(5)Because there is a high permeability to water and little permeability to urea and sodium ions in thin segment of descending limb,water is suctioned by osmosis and the tubular fluid reaches equilibrium with the surrounding interstitial fluid of the renal medulla,which is very hypertonic. 2.Countercurrent exchange in the vasa recta preserves hyperosmolarity of the renal medulla. 3.Process ofurinary concentrationand dilution (1)Concentrated urine:When body is in lack of water,the crystal osmotic pressure increases,and this promotes the release of ADH,thus,the permeability to water in collecting duct increases,the water in tubule is reabsorbed into the interstitial,so it forms concentrated urine. (2)Diluted urine:When there is a large excess of water in the body,the crystal osmotic pressure will decrease,and this reduces the release of ADH,so the collecting duct is impermeable to water,and the sodium ion is reabsorbed in distal tubule,all these lead to the tubular osmosis pressure decreasing and forming dilute urine. Seaction4 Factors Affecting Urine Formation 1.Factors affecting glomerular filtration (1)Effective filtrationpressure changes: 1 Glomerular capillary pressure ②Capsular pressure 3 Plasma colloid osmotic pressure (2)The permeability and area ofglomerular filtration membrane changes(5) Because there is a high permeability to water and little permeability to urea and sodium ions in thin segment of descending limb, water is suctioned by osmosis and the tubular fluid reaches equilibrium with the surrounding interstitial fluid of the renal medulla, which is very hypertonic. 2. Countercurrent exchange in the vasa recta preserves hyperosmolarity of the renal medulla. 3. Process of urinary concentration and dilution (1) Concentrated urine: When body is in lack of water, the crystal osmotic pressure increases, and this promotes the release of ADH, thus, the permeability to water in collecting duct increases, the water in tubule is reabsorbed into the interstitial, so it forms concentrated urine. (2) Diluted urine: When there is a large excess of water in the body, the crystal osmotic pressure will decrease, and this reduces the release of ADH, so the collecting duct is impermeable to water, and the sodium ion is reabsorbed in distal tubule, all these lead to the tubular osmosis pressure decreasing and forming dilute urine. Seaction4 Factors Affecting Urine Formation 1. Factors affecting glomerular filtration (1) Effective filtration pressure changes: ① Glomerular capillary pressure ② Capsular pressure ③ Plasma colloid osmotic pressure (2) The permeability and area of glomerular filtration membrane changes