The document summarizes urine formation and micturition. Urine is formed through glomerular filtration in the kidneys where water and small molecules pass through the glomerular membrane into the filtrate. Most of the filtrate is reabsorbed in the renal tubules, with less than 1% excreted as urine. Micturition is the process of emptying the bladder through contraction of the detrusor muscle and relaxation of the internal urethral sphincter.

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
1. Glomerular filtration
2. Tubular reabsorption
3. 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.

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

11. 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.

12. Formation Of Urea

13. 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

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

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

16. 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.

17. Micturition

18. Stimulation of parasympathetic nervous system
i. Retains urine in bladder
ii. Helps voiding of urine
iii. Neither
iv. Both
Normal specific gravity of urine is
i. 0.980
ii. 1.000
iii. 1.010
iv. 1.020

19. Thankyou