Urine Formation And Osmoregulation

Table of Contents

Every one of us, including plants and animals, depends on the excretion process for the removal of certain waste products from our bodies. During the process of excretion, both the kidneys play an important role in filtering the blood cells.

Let us have an overview of urine formation and osmoregulation.

What is Excretion?

Excretion is a biological process, which plays a vital role in eliminating toxins and other waste products from the body. In plants and animals, including humans, as part of metabolism, a lot of waste products are produced. Plants usually excrete through the process of transpiration and animals excrete the wastes in different forms such as urine, sweat, faeces and tears. Among all these, the usual and the main form of excretion is urine.

Urine Formation

Waste is excreted from the human body, mainly in the form of urine. Our kidneys play a major role in the process of excretion. Constituents of normal human urine include 95 per cent water and 5 per cent solid wastes. It is produced in the nephron, which is the structural and functional unit of the kidney. Urine formation in our body is mainly carried out in three phases namely

  1. Glomerular filtration
  2. Reabsorption
  3. Secretion

Mechanism of Urine Formation

Urine formation2

The mechanism of urine formation involves the following steps:

Glomerular Filteration

Glomerular filtration occurs in the glomerulus where blood is filtered. This process occurs across the three layers- the epithelium of Bowman’s capsule, the endothelium of glomerular blood vessels, and a membrane between these two layers.

Blood is filtered in such a way that all the constituents of the plasma reach the Bowman’s capsule, except proteins. Therefore, this process is known as ultrafiltration.


Around 99 per cent of the filtrate obtained is reabsorbed by the renal tubules. This is known as reabsorption. This is achieved by active and passive transport.


The next step in urine formation is tubular secretion. Here, tubular cells secrete substances like hydrogen ions, potassium ions, etc into the filtrate. Through this process, the ionic, acid-base and the balance of other body fluids are maintained. The secreted ions combine with the filtrate and form urine. The urine passes out of the nephron tubule into a collecting duct.


The urine produced is 95% water and 5% nitrogenous wastes. Wastes such as urea, ammonia, and creatinine are excreted in the urine. Apart from these, the potassium, sodium and calcium ions are also excreted.

Also Read: Counter-current Mechansim


Osmoregulation is the process of regulating body fluids and their compositions. It maintains the osmotic pressure of the blood and helps in homeostasis. This is why it is recommended to consume more water about 2-3 litres, which helps in the proper functioning of our kidneys. For example, we consume lots of water during summers, but still, we urinate fewer times in summers than in winters and the concentration of the urine is also more. The reason is that we lose lots of water from our body in summer through sweating. Thus, to maintain the fluid balance in the body our kidneys reabsorb more water.

Key Points on Urine Formation and Osmoregulation

  • Urine is formed in three main steps- glomerular filtration, reabsorption and secretion.
  • It comprises 95 % water and 5% wastes such as ions of sodium, potassium and calcium, and nitrogenous wastes such as creatinine, urea and ammonia.
  • Osmoregulation is the process of maintaining homeostasis of the body.
  • It facilitates the diffusion of solutes and water across the semi-permeable membrane thereby maintaining osmotic balance.
  • The kidney regulates the osmotic pressure of blood through filtration and purification by a process known as osmoregulation.

Also Read: Osmoregulation

To know more about urine formation, the mechanism of urine formation and how kidneys help in osmoregulation, keep visiting BYJU’S website or download the BYJU’S app for further reference.

Frequently Asked Questions


Define Excretion.

Excretion is the removal of all toxic and unwanted metabolic waste products from the body.

What is the difference between egestion and excretion?

Excretion is the removal of toxic materials, waste products of metabolism and excess substances from organisms.

Egestion is the passing out of undigested food as faeces, through the anus.


What are the 3 major processes involved in Urine Formation?

The 3 processes are:

a) Glomerular Filteration
b) Tubular Reabsorption
c) Tubular Secretion


Define Osmoregulation.

It is a process of maintaining the normal osmotic concentration of water and salts in the body.

Define Ultrafiltration.

The process of the filtration of blood in the glomerulus under great pressure during which the liquid part of the blood i.e. plasma along with urea, glucose, amino acids and other substances enter the renal tubule.
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