This ppt only for study purposes

1. Mechanism of
Urine formation -1
Dr. Pandian M.
Dept. of Physiology

2. Introduction
• The main function of the kidneys is to clear waste
products from the blood and excrete them in the urine.
• The kidneys accomplish their excretory function by
the formation of urine.
• Processes concerned with urine formation.

3. Three processes are involved in the urine
formation :
1. Glomerular filtration,
2. Tubular reabsorption and
3. Tubular secretion.

4. • The four basic components of
renal function.
• This figure is to illustrate only
the directions of reabsorption
and secretion, not specific
sites or order of occurrence.
• Depending on the particular
substance, reabsorption and
secretion can occur at various
sites along the tubule.

5. Renal handling of three hypothetical filtered substances X, Y, and Z.
• X is filtered and secreted but not reabsorbed.
• Y is filtered, and a fraction is then reabsorbed.
• Z is filtered and completely reabsorbed.
The thickness of each line in this hypothetical example suggests the magnitude of the process.

6. Three processes are involved in the urine
formation :
1. Glomerular filtration,
2. Tubular reabsorption and
3. Tubular secretion.

7. 1. GLOMERULAR FILTRATION
•Glomerular filtration refers to the process of
ultrafiltration of plasma from the glomerular
capillaries into the Bowman’s capsule.

8. The understanding of process of glomerular
filtration involves:-
A. Characteristics of filtration membrane,
B. Composition of glomerular filtrate,
C. Dynamics of glomerular filtration,
D. Glomerular filtration rate,
E. Filtration fraction,
F. Factors affecting glomerular filtration and
G. Measurement of glomerular filtration.

9. A. CHARACTERISTICS OF FILTRATION
MEMBRANE
•The filtration membrane consists of three
layers:
a.Capillary endothelium,
b.Glomerular basement membrane (GBM) &
c.Bowman’s visceral epithelium (podocytes).

10. Continue…..
•High permeability - The glomerular membrane is
highly permeable to water and 100% dissolved
substances because of its porous nature.
•Permeability selectivity - The filtration membrane
exhibits a high degree of permeability selectivity
based on two factors, i.e. pore size and electrical
charge of the molecule.

11. a. Pore size. The capillary endothelial cells 70−90 nm
in diameter,
• The GBM has no pores but its permeability
corresponds to pore size of 8 nm.
– Molecules less than 4 nm in size are freely
filtered.
– Molecules with diameter more than 8 nm are not
filtered at all (i.e. zero permeability).
– Filtration of molecules having diameter
between 4 and 8 nm is inversely proportional to their
diameter.
Continue…..

12. b. Electrical charge - The pores in the filtration membrane
are negatively charged due to the presence of glycoproteins
rich in sialic acid (sialo proteins).
•Thus, with the same molecular size, compared to anionic
particles,
•There is, in order, increasing permeability for neutral and
cationic particles.
•This explains why albumin (with a molecular diameter of 7
nm but a negative charge) is not filtered
Continue…..

13. Effect of electrical charge and effective molecular diameter on
filterability of dextran molecule through GFM.
A value of one indicates that it is filtered freely, whereas a value
of zero indicates that it is not filtered.
Continue…..

14. The understanding of process of glomerular
filtration involves:-
A. Characteristics of filtration membrane,
B. Composition of glomerular filtrate,
C. Dynamics of glomerular filtration,
D. Glomerular filtration rate,
E. Filtration fraction,
F. Factors affecting glomerular filtration and
G. Measurement of glomerular filtration.

15. What is the composition of glomerular
filtrate?
•Glomerular filtrate has almost the
same composition as that of plasma
except that it has no significant
amount of proteins.

16. The understanding of process of glomerular
filtration involves:-
A. Characteristics of filtration membrane,
B. Composition of glomerular filtrate,
C. Dynamics of glomerular filtration,
D. Glomerular filtration rate,
E. Filtration fraction,
F. Factors affecting glomerular filtration and
G. Measurement of glomerular filtration.

17. C. Explain dynamics of filtration through the
glomerular membrane.
The forces which cause filtration of fluid through
glomerular capillary are:
•Pressure inside the glomerular capillaries.
• It is 60 mmHg.
•Colloid osmotic pressure of the proteins in the
Bowman’s capsule.
•Very little protein is actually filtered. Therefore, this
factor is not very significant.

18. • The forces which oppose filtration are:
(a) Colloid osmotic pressure of plasma proteins.
• It varies from 28 to 36 mmHg as blood passes through capillaries.
• So, average colloid osmotic pressure is about 32 mmHg.
(b) Hydrostatic pressure in the fluid of Bowman’s capsule.
* It is about 18 mmHg.
• Normal filtration pressure therefore is equal to glomerular pressure
minus sum of colloid osmotic pressure of blood in glomerular
capillaries and capsular pressure which is equal to 60 − (32 + 18)
mmHg = 10 mmHg.
• Filtration pressure (10 mmHg) is the net pressure forcing fluid
through the glomerular membrane.

19. GFR = Kf [(PGC − PBS) − (πGS − πBS)],
Depiction of the Starling forces across the
glomerular filtration membrane.
(PGC = glomerular capillary hydrostatic
pressure;
PBS = Bowman’s space hydrostatic pressure;
πGC = glomerular capillary oncotic pressure;
πBS = Bowman’s space oncotic pressure.)

20. GFR = Kf [(PGC − PBS) − (πGS − πBS)],

21. The understanding of process of glomerular
filtration involves:-
A. Characteristics of filtration membrane,
B. Composition of glomerular filtrate,
C. Dynamics of glomerular filtration,
D. Glomerular filtration rate,
E. Filtration fraction,
F. Factors affecting glomerular filtration and
G. Measurement of glomerular filtration.

22. D. What is the normal glomerular
filtration rate (GFR)?
•In a normal person, quantity of glomerular
filtrate formed each minute by all the
nephrons of both the kidneys is 125 ml.
•Total quantity of glomerular filtrate formed
per day is equal to 180 liter.
GFR=K1 X Net filtration pressure
GFR = Kf [(PGC − PBS) − (πGS − πBS)],

23. The understanding of process of glomerular
filtration involves:-
A. Characteristics of filtration membrane,
B. Composition of glomerular filtrate,
C. Dynamics of glomerular filtration,
D. Glomerular filtration rate,
E. Filtration fraction,
F. Factors affecting glomerular filtration and
G. Measurement of glomerular filtration.

24. E. What is filtration fraction?
•Filtration fraction is the fraction of renal plasma
flow that becomes glomerular filtrate.
•Normal plasma flow to both the kidneys is 650
ml/min and
•rate of glomerular filtration is 125 ml/minute.
•So, an average filtration fraction is
approximately 1/5th or 19%.
Filtration fraction = GFR/ Renal plasma flow

25. What is filtration coefficient?
•Filtration coefficient is glomerular filtration rate
of both the kidneys per millimeter of mercury of
filtration pressure.
•Normal filtration coefficient is therefore 12.5
ml/min/mmHg of filtration pressure (125 ml/10
mm where 125 ml/min is GFR and
•10 mmHg is net fi ltration pressure). It is termed
K1.

26. The understanding of process of glomerular
filtration involves:-
A. Characteristics of filtration membrane,
B. Composition of glomerular filtrate,
C. Dynamics of glomerular filtration,
D. Glomerular filtration rate,
E. Filtration fraction,
F. Factors affecting glomerular filtration and
G. Measurement of glomerular filtration.

27. F. Enumerate the factors affecting GFR.
Various factors affect GFR in the following way:
1. Filtration coefficient (K1).
2. Hydrostatic pressure in Bowman’s capsular
fluid.
3. Glomerular capillary hydrostatic pressure.
4. Glomerular capillary colloid osmotic
pressure.
5. Sympathetic stimulation.

28. The understanding of process of glomerular
filtration involves:-
A. Characteristics of filtration membrane,
B. Composition of glomerular filtrate,
C. Dynamics of glomerular filtration,
D. Glomerular filtration rate,
E. Filtration fraction,
F. Factors affecting glomerular filtration and
G. Measurement of glomerular filtration.

29. G. MEASUREMENT OF GRF.
•Glomerular filtration rate can be measured by the
renal clearance of inulin, urea and creatinine.
•The renal clearance can be defined as volume of
plasma,
•i.e. cleared of substance in 1 min by excretion of the
substance in urine.

30. It’s calculated by the following formula:

31. Describe afferent arteriolar vasodilation feedback
mechanism for regulating GFR.
This negative feedback
mechanism keeps GFR
steady.
It also auto regulates
renal blood flow.

32. Describe efferent arteriolar vasoconstrictor mechanism for
regulating GFR

33. What is the effect of arterial pressure on urinary output?
• There is almost no change in GFR when arterial pressure varies in the
range of 75 to 160 mmHg (normal mean arterial pressure is 100
mmHg).
• When arterial pressure falls below 75 mmHg, it decreases urine
output.
• A pressure of about 50 mmHg causes almost complete cessation of
urinary output.
• When arterial pressure rises above 200 mmHg (double normal), urine
output increases about sevenfold.

34. Referred :-
• Text book of Medical Physiology
• Guyton, 14th edition,
• Text book of Medical Physiology
• Indu khurana,
• Text book of Medical Physiology
• Vander’s
• Text book of Medical Physiology
• Sembulingam &
• LPR

35. THANK YOU . . .