1. The Urinary System: Fluid and Electrolyte Balance

2. Exchanges Affecting Plasma Content
Figure 19.1
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3. Concept of Balance Figure 19.2
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4. Kidneys in Maintaining Balance
Water
Sodium
Potassium
Calcium
Acid-base
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5. Exchange of Water Intake Output Gastrointestinal tract Metabolism
Insensible loss
Sweating
Kidneys
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6. Factors Affecting Water Balance
Note: Kidneys can only minimize fluid loss
Intake is required in order to add fluids
Figure 19.3
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7. Osmolarity of Fluids Osmolarity of body fluids = 300 mOsm
No osmotic force for water to move between fluid compartments
Kidneys compensate for changes in osmolarity of extracellular fluid by regulating water reabsorption
Water reabsorption passive
Based on osmotic gradient
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8. Water Reabsorption Proximal tubules
70% filtered water is reabsorbed
Not regulated
Distal tubules and Collecting ducts
Most remaining water is reabsorbed
Regulated by ADH
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9. Sodium Reabsorption Figure 19.4
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10. Sodium Reabsorption
Na+ is actively transported across basolateral membrane, this establishes an osmotic gradient for water reabsorption
Figure 19.4
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11. Water Reabsorption Figure 19.5
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12. The Medullary Osmotic Gradient
Figure 19.6
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13. Counter-Current Multiplier in the Loop of Henle
Osmotic gradient established by counter-current multiplier
Dependent on loop of Henle
Ascending Limb
Impermeable to water
Active transport of Na+, Cl-, and K+
Descending Limb
Permeable to water
No transport of Na+, Cl-, or K+
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14. Counter-Current Multiplier
Cortex
Medulla
No osmotic
gradient
(mOsm)
Fluid
enters
tubule
Water moves
out of descending
limb by osmosis
Active transport
of Na+, Cl–, K+ ions
into medullary
interstitial fluid
increases
osmolarity
Peritubular fluid
Tubular fluid
Iso-osmotic state in
descending limb;
osmotic difference
between descending
and ascending limbs
More fluid
enters tubule,
pushing fluid
through by
bulk flow
Active
transport of
Na+, Cl–, K+
ions into
medullary
interstitial
fluid increases
300
Na+
Cl– K+
400
200
H2O
Osmotic
established
Iso-osmotic state in descending
limb; osmotic difference
between descending and
ascending limbs
More water
enters tubule
and process
continues
System
is in steady
state
350
150
500
100
900
700 1
1200
1100
1400 2 3 4 5 6 7
Figure 19.7
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15. Vasa Recta: Countercurrent Exchanger
Figure 19.8
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16. Regulated Water Reabsorption
When membrane of late DCT and CD is impermeable to water
Water cannot leave the tubules
No water reabsorption
More water is excreted in urine
Figure 19.9a
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17. Regulated Water Reabsorption
ADH stimulates the insertion of water channels (aquaporin-2) into apical membrane
Water is reabsorpbed by osmosis
Maximum urine concentration is 1400 mOsm
Figure 19.9b
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18. Minimum volume of water that must be excreted in the urine per day
Obligatory Water Loss
Minimum volume of water that must be excreted in the urine per day
Max osmolarity urine = 1400 mOsm
Some solute must be excreted
Minimum water loss = 440 mL
Obligatory water loss
Necessary to eliminate non-reabsorbed solutes
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19. Effects of ADH on Principal Cells
Figure 19.10
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20. Response: Increase in ECF Osmolarity
Figure 19.11
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21. Response: Decrease in Blood Volume
Figure 19.12
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22. Proximal Tubule Sodium Reabsorption
Coupled to the reabsorption of other solutes
Figure 19.13a
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23. Distal Tubule Sodium Reabsorption
Coupled to the secretion of K+ and H+
Figure 19.13b
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24. Effects of Aldosterone
Figure 19.14
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25. Renin-Angiotensin-Aldosterone
Figure 19.15
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26. Angiotensin II Effect on MAP
Adrenal cortex
Systemic arterioles
Posterior pituitary
Hypothalamic neurons
Aldosterone secretion
Sodium reabsorption
in late distal tubules
and collecting ducts
Water reabsorption
Extracellular fluid
osmolarity
Thirst stimulation
ADH secretion
Vasoconstriction
MAP
Plasma volume
Kidneys
Angiotensin II
Figure 19.16
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27. Stimulation of Renin Release
Baroreceptor reflex
Sympathetic activity
Afferent arteriole
pressure
GFR
[Na+] and [Cl–]
in distal tubules
Macula densa
Renin release
Juxtaglomerular cells
of afferent arteriole
MAP
Paracrine
secretion
Figure 19.17
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28. Role of Atrial Natriuretic Peptide
Figure 19.18
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29. PCT: Potassium Reabsorption
Figure 19.19a
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30. DCT: Potassium Secretion
Figure 19.19b
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31. Routes of Calcium Exchange
Figure 19.20
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32. PTH and Calcium Balance
[PTH] in plasma
PTH secretion
[1,25-(OH2)D3] in plasma
Ca2+ resorption
Phosphate reabsorption
Calcium excretion
in urine
Ca2+ reabsorption
1,25-(OH2)D3 activation
Parathyroid glands
Kidneys
Ca2+ absorption
Gastrointestinal tract
Bone
[Ca2+] in plasma
Negative
feedback
Figure 19.21
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33. Role of Vitamin D Figure 19.22
Conversion of
vitamin D3
to 25-OH D3
Vitamin D3 absorption
7-dehydrocholesterol
Sunlight
[Vitamin D3] in plasma
[25-OH D3]
in plasma
[1,25-(OH2)D3] in plasma
Conversion of 25-OH D3
to 1,25-(OH2)D3
Liver
Kidney
Ca2+ absorption
Kidneys
Gastrointestinal tract
[Ca2+] in plasma
[PTH] in
plasma
[Ca2+] in
Skin
Negative
feedback
Figure 19.22
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34. Inputs and Outputs of Acid
Figure 19.23
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35. Response to Decrease in pH
Figure 19.24
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36. Bicarbonate Reabsorption
Figure 19.25
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37. Hydrogen Ion Secretion
Figure 19.26
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38. Glutamine Metabolism in the PCT
Figure 19.27
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39. Metabolic Alkalosis Cause: decreased H+ independent of CO2
Compensation: respiratory and renal (unless renal problem)
Respiratory compensation
Decrease ventilation  increase CO2
Renal compensation
Decrease H+ secretion
Decrease HCO3- reabsorption
Decrease synthesis of new bicarbonate
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40. Acid-Base Disturbances/Compensation
Arterial pH
Acidosis
pH < 7.35
Alkalosis
pH > 7.45
PCO 2
< 40 mm Hg
Respiratory
compensation
Metabolic acidosis
[HCO3–] < 24 mM
[HCO3–] > 24 mM
Renal
Respiratory acidosis
> 40 mm Hg
Metabolic alkalosis
Respiratory alkalosis OR
Figure 19.28
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