MECHANISM OF URINE FORMATION -

Production of urine is uripoisis or diuresis. First of all it was explained by Kushney.

Nephron separates acidic urine from basic blood, so maintain acid base balance i.e. Homeostasis (w. cannon).

Process is completed in three stpes -

1.      ULTRAFILTRATION-

1. Blood along with waste products come in glomerulus by afferent arteriole.
2. Concentration of urea is more in afferent arteriole. By efferent arterioles only blood moves out.
3. Diameter of afferent arteriole is more than efferent one. So blood stops in glomerulus for little time.
4. Here +70 mm Hg GHP (glomerulur hydrostatic pressure) is exerted.
5. It is balanced by BCOP & CHP.
6. B.C.O.P. (Blood colloidal osmotic pressure) is due to blood proteins. It is - 30 mm Hg.
7. CHP (Capsular hydrostatic pressure) is of -20 mm Hg by nephric filtrate of Bowmann's capsule.
8. So at +20 mm Hg EFP (Effective filtration pressure) such elements having low molecular weight are filtered in glomerulus.
9. Nephric filtrate is protein less plasma, it is isotonic. Both kidney form 130 ml filtrate / minute.
10. According to Ludwig process is due to change in pressure.

2.      SELECTIVE REABSORPTION -

1. In nephric filtrate some essential chemicals are also present as H O, glucose, minerals, A.A.
2. 90% of nephric filtrate is reabsorbed. These essential chemicals again reach in blood.
3. Their reabsorption is based on Threshold value. i.e. needed chemicals.
4. In PCT 80% H₂O is reabsorbed by passive transport. It is obligatory absorption.
5. Rest water is absorbed in DCT under control of ADH. It is facultative absorption.
6. In PCT glucose, amino acid, minerals are absorbed by diffusion. In PCT solution remains isotonic.
7. Positive ions are absorbed by active transport.
8. Negative ions are absorbed by passive transport.
9. In descending loop H₂O is absorbed so solution becomes hypertonic.
10. In ascending loop NaCl is given out so solution becomes isotonic.
11. In DCT again NaCl is absorbed so solution becomes hypotonic. It comes in duct of Bellini.

3.      SECRETION -

1. Richard proved it by experiment. Renal tubule directly discharges heavy waste products in DCT.
2. It is active process. These chemicals are mainly creatine, pera amino hippuric acid.