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Introduction on Urine

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Urine is known to be a liquid or semi solid solution of waste produced by metabolic processes along with that also contain toxic substances which are produced from excretory organs during the circulation of fluid. As by examining the composition of urine we can know the need of water inside the body. In the case of freshwater organisms their urine is in very diluted form whereas marine animals tend to urinate in concentrated form to prevent loss of excess water. In case of terrestrial animals they excretes highly concentrated urine depending on their habitat, to usually retain water.  

Formation of Urine in Different Organism

In organisms like humans, urine formation begins in the nephrons of the kidneys. As filtration of plasma occurs in the nephron and fluid which is found within the nephron is similar to blood plasma without macromolecules like proteins. With passage of time fluid passes along the nephron tube by which water and various useful components like amino acids, glucose, and other nutrients are reabsorbed into the bloodstream, leaving a concentrated solution of waste material called final, or bladder, urine. Composition of urine is mainly made up of water, urea, inorganic salt, creatinine, ammonia and pigmented products which are formed by breakdown of blood and this is the reason the color of urine is yellow. Apart from that substance which is not usually reabsorbed  remains in the urine. 

Whereas in case of birds, reptiles and terrestrial insects the excretion product is insoluble uric acid instead of water soluble urea. And urine color of birds and reptiles are mainly whitish and suspension of uric acid crystals that is passed into the cloaca and mixed with fecal material before being expelled. Whereas incase of terrestrial insects they excrete solid urine. 

Organisms like amphibians and fishes excrete solutions of urea and their excretory organs do not reabsorb large quantities of water, so their urine remains dilute.

Steps Involved in Urine Formation

Waste material is excreted by the human body in the form of urine and in this excretion process the kidney plays a very crucial role. As human urine composition is made up of 95 percent water and 5 percent solid waste. And all this filtration process occurs in the nephron and because of this reason it is known as the structural and functional unit of the kidney. 

Urine Formation Occur in Three Steps

  1. Glomerular Filtration: 

This filtration occurs in the glomerulus, and here filtration of blood is done. This type of filtration occurs in three layers: epithelium of Bowman’s capsule, endothelium of glomerular blood vessels, and a membrane between these two layers.

  1. Reabsorption: 

Reabsorption process is majorly done in renal tubules and 99% of filtrate is obtained in this process only. And this process is both active and passive type. Secretion: This step is also known as tubular secretion as here tubular cells secretes different substances like hydrogen ion, potassium ion, etc. into the filtrate. As this process helps in maintaining acid-base regulation activities. Further this secreted ions combine with filtrate to form urine. Further this urine passes out of the nephron tubules into the collecting ducts. 

Composition of Urine

Urine is made up of different products like 95% water and 5% nitrogenous waste. These nitrogenous waste including urea, ammonia, creatinine are excreted in the urine. Along with that other substance like potassium and sodium is also present in the urine.

Amount of Urine Excreted by Human

All living organisms do excretion processes and urine is mainly sterile liquid and generally a waste product. They mainly contain water, salts, urea and other-soluble nitrogen products. This urine formation process is done by the kidney  and released through the urethra. Human urine composition is of water, urea, salts, and pigments. Excretion of urine varies on different factors like:

A. The total amount of water present in the body.

B. Amount of dissolved nitrogenous wastes present in the urine

C. Different hormones also control the movement of water and sodium ions into and out of the nephrons.

Because of all these reasons, total quantity of urine production is directly proportional to the volume of fluid and other nitrogenous substances. 

FAQ (Frequently Asked Questions)

1. Explain the Composition of Urine in the Human Body?

Ans. Human urine contain 95% of water and 5% of water substance which are listed below:

9.3 g / L urea.

1.87 g / L chloride.

1.17 g / L sodium.

0.750 g / L potassium

0.670 g / L of creatinine


All these substances reduce the concentration of urine. Along with that there are also various other components like dissolved ions, organic and inorganic compounds such as metabolites, proteins, hormones.

2. Explain the Steps involved in Urine Formation?

Ans. Urine formation majorly takes place in three steps, they are:

A. Glomerular filtration

B.  Reabsorption

C.  Secretion

Along with that there is one more process called osmoregulation which maintains homeostasis is the bodyn and it facilitates diffusion of solutes and water across the semipermeable membrane to maintain osmotic balance.