Physiology of urine formation

There are mainly 3 processes involved in the urine formation

  1. Glomerular filtration
  2. Tubular reabsorption
  3. Tubular secretion

Glomerular filtration: It is the filtration of the body fluids and solutes from the blood filtration is assisted by the difference between the blood pressure in the glomerulus and the pressure of filtration in the Glomerular capsule. The volume of filtration by both kidney in each minute is called Glomerular filtration rate (GFR) in a healthy individual GFR. Is 125ml/min i.e 180 ml of filtrate is formed each day by two kidneys but nearly all the filtrate is reabsorbed and 1-1.5 lts is expected as urine.

GFR rate depends on the following factors:

  • permeability of capillaries
  • area of filtration
  • intra capillary pressure
  • osmotic pressure of the blood.

Tubular reabsorption: Nearly 180lts of filtrate is form each day by two kidneys but nearly all the filtrate is reabsorbed and 1-1.5lts is excreted as urine. The following substances are reabsorbed by the renal tubules:

  • Glucose: It is the observed completely from the proximal tubules, galactose, fructose are also reabsorbed.
  • water: More reabsorbtion of water from glomerular tubules and proximal and distal tubule. This reabsorption is controlled antidiuretic hormone of the posterior pituitary gland.
  • salts:
    • Nacl is reabsorbed in the proximal and distal tubules
    • potassium, phosphate and bicarbonate is completely observed in the proximal tubules.
    • Aldosterone regulate the extracellular fluids by the adjusting the amount of sodium reabsorbed by the blood from the kidneys.
    • uric acid, creatin, amino acids, beta hydroxybutyrate acids, are observed from the loop of henle.

Tubular secretion: Further the substances which are not found in the filtrate are also found in the urine this are direct excreted from the blood by the thieves through the enzyme mechanism, this is called tubular secretion. It is an active process in in which substances like potassium, hydrogen, creating and drugs like penicillin, para amino hippuric acid etc are directly by the tubular cells from the blood.

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