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It is like eliminating unwanted substances from a living body.

Excretion definition in biology is a process through which living organisms expel waste or toxic substances from their bodies. It is like eliminating unwanted substances from a living body. The product that comes out of the excretion process is called excretory products. Excretory products are generated through metabolic activities or non-metabolic activities. Metabolism helps in stimulating the excretory process. In unicellular organisms, excretion takes place through cells whereas in multicellular organisms, like animals and human beings, excretion takes place through the body. 

Metabolic wastes are excreted in the form of solid, liquid, and gas like oxygen in plants, sweat in humans. Non-metabolic wastes are generally substances in a living organism’s body that are no longer required or are no more useful to the body like urine and excreta.

The human excretory system is also the urinary system of the human body. This system consists of the kidneys, the ureters, the bladder, and the urethra. Kidneys are the main functional unit of this system. Nephron, a part of the kidney, helps in filtering the blood and collecting waste from it and storing it into the kidney. The waste from the blood is stored in the form of urine in both the kidneys. Urine leaves the kidneys through the ureters and then passes into the bladder. The bladder stores the urine, but it cannot store for longer. The bladder pressurizes the urine to move to the urethra and straight out of our body in the form of urine.

In animals, bodies with a single layer of cells, excretion generally occurs through the process of diffusion. For example, sponges that are multicellular animals but with a single layer of cells excrete through specialized cells. It diffuses gaseous wastes into the water. It takes place at the site of the elimination of the outer environment of the organism. In more complex animals, the process of excretion takes place through an overly complicated excretory system. For example, in vertebrates, the excretion of wastes takes place through an excretory organ like the kidney and urinary ducts associated with it. For example, an elephant eats his food and swallows it down his stomach. This process is called ingestion. In the stomach, the food is broken down into simpler and soluble products. This is digestion. The soluble parts are absorbed in the body by the process of assimilation. After absorption, enzymes act on the product, and through metabolism, it undergoes a chemical reaction. This leads to the production of carbon dioxide which is exhaled to the environment with the help of the lungs in the respiratory system, oxygen that is produced is used, and other nitrogenous wastes like uric acid, ammonia, etc. are excreted out of the body through the excretory system and its organs-kidneys and various urinary ducts. This system is more or less similar to the excretory system of human beings.

We should not forget that plants are living things and, therefore, can excrete too. Small plants can simply go through their excretory process with the help of their cells. These plants excrete wastes on the surface of their cells. Large plants cannot use these cells as they do not have much access to the outside environment. Therefore, like animals, they use their cells to secrete wastes from the extracellular spaces, especially in the leaves. The main product of the excretion in plants is oxygen or O2, which is our life support. Plants exhale oxygen through large openings in the leaves called stomata. This very much sums up the process of photosynthesis in the plants as well. These stomata inhale the carbon dioxide that we give out and exhale the oxygen that we breathe. The major difference between the excretion in plants and animals through extracellular spaces is that the main byproduct of the excretory system of the plant is in a gaseous form, unlike animals.  

People tend to confuse between the two processes. In simpler terms, secretion is the process in which hormones and other important substances are secreted from the cell to fulfill important purposes like hormonal growth, whereas excretion is a process in which unwanted and harmful substances like uric acid and ammonia are eliminated from the living body. The product excreted are of no use but the products secreted are useful.

If kidneys are not functioning properly in any human being, it can lead to problems. In the most common cases, excess wastes start to store in the kidneys, and the extra wastes start crystallizing themselves as kidney stones. These stones can grow and become bigger. It can cause problems in your urinary tract, can lead to bleeding or infections. These can be very painful and require immediate treatment by the doctors. The small stones can be made to pass through the urethra, but the bigger ones require surgery. Many kidney functions are treated by dialysis.