Urine Formation:

The process of formation of urine is called Uropoiesis. This process involves three steps:  

Glomerualr filtration, re-absorption, and tubular secretion.

1. Glomerular Filtration: It is the first step of urine formation. Blood is filtered through 3 layers is through layer of Bowman’s capsule, through layer of capillaries of glomerular and through them so finally, so that this filtration is called ultra-filtration. Here, Blood is left only with corpuscles (RBC, WBC and platelets) and proteins and remaining components of blood are passed into Lumen of nephron. (The filtered out fluid is called Glumerular filtrate which contains sodium, potassium, chloride ions, glucose, amino acids, urea, uric acid, ketone bodies and large amount of water.

Glomerular filtration rate: The amount of filtrate formed by kidneys per minute is called GFR. 180 litres per day.

2. Tubular re-absorption: From Bowman’s capsule, ultra-filtrate passes to lumen of Bowman’s capsule and passes into the tubules and flow to collecting duct.

1. PCT: Cells of proximal convoluted tubule causes re-absorption of materials from filtrate. These cells reabsorb glucose, amino acids, (ions, like K⁺, Cl^-, Na⁺). Much water, some urea from filtrate.
2. Loop of Henle:

1. First wide part of descending loop of Henle, is impermeable to ay ion. Here, no re-absorption takes place.
2. Second, thin part of descending loop of Henle is permeable to water and reabsorbs water from filtrate.
3. Ascending limb is impermeable to water along entire length but permeable to ions. Like (Na⁺, K⁺, and Cl^-) and reabsorb these ions and urea from filtrate.

3. DCT: Distal Convoluted tubule. When level of water in plasma falls, then pituitary releases a hormone called ADH (Anti diuretic hormone). It increases the permeability of DCT and collecting duct to reabsorb water. Now when water level becomes normal, ADH stops releasing.
4. Collecting duct: DCT and collecting duct reabsorb sodium from filtrate. And now urine is completely formed which is stored in this collecting duct.

3. Tubular secretion: Pigments, drugs etc are secreted into filtrate in the PCT by intestinal fluid. NH₃ and H⁺ ions are also secreted into filtration in the PCT.

• Blood enters by afferent arteriole and after filtration goes through efferent arteriole.