Urine formation and micturition

process of urine formation and micturition

1. URINE FORMATION AND MICTURITION By Soni Kumari Shah

2. Urine Formation Process of blood cleansing Urine is formed by filtering the blood at renal corpuscle and reabsorption of necessary substances in renal tubule. Mechanism Of Urine Formation Glomerular filtration Tubular reabsorption Tubular secretion

3. Glomerular Filtration Takes place through semi permeable membrane of glomerulus and glomerular capsule. Water and large number of small molecules pass through Blood cells, plasma protein and other large molecules are unable to filter so remain in the capillaries. Filtration is assisted by the differences between blood pressure in glomerulus and pressure of filtrate in glomerular capsule.

4. Because the efferent arteriole is narrower than the afferent arteriole, a capillary hydrostatic pressure of about ( 55 mmHg) builds up in the glomerulus. This pressure is opposed by the osmotic pressure of the blood , provided mainly by plasma proteins, about (30 mmHg), and by filtrate hydrostatic pressure of about (15 mmHg) in the glomerular capsule. The net filtration pressure is, therefore: Glomerular filtration pressure = capillary hydrostatic pressure – (osmotic pressure of blood + filtrate hydrostatic pressure) GFP= 55 – (30+15) = 10mmHg

5. Glomerular Filtration Rate (GFR) Volume of filtrate formed by both kidney in one minute is GFR. GFR=125ml/min. i.e. 180 litres Most of the filtrate is reabsorbed. Less than 1% i.e. 1-1.5 litres is excreted as urine.

6. Tubular Reabsorption/Selective Reabsorption Process by which composition and volume of glomerular filtrate is altered during passage through renal tubule. Reabsorb into blood those filtrate constituents needed by the body to maintain fluid and electrolyte balance and pH balance. Reabsorption is achieved by active transport, passive transport, osmosis and diffusion. Some constituents of glomerular filtrate do not appear normally in urine because they are completely reabsorbed unless they are present in blood in excessive quantities. PCT: glucose, amino acid, vitamin, hormone, calcium, and some urea PCT, LH and DCT: 99% water, Na + , Cl - ions

7. Renal Threshold/ Transport Maximum Kidneys maximum capacity for reabsorption of a substance. If the level rises above the transport maximum, the substance will appear in urine. Hormones Regulating Reabsorption: Parathyroid hormone and calcitonin regulates reabsorption of calcium and phosphate. Antidiuretic hormone(ADH) increases permeability of DCT and collecting tubule, increasing water reabsorption. Aldosterone increases reabsorption of sodium and excretion of potassium. Atrial natriuretic peptide(ANP) inhibits NaCl reabsorption.

8. Tubular Secretion Substances not required and foreign materials eg . Drugs including penicillin and aspirin may not be cleared from the blood by filtration because of short time it remains in the glomerulus. Such substances are cleared by secretion into the convoluted tubules and excreted from the body in urine. Tubular secretion of hydrogen ion (H + ) is important in maintaining homeostasis of blood pH.

9. The normal GFR is 50 ml per min. 125 ml per min. 250 ml per min. 500 ml per min. Normal daily urine output of an adult is 500ml 1000ml 1500ml 2500ml In kidney glucose is mainly absorbed by PCT Loop of Henle DCT Collecting tubule Volume of urine is primarily controlled by ADH or Vasopressin Oxytocin ACTH Growth Hormone

10. Formation Of Uric Acid In the liver, purines are metabolized to uric acid. The last step in purine metabolism involves an enzyme called xanthine oxidase.

11. Formation Of Urea

12. Urine Clear amber colored fluid of slight acidic reaction excreted by kidney. pH : around 6 (normal range: 4.5-8) Color : light yellow Volume excreted : 1-1.5 liters per day The volume depends on: i . Amount of water intake ii. Environmental temperature iii. Diet iv. Mental state Odour : faintly aromatic Specific gravity : 1.020 – 1.030

13. Composition Of Urine Water - 96 % Urea - 2 % Uric acid Creatinine Ammonia Sodium Potassium 2 % Chlorides Phosphates Sulphates Oxalates

14. Micturition Process of emptying of bladder Also called voiding Structures Involved In Micturition: Urinary bladder (detrusor muscle) Urethra (sphincter muscle) Parasympathetic nerves supplying urinary bladder and urethra

15. Mechanism Of Micturition In infants , accumulation of urine in the bladder activates stretch receptors in the bladder wall generating sensory (afferent ) impulses that are transmitted to the spinal cord, where a spinal reflex is initiated. This stimulates involuntary contraction of the detrusor muscle and relaxation of the internal urethral sphincter, and expels urine from the bladder – this is micturition or voiding of urine. In adult , micturition occurs when detrusor muscle contracts, and there is reflex relaxation of internal sphincter and voluntary relaxation of external sphincter.

16. Micturition

17. Stimulation of parasympathetic nervous system Retains urine in bladder Helps voiding of urine Neither B oth Normal specific gravity of urine is 0.980 1.000 1.010 1.020

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