There are three main processes involved in the process formation of urine. They are

  1. Ultrafiltration
  2. Selective reabsorption
  3. Secretion.


Process of Ultrafiltration in urine production 

                In the process of ultrafiltration, the filtration of blood under high pressure into the cavity of the Bowman’s capsule happens. Filtration occurs through the capillary walls of the glomerulus and the inner wall of Bowman’s capsule.  Blood capillaries of the glomerulus are porous and the cells lining the Bowman’s capsule (Podocytes) are specialized for filtration of small size molecules and ions. These specializations allow the passage of water and small molecules through the blood capillary walls into Bowman’s capsule. Blood cells, platelets and large molecules such as plasma proteins do not pass into the Bowman’s capsule because they are large in size. Filtration contains salts, glucose, amino acids, vitamins, nitrogenous wastes and other small molecules (H2O, HCO3-, NaCl, K+, Urea, Glucose).

Ultrafiltration occurs under three main pressure gradients. They are hydrostatic pressure, colloid osmotic pressure gradient and fluid pressure.

Process of Selective reabsorption in urine production 

                This is the process by which useful molecules, ions and water from the glomerulus filtration are recovered and returned to interstitial fluid and then into a capillary network of the tubules. Most of the reabsorption takes place in the convoluted tubules. Ions, nutrients. Water is reabsorbed either inactive or passive transport from the filtrate.

Active transport

Proximal tubule – NaCl, Glucose

The thick segment of ascending limb – NaCl

Distal tubule – NaCl, HCO3-

Collective duct – NaCl


Passive transport

Proximal tubule – HCO3-, H2O, K+

Descending limb – H2O

The thin segment of ascending limb – NaCl

Distal tubule – H2O

Collective duct – Urea, H2O

Process of Secretion in urine production 

                Foreign materials and substances which is not required to the body including wastes return from the peritubular capillaries into the filtrate. Secretion is required because such substances may not be entirely filtered because they remain a short time in the glomerulus and it occurs in the proximal and distal convoluted tubules. Secretion may be either active or passive depending on the location of the substance.

Active transport

Proximal tubule – H+

Distal tubule – K+, H+

Passive transport

Proximal tubule – NH3

 After the formation of urine, urine travels through the collective ducts and finally gets into the bladder through ureters.

 Featured image by Robina Weermeijer on Unsplash

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