Question

Briefly state the mechanism of urine formation in the human kidney.

Answer 1

The formation of urine in human kidneys occurs by a special type of process called a counter current mechanism. The process involves absorption and resorption of various elements of the blood that enters the kidney at various points within a nephron.

Complete step by step answer:
Kidneys in human beings are 10- 12 cm × 5-6 cm × 3-4 cm and 150g to 135 g in weight. Histologically, the kidney is divided into the outer cortex and an inner medulla. Projections or cortex into the medulla are called as columns of Bertini. The proximal portion of the ureter, lying in the hilum, is expanded in the form of a funnel called the pelvis. This pelvis breaks into 2 to 3 major branches proximally towards the kidney and is called as major calyces. They further divide into smaller calyces. Nephrons are embedded in the medulla and some also extend up to the cortex.
The counter- current mechanism of urine formation occurs as:
- When blood entered the first region of nephron i.e. Descending limb of Henle’s loop, the water content is reduced as the walls of this region are permeable to water but are impermeable to electrolytes. This concentrates the filtrate as it moves down towards the ascending limb. Initially, at this point the tubular fluid is isotonic and the tissue fluid is hypertonic in nature.
- As the blood moves towards the ascending limb, the tubular fluid becomes hypertonic and the tissue fluid becomes hypotonic gradually. This is due to the fact that the walls of the ascending limb of Henle’s loop are impermeable to water but are permeable to electrolytes.
- The flow of blood in these two limbs is always in the opposite direction and hence forms a counter current.
- As the fluid further moves into the distal convoluted tubule, the tubular fluid becomes isotonic again and when it enters into the collecting duct it is called urine, and it's the most concentrated form. The tubular fluid becomes hypertonic again.

Note: The current in the limbs of the vasa recta is also in the opposite direction i.e. forms a counter current pattern. This counter- current situation and the proximity between vasa recta and Henle’s loop helps in maintaining increasing osmolarity towards the inner medullary tissue.