Urine Formation 101: Filtration, Reabsorption, And Secretion

Explain the formation of urine.
Answer:

Formation of urine:

The urine formation takes place in three steps.

1. Glomerular filtration:

It occurs in the glomerulus
It is the initial step in urine formation, the plasma that transverses the glomerular capillaries is filtered by the highly permeable glomerular membrane’ and the resultant fluid the glomerular filtrate, is passed into Bowman’s capsule.
Glomerular filtrate is a proteinfree filtrate of plasma.
All the substances of plasma are filtered except the plasma proteins.
During the filtration, the substances pass through a filtering membrane This filtration depends upon
- Glomerular hydrostatic pressure (HP)
  - It provides driving force for the filtration
- Colloidal osmotic pressure
  - Oppose the process of filtration
- Intracapsular pressure
  - Oppose the process of filtration
  - The resultant of these factors is termed as the effective filtration pressure
Glomerular filtration rate is the rate at which the filtrate is formed

2. Tubular reabsorption:

When the glomerular filtrate flows through the tubular portion of nephron, both quantitative and qualitative changes occurs
Large quantity of water, electrolytes and other substances are reabsorbed by the tubular epithelial cells
The reabsorbed substances move into the interstitial fluid of renal medulla
From here, the substances move into the blood in peritubular capillaries.

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