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Preface and Contents

Kidney and its Function

Urine formation

How is blood purified and urine formed?

In the process of blood purification, the kidneys retain all necessary substances and selectively removes excess fluid, electrolytes and waste products.

Let us understand this complex and amazing process of urine formation.

  • Did you know that every minute, 1200 ml of blood enters the kidneys for purification, which is 20% of the total blood pumped by the heart? So in one day, 1700 liters of blood is purified!
  • This process of purification occurs in small filtering units known as nephrons.
  • Each kidney contains about one million nephrons, and each nephron is made up of glomerulus and tubules.
  • Glomeruli are filters with very tiny pores with the characteristic of selective filtration. Water and small-sized substances are easily filtered through them. But larger-sized red blood cells, white blood cells, platelets, protein etc. cannot pass through these pores. Therefore such cells are normally not seen in the urine of healthy people.
The kidney’s chief function is to remove waste and harmful products and excess water in the form of urine.
  • The first step of urine formation occurs in the glomeruli, where 125 ml per minute of urine is filtered. It is quite astonishing that in 24 hours, 180 liters of urine is formed. It contains not only waste products, electrolytes and toxic substances, but also glucose and other useful substances.
  • Each kidney performs the process of reabsorption with great precision. Out of 180 liters of fluid that enters the tubules, 99% of fluid is selectively reabsorbed and only the remaining 1% of fluid is excreted in the form of urine.
  • By this intelligent and precise process, all essential substances and 178 liters of fluid are reabsorbed in the tubules, whereas 1-2 liters of fluids, waste products, and other harmful substances are excreted.
  • Urine formed by the kidneys flow to the ureters, and passes through the urinary bladder and is finally excreted out through the urethra.

Can there be variation in the volume of urine in a person with healthy kidney?

  • Yes. The amount of water intake and atmospheric temperature are major factors which determine the volume of urine that a normal person makes.
  • When water intake is low, urine tend to be concentrated and its volume is decreased(about 500 ml) but when a large volume of water is consumed, more urine is formed.
  • During the summer months, because of perspiration caused by high ambient temperature, the volume of urine decreases. During winter months it is the other way round – low temperature, no perspiration, more urine.
  • In a person with a normal intake of water, if the volume of urine is less than 500 ml or more than 3000 ml, it could indicate that the kidneys need closer attention and further investigation.
Too little or too much volume of urine formation, could be an indication that the kidney seeks attention and investigation.