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](https://homeoflearning.in/wp-content/uploads/2021/07/kidney-147499_640_37.png)
Formation of Urine
The process of urine formation occurs in two major steps:
Ultrafiltration:
- 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.
Reabsorption:
- 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.
Osmoregulation:
- 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.
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