Introduction

Depending on the product produced, animals exhibit five types of nitrogenous discharge where ammonotelism, ureotelism and uricotelism are the major types, and aminotelism and guanotelism are the minor types. Nitrogen waste such as ammonia, urea or uric acid is produced during protein metabolism according to the species.

Small amounts of nitrogenous wastes are also produced during the conversion of nucleic acids. Ammonia is very dangerous, followed by urea and uric acid. The latter is less toxic.

Definition of Excretion

The definition of excretion in biology states that the process of removing metabolic wastes from an organism. All animals, including humans, are eating food to survive. Thereafter, several biochemical reactions occur within the body. During the process it produces certain toxic products. Therefore, the body releases those products by releasing feces. Different animals use different organs to continue this process. In humans, the major excrement organs are the liver, lungs, large intestines, kidneys, and skin. The following is a list of these organs that work in extraction.

 Modes of Excretion

1. Ammonotelism – the process of releasing ammonia. Many aquatic insects, aquatic and aquatic animals and reptiles reflect the Ammonotelic environment. As ammonia dissolves easily, a few diffusions on the surface of the body are sufficient to repel the substance. In the case of fish, however, the surface of the gill does the same, releasing ammonium ions. It involves the most toxic excrement in animals. Many aquatic animals such as protozoans, (e.g., Amoeba, Paramecium), sponges (e.g., Sycon), cnidarians or coelenterates (e.g., Hydra), liver fluke, worms tape, Ascaris, Nereis, Earthworm, Leech, many aquatic arthropods (e.g., Prawn), many aquatic fish (e.g., Pila) fish with bones (e.g., Labeo), Amphibian tadpole (e.g., frog tadpole), marine mammals (e.g Salamanders), and crocodiles emit ammonia.
2.   Ureotelism – A number of Ureotelic animals such as mammals, marine fish, terrestrial animals, etc. they release Urea using this process. As a result of metabolism, ammonia converts into Urea in the liver of these animals, which are eventually absorbed into the bloodstream. Afterward, the kidneys filter the blood to remove impurities. However, some of the contents of the urea may remain in the kidneys which later help maintain osmolarity. In addition, the most important human nitrogen waste is urea which is excreted in the urine.
3.  Uricotelism – Other animals such as birds, reptiles, insects, land snails, etc. they remove nitrogenous wastes in the form of uric acid. The whole process happens with a small amount of water loss. Uric acid is a minor toxin among other substances.
4. Aminotelism – A method of extraction where nitrogenous waste is released in the form of amino acids is known as aminotelism. Animals that follow this release process are known as aminotelic animals. Example: Unio, Asterias etc.
5. Guanotelism – Animals such as Scorpio, spiders, etc. they excrete guanine (uric acid) using this process. It is insoluble and non-toxic. This excreta comes out solid. Many arid wildlife species agree with this outgrowth.

Human Excretory System

The excretory system is a system of living organisms that performs the function of excretion, the body’s process of excreting waste. There are several organs involved in this process, such as sweat glands, liver, lungs and kidney system. The human excretion system consists of the following structures: two bean-shaped kidneys, two ureters, one bladder, and urethra. The kidneys are a major part of the human excretion system.

Kidneys

The human kidneys are the major organs in the body. They are bean-shaped organs found on both sides of the spine at about the level of the abdomen and liver. Blood enters the kidneys through the kidneys and then exits through the kidneys. Tubes called ureters carry waste from the kidneys to another for storage or extraction. Kidneys produce urine, aqueous solution of waste products, salt, organic compounds, and two important nitrogen compounds: uric acid and urea. Uric acid is caused by the decay of nucleic acid, and urea is caused by the breakdown of amino acids in the liver. Both of these nitrogen products can be toxic to the body and must be excreted in the urine.

The Nephron

Nephrons are active kidney units. Each of our kidneys contains up to 1 million nephrons per human kidney. The basic functions of nephrons are to make fluids and the excretion process. 

Each nephron is made up of a Malpighian body with three parts-

1. Glomerulus– Glomerulus is a series of capillaries that absorb proteins in the blood that flow through the Malpighian Body.
2. Renal Tubule – The Renal Tubule is made up of a Proximal convoluted tubule lying in the cortex and absorbs water, sodium, and glucose back into the blood, distal convoluted tubule which returns sodium to the blood and absorbs potassium and acid in the blood and Loop of Henle.
3. Loop of Henle– A long, U-shaped tube that is mainly involved in getting water and sodium chloride in the urine is the Loop of Henle. It is a ‘U’ shaped like an ascending and descending limb and continues as a distant integrated tube. The Henle Loop produces highly concentrated urine. Therefore, desert animals have a very effective Henle Loop for removing dirt with very little water. The Henle Loop lies in the medulla region.               

Ureter

The ureter is a tube that carries urine from the kidneys to the bladder. There are two ureters, one connected to each other. The upper part of the ureter is located in the abdomen and the lower part is located in the pelvic area.

Human Excretion Process

The urinary system filters the blood of nitrogenous waste urea, salt, and water and excrets it in the urine. The kidneys also help to digest nutrients.

Blood filtration occurs in the kidney nephrons. Millions of nephrons work together to absorb minerals such as glucose, amino acids, salts, and allow pure blood to flow to other parts of the body. Urea and extra water are converted into urine which is transferred to another through the ureters and excreted in the body through a tube. The bladder holds the urine for a while, until it fills up and you get signals from the brain. Upon receiving the signal, the sphincters at the opening of the bladder relax and urine is expelled.

Conclusion

Excretion is the process by which metabolic waste is released from an organism. In animals, the main products are carbon dioxide, ammonia (ammoniotelics), urea (ureotelics), uric acid (uricotelics), guanine (Arachnida), and creatine . The liver and kidneys remove excess material from the bloodstream and remove the excreted material from the body through the urine and feces. Marine animals tend to emit ammonia directly from the outside, as this element has a high solubility and there is plenty of water available for purification. In terrestrial animals compounds such as ammonia are converted into other nitrogenous compounds, namely urea, which are less harmful as there is less water in the environment and ammonia itself is toxic.