1. Urine Formation by the Kidneys II. Tubular Reabsorption and Secretion
L3-L4-L5

2. Objectives
To understand the principle of Clearance and its applications
To describe the mechanism of tubular reabsorption and secretion.
To follow up the reabsorption of glomerular filtrate along the tubules.
To understand the regulation of tubular reabsorption and secretion.

3. Basic Mechanisms of
Urine Formation

4. Clearance “Clearance” describes the rate at which substances
are removed (cleared) from the plasma.
Renal clearance of a substance is the volume of
plasma completely cleared of a substance
per min by the kidneys.

5. Renal Clearance
Definition: The volume of plasma completely cleared of a substance (x) per minute
Units are ml/min
Clearance,
UX= Concentration of X in the urine (mg/ml)
PX= concentration of X in the plasma (mg/ml)
V= urine flow rate (ml/min)
CInulin – is equal to the GFR.(Glomerular Marker) Inulin is a Fructose polymer ; freely filtered across the membrane and neither reabsorbed nor secreted.

6. Clearance Technique
Renal clearance (Cs) of a substance is the volume of
plasma completely cleared of a substance per min.
Cs x Ps = Us x V
Cs = Us x V = urine excretion rate s
Ps Plasma conc. s
Where : Cs = clearance of substance S
Ps = plasma conc. of substance S
Us = urine conc. of substance S
V = urine flow rate

7. Clearances of Different Substances
Substance Clearance (ml/min)
glucose
albumin
sodium
urea
inulin
creatinine
PAH

8. Use of Clearance to Measure GFR
For a substance that is freely filtered, but not reabsorbed or secreted (inulin, 125 I-iothalamate, creatinine), renal clearance is equal to GFR
amount filtered = amount excreted
GFR x Pin
= Uin x V
GFR =
Pin
Uin x V

9. Calculate the GFR from the following data:
Pinulin = 1.0 mg / 100ml
Uinulin = 125 mg/100 ml
Urine flow rate = 1.0 ml/min
GFR = Cinulin =
Pin
Uin x V
GFR =
125 x 1.0
1.0
= 125 ml/min

10. Use of Clearance to Estimate Renal Plasma Flow
Theoretically, if a substance is completely cleared from the plasma, its clearance rate would equal renal plasma flow
Cx = renal plasma flow

11. Use of PAH Clearance to Estimate Renal Plasma Flow
Paraminohippuric acid (PAH) is freely filtered and secreted and is almost completely cleared from the renal plasma
1. amount enter kidney =
RPF x PPAH
2. amount entered = amount excreted ~
~ 10 % PAH
remains
3. ERPF x PPAH = UPAH x V
ERPF =
UPAH x V
PPAH
ERPF = Clearance PAH

12. To calculate actual RPF , one must correct for incomplete extraction of PAH
APAH =1.0
APAH - VPAH
APAH
EPAH =
= 1.0 – 0.1
1.0
= 0.9
normally, EPAH = 0.9
i.e PAH is 90 % extracted
VPAH = 0.1
RPF =
ERPF
EPAH

13. Calculation of Tubular Reabsorption
Reabsorption = Filtration -Excretion
Filt s = GFR x Ps
Excret s = Us x V

14. Calculation of Tubular Secretion
Secretion = Excretion - Filtration
Filt s = GFR x Ps
VPAH = 0.1
Excret s = Us x V

15. Use of Clearance to Estimate Renal Plasma Flow
Theoretically, if a substance is completely cleared from
the plasma, its clearance rate would equal renal plasma flow
Cx = renal plasma flow

16. Clearances of Different Substances
Substance Clearance (ml/min
inulin
PAH
glucose
sodium
urea
Clearance of inulin (Cin) = GFR
if Cx < Cin : indicates reabsorption of x
if Cx > Cin : indicates secretion of x
Clearance creatinine (Ccreat) ~ 140 (used to estimate GFR)
Clearance of PAH (CPAH) ~ effective renal plasma flow

17. Effect of reducing GFR by 50 % on serum creatinine concentration and creatinine excretion rate

18. Plasma creatinine can be used to estimate changes in GFR
If muscle mass remains constant, creatinine production and therefore creatinine excretion (UCr x V) will be relatively constant. Thus, GFR is directly proportional to the reciprocal of the plasma creatinine concentration. In the clinic, monitoring the reciprocal plasma creatinine concentration over time can be used to evaluate temporal changes in renal function.
Serum creatinine can only be used in patients with stable kidney function. With severe acute renal failure, for example, the GFR is markedly reduced but there has not yet been time for creatinine to accumulate and for the serum creatinine to reflect the degree of renal dysfunction.
Note that there is a large change in magnitude of the GFR and therefore the reciprocal plasma creatinine with small changes in plasma creatinine in patients with relatively normal renal function. Thus, plotting the reciprocal plasma creatinine is not very useful until the plasma creatinine is above 2.0 mg/dL
The production of creatinine differs among and within people over time. For example, individuals with significant variations in dietary intake (vegetarian diet or muscle mass (amputation, malnutrition, muscle wasting) produce different amounts of creatinine.
There are certain settings in which there may be an acute increase in creatinine production. One example is a recent meat meal.

19. Example: Given the following data, calculate the rate
of Na+ filtration, excretion, reabsorption, and secretion
GFR =100 ml/min (0.1 L/min)
PNa = 140 mEq/L
urine flow = 1 ml/min (.001 L/min)
urine Na conc = 100 mEq/L
Filtration Na =
GFR x PNa
= 0.1 L/min x 140 mEq/L = 14 mEq/min
Excretion Na =
Urine flow rate x Urine Na conc
=.001 L/min x 100 mEq/L
= 0.1 mEq/min

20. Example: Given the following data, calculate the rate
of Na+ filtration, excretion, reabsorption, and secretion
GFR =100 ml/min;
PNa = 140 mEq/L
urine flow = 1 ml/min;
urine Na conc = 100 mEq/L
Filtration Na = 0.1 L/min x 140 mEq/L = 14 mEq/min
Excretion Na = .001 L/min x 100 mEq/L = 0.1 mEq/min
Reabsorption Na =
Filtration Na - Excretion Na
Reabs Na = = 13.9 mEq/min
Secretion Na =
There is no net secretion of Na since
Excret Na < Filt Na

21. Reabsorption of Water and Solutes

22. Primary Active Transport of Na+

23. Mechanisms of secondary active transport.

24. Glucose Transport Maximum

25. Transport Maximum
Some substances have a maximum rate of tubular transport
due to saturation of carriers, limited ATP, etc
Transport Maximum: Once the transport maximum is
reached for all nephrons, further increases in tubular
load are not reabsorbed and are excreted.
Threshold is the tubular load at which transport maximum is
exceeded in some nephrons. This is not exactly the same
as the transport maximum of the whole kidney because
some nephrons have lower transport max’s than others.
Examples: glucose, amino acids, phosphate, sulphate

26. Filtered Load of Glucose
A uninephrectomized patient with uncontrolled diabetes has a GFR of 90 ml/min, a plasma glucose of 200 mg% (2mg/ml), and a transport max (Tm) shown in the figure. What is the glucose excretion for this patient?
250
200
150
100 50
Transport
Maximum
(150 mg/min)
1. 0 mg/min
2. 30 mg/min
3. 60 mg/min
4. 90 mg/min
mg/min
Reabsorbed
Glucose
(mg/min)
Excreted .
Threshold
Filtered Load of Glucose
(mg/min)

27. Filtered Load of Glucose
Answer: Filt Glu =
Reabs Glu =
Excret Glu =
(GFR x PGlu) = (90 x 2) = 180 mg/min
Tmax = 150 mg/min
30 mg/min
250
200
150
100 50
Transport
Maximum
(150 mg/min)
GFR = 90 ml/min
PGlu = 2 mg/ml
Tmax = 150 mg/min
Reabsorbed
Glucose
(mg/min)
Excreted
a. 0 mg/min
b. 30 mg/min
c. 60 mg/min
d. 90 mg/min
e. 120 mg/min .
Threshold
Filtered Load of Glucose
(mg/min)

28. Reasorption of Water and Solutes is Coupled to Na+ Reabsorption
Interstitial
Fluid
Tubular
Cells
Tubular
Lumen
- 70 mV
Na +
H +
glucose, amino
acids
Na +
3 Na +
ATP
2 K+
Urea
3Na +
H20
- 3 mV
ATP
Na +
2K+
Cl-
0 mv
- 3 mV

29. Mechanisms by which water, chloride, and urea reabsorption are coupled with sodium reabsorption

30. Transport characteristics of proximal tubule.

31. Changes in concentration in proximal tubule

32. Thank you

33. Transport characteristics of thin and thick loop of Henle.
very permeable to H2O)
Reabsorption of
Na+, Cl-, K+, HCO3-,
Ca++, Mg++
Secretion of H+
not permeable to H2O
~ 25% of filtered load

34. Sodium chloride
and potassium
transport in thick
ascending loop
of Henle

35. Early Distal Tubule

36. Early Distal Tubule Functionally similar to thick ascending loop
Not permeable to water (called diluting segment)
Active reabsorption of Na+, Cl-, K+, Mg++
Contains macula densa

37. Early and Late Distal Tubules and Collecting Tubules.
~ 5% of filtered load
NaCl reabsorbed
not permeable
to H2O
not very
permeable to
urea
permeablility
to H2O depends
on ADH
not very
permeable to
urea

38. Late Distal and Cortical Collecting Tubules Principal Cells – Secrete K+

39. Late Distal and Cortical Collecting Tubules Intercalated Cells –Secrete H+
Tubular Cells
Tubular Lumen
H2O (depends on
ADH)
H + K+
ATP
ATP K+
Na + H+
ATP
ATP K+
ATP
Cl -

40. Transport characteristics of medullary collecting ducts

41. Normal Renal Tubular Na+ Reabsorption
(1789 mEq/d)
5-7 %
(16,614 mEq/day)
65 %
25,560
mEq/d
25 %
(6390 mEq/d)
2.4%
(617 mEq/day)
0.6 %
(150 mEq/day)

42. Concentrations of solutes in different parts of the
tubule depend on relative reabsorption of the
solutes compared to water
If water is reabsorbed to a greater extent than the
solute, the solute will become more concentrated in
the tubule (e.g. creatinine, inulin)
If water is reabsorbed to a lesser extent than the
solute, the solute will become less concentrated in
the tubule (e.g. glucose, amino acids)

43. Changes in concentrations of substances in the
renal tubules

44. The figure below shows the concentrations of inulin at different points along the tubule, expressed as the tubular fluid/plasma (TF/Pinulin) concentration of inulin. If inulin is not reabsorbed by the tubule, what is the percentage of the filtered water that has been reabsorbed or remains at each point ? What percentage of the filtered waterhas been reabsorbed up to that point? A
3.0 B
8.0 C 50
A =
B =
C =
1/3 (33.33 %) remains
66.67 % reabsorbed
1/8 (12.5 %) remains
87.5 % reabsorbed
1/50 (2.0 %) remains
98.0 % reabsorbed

45. Regulation of Tubular Reabsorption
Glomerulotubular Balance
Peritubular Physical Forces
Hormones
- aldosterone
- angiotensin II
- antidiuretic hormone (ADH)
- natriuretic hormones (ANF)
- parathyroid hormone
Sympathetic Nervous System
Arterial Pressure (pressure natriuresis)
Osmotic factors

46. Glomerulotubular Balance
Reabsorption
Tubular Load

47. Importance of Glomerulotubular Balance in Minimizing Changes in Urine Volume
GFR Reabsorption Urine Volume % Reabsorption
no glomerulotubular balance
“perfect” glomerulotubular balance

48. Peritubular capillary reabsorption

49. Peritubular Capillary Reabsorption
Reabs = Net Reabs Pressure (NRP) x Kf
= (10 mmHg) x (12.4 ml/min/mmHg)
Reabs = 124 ml/min

50. Determinants of Peritubular Capillary Reabsorption
Kf Reabsorption
Pc Reabsorption
c Reabsorption

51. Determinants of Peritubular Capillary Hydrostatic Pressure
Glomerular
Capillary
Peritubular
Capillary (Pc) Ra Re
Arterial
Pressure
Arterial Pressure Pc
Reabs. Ra Pc
Reabs. Re Pc
Reabs.

52. Determinants of Peritubular Capillary Colloid OsmoticPressure
c Reabsorption
Plasm. Prot a c Filt. Fract. c
Filt. Fract. = GFR / RPF

53. Factors That Can Influence Peritubular Capillary Reabsorption
Kf Reabsorption
Pc Reabsorption
Ra Pc ( Reabs)
Re Pc ( Reabs)
Art. Press Pc ( Reabs)
c Reabsorption
a c
Filt. Fract. c

54. Effect of increased
hydrostatic pressure
or decreased colloid
osmotic pressure
in peritubular
capillaries to reduce
reabsorption

55. Question
Which of the following changes would tend to increase peritubular reabsorption ?
1. increased arterial pressure
2. decreased afferent arteriolar resistance
3. increased efferent arteriolar resistance
4. decreased peritubular capillary Kf
5. decreased filtration fraction

56. Increases Na+ reabsorption - principal cells
Aldosterone actions on late distal, cortical and medullary collecting tubules
Increases Na+ reabsorption - principal cells
Increases K+ secretion - principal cells
Increases H+ secretion - intercalated cells

57. Late Distal, Cortical and Medullary Collecting Tubules
Principal Cells
Tubular Lumen
H20 (+ ADH)
Na +
ATP K+
Na + K+
Cl -
Aldosterone

58. Abnormal Aldosterone Production
Excess aldosterone (Primary aldosteronism
Conn’s syndrome) - Na+ retention,
hypokalemia, alkalosis, hypertension
Aldosterone deficiency - Addison’s disease
Na+ wasting, hyperkalemia, hypotension

59. Control of Aldosterone Secretion
Factors that increase aldosterone secretion
Angiotensin II
Increased K+
adrenocorticotrophic hormone (ACTH)
(permissive role)
Factors that decrease aldosterone secretion
Atrial natriuretic factor (ANF)
Increased Na+ concentration (osmolality)

60. Angiotensin II Increases Na+ and Water Reabsorption
Stimulates aldosterone secretion
Directly increases Na+ reabsorption
(proximal, loop, distal, collecting tubules)
Constricts efferent arterioles
- decreases peritubular capillary
hydrostatic pressure
increases filtration fraction, which increases
peritubular colloid osmotic pressure)

61. Angiotensin II increases renal tubular sodium reabsorption

62. Effect of Angiotensin II on Peritubular Capillary Dynamics
Glomerular
Capillary
Peritubular
Capillary Ra Re
Arterial
Pressure Re
Pc (peritubular cap. press.)
Ang II
renal blood flow FF c

63. Ang II constriction of efferent arterioles causes Na+ and water retention and maintains excretion of waste products
Na+ depletion
Ang II
Resistance efferent arterioles
Glom. cap. press
Renal blood flow Peritub. Cap. Press.
Prevents decrease
in GFR and
retention of
waste products
Filt. Fraction
Na+ and H2O Reabs.

64. Angiotensin II blockade decreases Na+ reabsorption and blood pressure
ACE inhibitors (captopril, benazipril, ramipril)
Ang II antagonists (losartan, candesartin, irbesartan)
Renin inhibitors (aliskirin)
decrease aldosterone
directly inhibit Na+ reabsorption
decrease efferent arteriolar resistance
Natriuresis and Diuresis Blood Pressure

65. Antidiuretic Hormone (ADH)
Increases H2O permeability and reabsorption in
distal and collecting tubules
Allows differential control of H2O and solute
excretion
Important controller of extracellular fluid
osmolarity
Secreted by posterior pituitary

66. ADH synthesis in the magnocellular neurons of hypothalamus, release by the posterior pituitary, and action on the kidneys

67. Mechanism of action of ADH in distal and collecting tubules

68. Feedback Control of Extracellular Fluid Osmolarity by ADH
Extracell. Osm
(osmoreceptors-
hypothalamus
ADH secretion
(posterior pituitary)
Tubular H2O permeability
(distal, collecting)
H2O Reabsorption
(distal, collecting)
H2O Excretion

69. Abnormalities of ADH Inappropriate ADH syndrome (excess ADH)
- decreased plasma osmolarity, hyponatremia
“Central” Diabetes insipidus (insufficient ADH)
- increased plasma osmolarity, hypernatremia,
excess thirst

70. Atrial natriuretic peptide increases Na+ excretion
Secreted by cardiac atria in response to stretch
(increased blood volume)
Directly inhibits Na+ reabsorption
Inhibits renin release and aldosterone formation
Increases GFR
Helps to minimize blood volume expansion

71. Atrial Natriuretic Peptide (ANP)
Blood volume
ANP
Renin release aldosterone GFR
Ang II
Renal Na+ and H2O reabsorption
Na+ and H2O excretion

72. Parathyroid hormone increases renal Ca++ reabsorption
Released by parathyroids in response to
decreased extracellular Ca++
Increases Ca++ reabsorption by kidneys
Increases Ca++ reabsorption by gut
Decreases phosphate reabsorption
Helps to increase extracellular Ca++

73. Control of Ca++ by Parathyroid Hormone
Extracellular
[Ca++]
Vitamin D3
Activation
PTH
Intestinal Ca++
Reabsorption
Renal Ca++
Reabsorption
Ca++ Release
From Bones

74. Sympathetic nervous system increases Na+ reabsorption
Directly stimulates Na+ reabsorption
Stimulates renin release
Decreases GFR and renal blood flow
(only a high levels of sympathetic
stimulation)

75. Increased Arterial Pressure Decreases Na+ Reabsorption (Pressure Natriuresis)
Increased peritubular capillary hydrostatic
pressure
Decreased renin and aldosterone
Increased release of intrarenal natriuretic factors
- prostaglandins
- EDRF

76. Osmotic Effects on Reabsorption
Water is reabsorbed only by osmosis
Increasing the amount of unreabsorbed solutes
in the tubules decreases water reabsorption
i.e. diabetes mellitus : unreabsorbed glucose in
tubules causes diuresis and water loss
i.e. osmotic diuretics (mannitol)

77. Abnormal Tubular Function : Increased Reabsorption
Conn’s Syndrome: primary aldosterone excess
Glucocorticoid Remediable Aldosteronism (GRA):
excess aldosterone secretion due to abnormal
control of aldosterone synthase by ACTH
(genetic)
Renin secreting tumor: excess Ang II formation
Inappropriate ADH syndrome: excess ADH
Liddle’s Syndrome: excess activity of amiloride
sensitive Na+ channel (genetic)

78. “Escape” from Sodium Retention During Excess Aldosterone Infusion
130
Mean
Arterial
Pressure
(mmHg)
100
Urinary
sodium
Excretion
(x normal) 1
Time (days)

79. Conn’s syndrome: (primary aldosteronism)
Na+ reabsorption (distal & coll. tub.)
Na+ excretion (in steady-state)
K+ secretion (transient)
K+ excretion (in steady-state)
plasma K+
blood pressure
- plasma renin

80. Abnormal Tubular Function:
Renin secreting tumor : (increased angiotensin II + increased aldosterone)
Na+ reabsorption (distal & coll. tub.)
Na+ excretion (in steady-state)
K+ secretion (transient)
K+ excretion (in steady-state)
plasma K+
blood pressure
- aldosterone

81. Abnormal Tubular Function:
Inappropriate ADH syndrome:
water reabsorption
water excretion (urine volume)
plasma Na+

82. Liddle’s Syndrome: Excess Activity of Amiloride Sensitive Na+ Channel in Late Distal and Cortical Collecting Tubules
Tubular Lumen
Tubular Cells
H20 K+
Na +
Na +
ATP
ATP
Na+ channel blockers:
- Amiloride
- Triamterene
Treatment K+
Cl -

83. Abnormal Tubular Function:
Liddle’s syndrome: (excess Na+ channel activity in late distal and collecting)
Na+ reabsorption (distal & coll. tub.)
Na+ excretion (in steady-state)
blood pressure
plasma renin
aldosterone

84. Assessing Kidney Function
Plasma concentration of waste products
(e.g. BUN, creatinine)
Urine specific gravity, urine concentrating ability;
Urinalysis test reagent strips (protein, glucose, etc)
Biopsy
Albumin excretion (microalbuminuria)
Isotope renal scans
Imaging methods (e.g. MRI, PET, arteriograms,
iv pyelography, ultrasound etc)
Clearance methods (e.g. 24-hr creatinine clearance)
etc

85. Thank you