Urine formation ( मूत्र निर्माण)

*Urine formation

1. Glomerular filtration
Blood pressure pushes plasma through the thin walls of the glomerular capillaries into Bowman’s capsule.
Small solutes (water, ions, glucose, amino acids, urea) pass through; cells and large proteins stay in the blood.
The fluid that enters the capsule is called *glomerular filtrate*.

2. Tubular reabsorption
As filtrate travels through the proximal convoluted tubule, most of the water, glucose, amino acids, and needed ions are actively or passively taken back into the peritubular capillaries.
About 65‑70 % of the filtrate is reabsorbed here.

3. Tubular secretion
The tubular cells add extra waste products, drugs, and excess ions (e.g., H⁺, K⁺, creatinine) from the blood into the filtrate.
This fine‑tunes electrolyte balance and pH.

4. Concentration in the Loop of Henle
The descending limb lets water out, concentrating the filtrate.
The ascending limb pumps out Na⁺ and Cl⁻ but is impermeable to water, creating a salty medullary interstitium.
This counter‑current system builds an osmotic gradient that later allows water reabsorption.

5. Distal convoluted tubule & collecting duct
Distal tubule adjusts Na⁺, K⁺, and Ca²⁺ levels under hormonal control (aldosterone, parathyroid hormone).
Collecting duct is highly permeable to water only when antidiuretic hormone (ADH) is present.
Water is reabsorbed into the hypertonic medulla, concentrating the urine.

6. Excretion
The final, concentrated fluid (urine) moves into the renal pelvis, then through the ureters to the bladder for storage until voided.

Key points to remember
Filtration creates a large volume of filtrate; reabsorption reduces it to ~1 % of the original.
Secretion adds substances the body wants to eliminate.
Concentration is regulated by the loop of Henle and ADH in the collecting duct.



Glomerular filtration – pressure‑driven movement of plasma into Bowman’s capsule, producing filtrate.
Filtrate – the fluid that enters the renal tubule after filtration.
Proximal convoluted tubule (PCT) – reabsorbs ~65 % of water, glucose, amino acids, Na⁺, and other solutes.
Tubular reabsorption – selective transport of needed substances from filtrate back into the blood.
Tubular secretion – active addition of waste, drugs, H⁺, K⁺, etc., from blood into the tubular fluid.
Loop of Henle – creates a medullary osmotic gradient; descending limb is water‑permeable, ascending limb pumps out Na⁺/Cl⁻.
Counter‑current multiplier – the mechanism that builds the hypertonic medulla.
Distal convoluted tubule (DCT) – fine‑tunes Na⁺, K⁺, Ca²⁺, and pH under hormonal control.
Collecting duct – adjusts water reabsorption; highly permeable to water only when ADH is present.
Antidiuretic hormone (ADH) – increases water permeability of the collecting duct, concentrating urine.
Aldosterone – enhances Na⁺ reabsorption (and K⁺ secretion) in the DCT and collecting duct.
Urea recycling – urea moves from the collecting duct into the medullary interstitium, aiding concentration.
Urine – the final, concentrated waste fluid excreted from the body.
Excretion – the process of eliminating urine from the renal pelvis to the bladder and out through the urethra.