Urine ~ Formation, Excretion, Properties

Urine is the yellowish liquid waste containing water and waste products that is secreted by the Kidneys, stored in the bladder, and discharged by way of the urethra.

Urine Formation by Kidney

Formation of Urine

The process of urine formation occurs in two major steps:


  • Due to the hydrostatic pressure built in the glomerulus, the liquid part of the blood filters out from the glomerulus and passes into the Bowman’s capsule.
  • This filtration under extraordinary force is called ultrafiltration.
  • The filtrate is known as glomerular filtrate.


  • The glomerular filtrate passes down the tubule; water and other substances required by the body are reabsorbed.
  • This entire process is called selective reabsorption.
  • Potassium ions and certain substances such as penicillin are passed into the urine through the distal convoluted tubule.
  • The cells of the walls of DCT are involved in bringing back into the renal tubule potassium ions and other substances; hence, this process is known as tubular secretion.

Urine Excretion:

  • The filtrate left after reabsorption and tubular secretion is called urine.
  • Then it passes from the collecting duct to the pelvis of the kidneys. From there, it is sent to the urinary bladder through the ureters.
  • By relaxing the sphincters present at the opening of the urethra, the urine is expelled from the body. This process is known as micturition or urination.

Physical Properties of Urine:

  • Colour: Yellow. It is due to urochrome.
  • Volume: 1 to 1.5 litres. However, the volume can vary depending on the liquid intake of the person.
  • pH: 5 to 8. Slightly acidic.
  • Odour: Faint smell. It is ammonia-like due to bacterial activity.
  • Specific gravity: 3 to 1.035
  • It is made of 95% of water and 5% of solid wastes.

Abnormal Constituents in Urine:

  • Haematuria: Due to infection in the urinary tract, kidney stone or tumour, blood passes out with it.
  • Glycosuria: Excess glucose passes out with it due to diabetes mellitus.
  • Due to anaemia, hepatitis or liver cirrhosis, bile is passed out in the urine.

Regulation of Urine Output:

  • The water content in it is controlled by Anti-diuretic Hormone (ADH).
  • Reduction in the secretion of ADH results in more production of it. This condition is called diuresis.
  • Substances which increase the production of it are called diuretics.


  • While removing urea from the blood, the kidneys also regulate the composition of blood, i.e. the water and salt concentration in the blood. This function is called osmoregulation.
  • Drinking enough water helps the kidneys to function properly.
  • In summer, we lose a considerable amount of water through perspiration which makes the urine thicker and concentrated.
  • Hence, the kidneys have to reabsorb more water from the urine.

Related Articles

The Excretory System

The Nervous System

Endocrine System

Digestive System

The Respiratory System

The Circulatory System

The Reproductive System

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