[Biology Class Notes] on Urine Pdf for Exam

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. By examining the composition of urine, we can know the need for 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 the case of terrestrial animals, they excrete 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 the 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. The composition of urine is mainly made up of water, urea, inorganic salt, creatinine, ammonia, and pigmented products which are formed by the 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 the 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 is mainly whitish and suspension of uric acid crystals that are passed into the cloaca and mixed with fecal material before being expelled. Whereas in the case 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 to 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.

2. Reabsorption: 

The 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 secrete different substances like hydrogen ions, potassium ions, etc. into the filtrate. This process helps in maintaining acid-base regulation activities. Further, the 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 wastes 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 a 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:

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

2. Amount of dissolved nitrogenous wastes present in the urine

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

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

What is Excretion?

Waste disposal is a biological process, which plays a key role in removing toxins and other wastes from the body. In plants and animals, including humans, as part of metabolism, a large amount of waste is produced. Plants often come out through the respiratory system and animals excrete waste in various ways such as urine, sweat, feces, and tears. In the midst of all this, the most common and primary way to urinate.

Urine Formation Occurs in 3 steps

1. Waste Ions and Hydrogen ions Hidden in Blood Complete Urine Formation

The next step in urine formation is tubular extraction. Here, tubular cells produce substances such as hydrogen ions, potassium ions, etc. in the filtrate. Through this process, ionic, acid-base, and balance of other body fluids are maintained. Concealed ions mix with the filtrate and form urine. Urine flows from the nephron tubule to the collection canal.

2. Urine is 95% water

The urine produced is 95% water and 5% waste nitrogen. Waste such as urea, ammonia, creatinine is excreted in the urine. Apart from this, potassium, sodium, and calcium ions are also released.

3. Osmoregulation

Osmoregulation is the process of controlling body fluids and their songs. It maintains the osmotic pressure of the blood and contributes to homeostasis. That is why it is recommended to consume about 2 liters of water, which contributes to the proper functioning of our kidneys. For example, we use a lot of water during the summer, but still, we urinate fewer times in the summer than in winter and the urine volume is also higher. The reason is that we lose a lot of water in our bodies in the summer due to sweating. Thus, to keep the body hydrated, our kidneys absorb large amounts of water.

Key Points for Urine Formation and Osmoregulation

• Urine is formed in three main stages – glomerular filtration, reabsorption, and excretion.

• It makes up 95% water and 5% waste such as sodium ion, potassium, and calcium, as well as nitrogenous wastes such as creatinine, urea, and ammonia.

• Osmoregulation is the process of maintaining the body’s homeostasis.

• It facilitates the dispersal of solvents and water into the lining, thus maintaining an osmotic balance.

• The kidneys control the osmotic pressure of the blood by filtration and purification by a process known as osmoregulation.

What is the Cause?

First, it helps to know what is down there and what is really going on when you urinate.

Your bladder holds your urine and is part of your lower urinary tract, which includes Trusted Source:

• The Urethra- This is a tube that transports urine from the bladder to the outside of your body.

• Internal Sphincter- The inner sphincter is located near the junction of the bladder and urethra (known as the bladder outlet). This keeps the urine from leaking out of the urethra very quickly. You cannot control this sphincter.

• External Sphincter– The external sphincter is located in the urethra to retain urine and excrete urine when urinating. It opens and closes to control when urine can come out of the bladder. You can control this sphincter – the one you use when you try to “hold it.”