secretion. After leaving the cell, the substance could still have specific functions.
In mammals, urine is excreted through the urethra and this is part of the excretory system. In single-celled organisms, waste products get discharged through the cell surface.
In the process of various activities of life like respiration, there are many chemical reactions that take place in the body. This process is called metabolism. This process produces waste products like carbon dioxide, water, salts, etc: When these wastes exceed a certain level, they become harmful to the organism and they need to be expelled. The excretory organs remove these wastes.
In the case of green plants, carbon dioxide and water are the products of respiration. The carbon dioxide thus released gets used during photosynthesis. Oxygen is a release during photosynthesis, and thus exits through the stomata or the root cell's walls. Plants use the process of transpiration and guttation to excrete excess water waste.
In the case of animals, main excretory products are ammonia, carbon dioxide, urea, to name a few. The liver and the kidneys remove many substances from the blood. The cleared waste products are removed or excreted from the body in the form of urine and feces.
Aquatic animals like fish excrete the ammonia directly into the external environment. Given the high level of water, they easily dissolve. Birds release their nitrogenous waste in the form of uric acid in the form of a paste. This process is more energy consuming but still, it allows for very efficient and effective water retention.
Burping is also a form of excretion but here it is mainly gas that is released. The expelled gas is generally a mix of nitrogen and oxygen. When we consume carbonated drinks, such as beer or other soft drinks, the gas gets expelled in the form of burping and it is mainly carbon dioxide in such cases.
Every human has a pair of kidneys. The kidney is made up of three sections named renal cortex, renal medulla, and renal pelvis. The blood reaches the kidney through the renal artery. Each artery splits or further divides into many arterioles which then go into the Bowman's capsules of nephrons where the waste is removed by pressure filtration mechanism. The renal pelvis takes urine away from the kidney via ureter. From there urine is removed through the urethra and out of the human body.
Sweat glands in the skin produce sweat. The body is kept cool during hot conditions by the water in the sweat. The sweat contains salts and urea.
Failure of the excretory system can result in major problems. Stones form in the kidneys. They can be found in any part of the urinary system. When urine gets concentrated, it allows minerals to crystallize and stick together forming stones. They are of different sizes, some of them can flow through the urinary system and some cannot. Some stones can cause great pain while others may cause little or no pain. Stones may at times need to be removed using surgical methods or a treatment method called ultrasound. If the kidneys cannot remove wastes from the blood its called kidney failure. The wastes will then build up in the body. Failure of the kidney can be caused by accidents or by drugs or something that is toxic. A slow decrease in kidney functioning is a chronic kidney disease. A person who has lost kidney function may need to get kidney dialysis. This involves the process of filtering the blood wastes using a machine. Urinary tract infection is another form of reason for failure and this is caused by bacteria getting into the bladder or kidney.