Urinary system

1. MECHANISM OF URINE
FORMATION
KHIRENDRA CHOUDHARY
LECTURER
Department of Physiology 1

2. Urine Formation
Urine passes through the collecting ducts to the renal papillae
Then to the minor and major calyces
Out the renal pelvis to the ureters
To the bladder
To the urethra and out of the body

3. Mechanism of Urine formation
• Urine formation
involves three
major processes
1. Glomerular
filtration
2. Tubular
reabsorption
3. Secretion
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4. Glomerular Filtration
Ultra filtration of plasma in the glomerulus
Governed by 2 major factors:
1. Filtration coefficient (Kf)
2. Pressure gradient/ Starling forces (hydrostatic and osmotic
pressure gradients) 4

5. GLOMERULAR FILTRAION RATE
It is the amount of plasma filtered by all nephrons of
both the kidney in a unit time i.e. per min or per day
125ml/min or 180 lit /day
An important measurement in the evaluation of
kidney function
The kidney filter in one day an amount equal to-4
times the body water-15 times the ECF-60 times the
plasma volume.
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6. DYNAMICS OF FILTERATION
NFP= sum of opposing forces and favouring forces
Favouring forces= hydrostatic pressure in glomerular
capillary ( PGC=50)
colloid osmotic pressure in
bowmans space (ᴫBS=0)
Opposing forces = Glomerular capillary oncotic pressure
(ᴫGC= 30)
hydrostatic pressure in bowmans space
(PBS =10)
NFP= favoring forces – opposing forces = ( 50+0) – (30 +10)
= 10 mm of Hg
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GFR NFP KF

7. Filterartion coefficient ( Kf)
KF = permeability total surface area
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NFP=10 NFP=10
NFP=10
NFP=10
2
1
1
2

8. FILTRATION FRACTION
Filtration fraction is the fraction (portion) of the renal
plasma, which becomes the filtrate.
It is the ratio between glomerular filtration rate &
renal plasma flow . It is expressed in percentage.
Filtration fraction = GFR/Renal plasma flow× 100
125 mL/min / 650 mL/min × 100
= 19.2%.
Normal filtration fraction varies from 15% to 20%.
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9. FACTORS AFECTING GFR
1) Renal Blood Flow
2) Glomerular Capillary Pressure
3) Colloidal Osmotic Pressure
4) Hydrostatic Pressure in Bowman Capsule
5) Constriction of Afferent Arteriole
6) Constriction of Efferent Arteriole
7) Systemic Arterial Pressure
8) Sympathetic Stimulation
9) Surface Area of Capillary Membrane
10) Permeability of Capillary Membrane
11) Contraction of Glomerular Mesangial Cells
12) Hormonal and Other Factors
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10. Tubular Reabsorption
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Tubular reabsorption is known as selective reabsorption because the
tubular cells reabsorb only the substances necessary for the body.
Essential substances such as glucose, amino acids and vitamins are
completely reabsorbed from renal tubule. Whereas the unwanted
substances like metabolic waste products are not reabsorbed and
excreted through urine.

11. Tubular Reabsorption
Reabsorption of the substances occurs in almost all the
segments of tubular portion of nephron.
1. Substances Reabsorbed from ProximalConvoluted Tubule
About 7/8 of the filtrate (about 88%) is reabsorbed in proximal
convoluted tubule. The brush border of epithelial cells in
proximal convoluted tubule increases the surface area and
facilitates the reabsorption.
Substances reabsorbed are glucose, amino acids, sodium,
potassium, calcium, bicarbonates, chlorides, phosphates,
urea, uric acid and water.
2. Substances Reabsorbed from Loop of Henle
sodium and chloride.
3. Substances Reabsorbed from Distal Convoluted Tubule
Sodium, calcium, bicarbonate and water are reabsorbed
from distal convoluted tubule
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12. Tubular secretion
Tubular secretion is the process by which the
substances are transported from blood into
renal tubules. It is also called tubular excretion
Substances secreted are
• 1. Paraaminohippuric acid (PAH)
• 2. Diodrast
• 3. 5hydroxyindoleaceticacid (5HIAA)
• 4. Amino derivatives
• 5. Penicillin.
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13. Summary
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