The differential permeability or impermeability of specific sections of the nephron tubules is necessary for urine formation.
Generally, the process involved in the formation of urine are as follows:
Ultrafiltration:
- This occurs at the glomerulus.
- Small molecules such as water, urea, mineral salts and plasma solutes are permeable ( can pass) through the one cell thick wall of the glomerulus.
Selective reabsorption :
- this takes place at the proximal part of the nephron tubule.
- The glomerular filtrate contains water, sugar, amino acids and salts which are reabsorbed into the surrounding blood capillaries.
Tubular secretion:
- this occurs at the distal part of the nephron tubule.
- Large waste molecules are secreted into the tubule.
- This is to keep the osmotic concentration of the blood constant.
- The fluid that eventually remains in the tubule is concentrated and is known as urine.
Therefore, the varying permeability and impermeability of various regions of the nephron tubule leads to urine formation.
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