How The Kidneys Concentrate Urine: The Countercurrent Mechanism Explained

How The Kidneys Concentrate Urine: The Countercurrent Mechanism Explained

Mechanism of urine concentration (or) Describe counter current mechanism operating in the kidney to produce hypertonic and hypotonic urine.
Answer:

Mechanism of urine concentration:

It involves two processes

1. Medullary gradient

• Osmolarity of different parts changes

• This increase in osmolarity is called medullary gradient
• It is maintained through counter current mechanism

Counter current mechanism

• It constitute of loop of Henle and vasa recta
• Flow of fluid is in opposite direction in two limbs of U shaped tubules

1. Loop of Henle:

• Acts as countercurrent multiplier
• It works as follows

2. Vasa recta:

2. Role of ADH

• Due to presence of ADH, distal convoluted tubule and collecting duct becomes permeable to water and causes reabsorption of water
• Thus urine becomes hypertonic