1. Body water
Fluid and electrolyte balance
© Department of Biochemistry, 2011 (J.D.)
1

2. Average total body water (TBW) as body weight percentage
Age Males Both Females
0 - 1 month 76
1 - 12 months 65
1 - 10 y 62
10 - 16 y 59 57
17 - 39 y 61 50
40 – 59 y 55 47
60 y and older 52 46
The water content of the body changes:
with age - about 75 % in the newborn, usually less than 50 % in the elders,
with the total fat (10 % H2O) and muscle (75 % H2O) content,
a lean person has high TBW, an obese person has low TBW 2

3. Total body water compartments
extracellular fluid ECF ICF intracellular fluid
ISF
1/3 2/3
interstitial fluid (and lymph)
IVF
blood plasma
3

4. Quantification of body fluid compartments
(in adult male, 70 kg)
Percentage of
Fluid Volume (Liters)
Body fluid Body weight
TBW 42 100 % 60 %
ICF 28 67 % 40 %
ECF 14 33 % 20 %
ISF 10.5 25 % 15 %
IVF 3.5 8% 5%
(blood plasma)
The contribution of lymph and transcellular fluid is negligible.
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5. Transcellular fluids
are secreted by specialized cells into particular body cavities
• cerebrospinal fluid
• intraocular fluid
• synovial fluid
• pericardial, intrapleural, peritoneal fluids
• digestive juices
• bile
• urine
they are usually ignored in fluid balance
Exceptions:
heavy vomiting or diarrhea, ascites or exudates may cause fluid imbalances
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6. Daily water balance (average data, 70 kg male)
Water input Water output
beverages 1200 ml insensible (skin, lungs) 800 ml
food 1000 ml sweat 100 ml
metabolic water 300 ml feces 100 ml
urine 1500 ml
total input 2500 ml total output 2500 ml
Attention should be paid to water intake in children:
– they have higher metabolic rate and large body surface (per body weight)
– they are more sensitive to water depletion
in the elders: the sensation of thirst is often impaired or lacking
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7. Metabolic water 100 g of glucose → 55 ml
100 g of protein → 41 ml
100 g of fat → 107 ml
generally:
nutrients + O2 → CO2 + H2O + chemical energy + heat
Examples of metabolic dehydration (1,3) or condensation (2) reactions:
• glycolysis: 2-P-glycerate → H2O + phosphoenolpyruvate
• glutamine synthesis: glutamate + NH3 + ATP → H2O + glutamine + ADP + Pi
• FA synthesis:
R-CH(OH)-CH2-CO-S-ACP → H2O + R-CH=CH-CO-S-ACP
7

8. Average ion concentrations in blood plasma, ISF, and ICF (mmol/l)
Ion Blood plasma ISF ICF
Na+ 142 142 10
K+ 4 4 155
Ca2+ 2.5 1.3 traces
Mg2+ 1.5 1 15
Cl- 103 115 8
HCO3- 25 28 10
HPO42- + H2PO4- 1 1 65 *
SO42- 0.5 0.5 10
Organic anions 4 5 2
Proteins 2 - 6
* Most of them are organic phosphates (hexose-P, creatine-P, nucleotides, nucleic acids)
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9. The average molarity of ions in blood plasma
Molarity (mmol/l) Molarity (mmol/l)
Cation Anion
Cation Charge Anion Charge
Na+ 142 142 Cl- 103 103
K+ 4 4 HCO3- 25 25
Ca2+ 2.5 5 Protein- 2 18
Mg2+ 1.5 3 HPO42- 1 2
total positive charge 154
SO42- 0.5 1
OA 4 5
total negative charge 154
65 – 85 g/l Total buffer bases: 42 ± 3 mmol/l 9

10. Comments to ionic composition of body fluids
• blood plasma and ISF have almost identical composition, ISF does not contain proteins
• the main ions of blood plasma are Na+ and Cl-, responsible for osmotic properties of ECF
• the main ions of ICF are K+, organic phosphates, and proteins
• every body fluid is electroneutral ⇒ [total positive charge] = [total negative charge]
• molarity of charge (mmol/l) = mEq/l (miliequivalent per liter)
• in univalent ionic species (e.g. Na+, Cl-, HCO3-) ⇒ molarity of charge = molarity of ion
• in polyvalent ionic species ⇒ molarity of charge = charge × molarity of ion,
e.g. SO42- ⇒ 2 × [SO42-] = 2 × 0.5 = 1 mmol/l
• plasma proteins have pI around 5 ⇒ at pH 7.40 they are polyanions
• OA = low-molecular organic anions: lactate, oxalate, citrate, malate, glutamate, ascorbate, KB ...
• hydrogen carbonate, proteins, and hydrogen phosphate are buffer bases
• total plasma proteins are usually expressed in g/l (mass concetration)
• charge molarity of proteins and org. anions is estimated by empirical formulas
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11. Sodium balance
150-280 mmol/day
Input
table salt (NaCl), salty foods (sausages etc.), some mineral waters
ECF (50 %), bones (40 %), ICF (10 %)
Distribution
the main cation of ECF, responsible for osmolality and volume of ECF
130-145 mmol/l
Blood level
gradient between ECF and ICF is created and maintained by Na+, K+-ATPase
aldosterone = salt conserving hormone
Regulation
ANP = atrial natriuretic peptide = antagonist of aldosterone
120-240 mmol/day (urine)
Output [99 % of filtered Na+ is reabsorbed in kidneys]
~10 mmol/day (stool), 10-20 mmol/day (sweat)
11

12. 12

13. Hormones regulating sodium
Feature Aldosterone Atrial natriuretic peptide
Produced in adrenal cortex heart atrium
Chemical type steroid peptide
resorption of Na+
Main effects
excretion of K+ excretion of Na+ and water
(in kidneys)
13

14. The loss of sodium under special or pathological situations
The loss by urine
• diuretics (furosemid, thiazides)
• osmotic diuresis (hyperglycemia in diabetes)
• renal failure
• low production of aldosterone
The loss by digestion juices
• vomiting, diarhea, fistulas etc.
The loss by sweating
• work or sports in hot and dry conditions
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15. Potassium balance
40-120 mmol/day
Input
mainly from plant food: potatoes, legumes, fresh and dried fruits, nuts etc.
ICF (98 %), ECF (2 %)
Distribution
the main cation of ICF, associated with proteins (polyanions) and phosphates
3.8-5.2 mmol/l,
Blood level the gradient between ECF and ICF is maintained by Na+, K+-ATPase
blood level of K+ depends on pH
the secretion of K+ into urine (in distal tubule) depends on many factors:
Regulation
K intake, aldosterone production, alkalosis/acidosis, anions in urine
45-90 mmol/day (urine)
Output
5-10 mmol/day (stool)
15

16. Potassium blood level depends on acid-base status
pH = 6.8 ~ 7.0 mmol K+ / l
K+
pH = 7.4 ~ 4.4 mmol K+ / l
pH
pH = 7.7 ~ 2.5 mmol K+ / l
cell
cell
H+ H+ alkalosis →
acidosis → H + H+
hyperkalemia cation exchange hypokalemia
K+ K+ K+ K+
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17. Ion diameter (nm)
Ion
Free Hydrated
The hydration of Na and K cation + +
Na+ 0,19 0,52
K+ 0,27 0,46
• Na+ is the most hydrated ion, typically with 4 or 6 water molecules in the first layer,
depending on the environment, Na+ binds water strongly, the hydration shell is stable and
moves together with the cation
• any Na+ movement (retention, excretion) is followed by H2O movement
• the more salt in ECF, the more water in ECF, the higher volume and blood pressure
• potassium ion is larger, has 8 more electrons shielding positively-charged nucleus,
so K+ makes transient associations with water rather than a discrete hydration layer
• it also helps to explain why K+ has higher permeability across cell membrane than Na+
K+
17

18. The main species determining the osmolality of blood plasma
IVF ISF ICF
water
urea
glucose
×
Na+
×
proteins × ×
barrier: barrier:
blood vessel walls cell membrane
pore-lined highly selective 18

19. The movement of ions and polar neutral molecules across cell membranes
is due to the existence of specific transport proteins (including ion pumps).
Diffusion of water molecules is possible, but it is slow and not efficient.
Aquaporins are membrane proteins that form water channels and account for
the nearly free and rapid two-way moving of water molecules across most cell
membranes (about 3 × 109 molecules per second).
Aquaporin channel structure
Aquaporins consist of six membrane-spanning segmenst arranged in two hemi-pores
which fold together to form the "hourglass-shaped" channel.
The highly conserved NPA motifs (Asn-Pro-Ala) may form a size-exclusion pore, giving the channel its
high specifity.
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20. In membranes, some of aquaporin types exist as homotetramers, or form
regular square arrays.
Aquaporins are controlled by means of gene expression, externalization of silenced channels
in the cytoplasmic vesicles, and also by the changes in intracellular pH values (e.g., increase
in proton production inhibits water transport through AQP-2 and increases the permeability
of AQP-6).
More than 12 isoforms of aquaporins were identified in humans, 7 of which are
located in the kidney.
Examples:
AQP1 (aquaporin-1), opened permanently, is localized in red blood cells, endothelial and
epithelial cells, in the proximal renal tubules and the thin descendent limb of the loop of
Henry.
AQP2 is the main water channel in the renal collecting ducts. It increases tubule wall
permeability to water under the control of ADH: If ADH binds onto the V2 receptors
located in the basolateral membrane, AQP2 in the membranes of cytoplasmic vesicles is
phosphorylated and exposed in the apical plasma membrane. Reabsorbed water leaves
cells through AQP3 and AQP4 in the basolateral plasma membrane.
20

21. males 290 ± 10 mmol/kg H2O
Osmolality of blood plasma females 285 ± 10 mmol/kg H2O
Osmolality of biological fluids is measured by osmometers based mostly on the
cryoscopic principle.
Osmolality of blood plasma depends predominantly on the concentrations of Na+,
glucose, and urea. Even if the osmolality of a sample is known (it has been
measured), it is useful to compare the value with the approximate assessment:
osmolality (mmol/kg H2O) ≈ 2 [Na+] + [glucose] + [urea] (mmol/l)
An osmotic gap can be perceived in this way. The measured value is higher
than the calculated rough estimate, if there is a high concentration
of an unionized compound in the sample (e.g. alcohol, ethylene glycol, acetone).
[One gram of ethanol per liter increases the osmolality by about 22 mmol/kg H2O]
21

22. Distinguish
Osmolarity = i c = i × molarity (mmol/l)
Osmolality = i cm = i × molality (mmol/kg H2O)
22

23. Oncotic pressure – colloid osmotic pressure
Within the extracellular fluid, the distribution of water between blood
plasma and interstitial fluid depends on the plasma protein concentration.
The capillary wall, which separates plasma from the interstitial fluid,
is freely permeable to water and electrolytes, but restricts the flow of proteins.
Oncotic pressure is a small fraction of the osmotic pressure
that is induced by plasma proteins.
plasma proteins: 62 - 82 g/l (1.3 - 2.0 mmol/l)
plasma albumin: 35 - 50 g/l (0.5 - 0.8 mmol/l)
Albumin makes about 80 % of oncotic pressure.
Osmotic pressure of blood plasma: 780 - 795 kPa
Oncotic pressure of blood plasma: 2.7 - 3.3 kPa
2.7 – 1.4 kPa ... sizable edemas, imminent danger of pulmonary edema
< 1.4 kPa ... unless albumin is given i.v., survival is hardly possible
23

24. The significance of oncotic pressure
The capillary wall is permeable for small molecules and water but not
permeable for proteins.
The hydrostatic pressure of a blood tends to push water out of the
capillary – filtration.
The oncotic pressure pulls the water from the interstitial space back
into the capillary - resorption.
Endothelial cells
Blood capillary
24

25. The movement of fluid between plasma and interstitial fluid
Oncotic pressure can be measured by means of colloid osmometers.
25

26. Six cases of water/sodium imbalance
The loss of The overload by
isotonic fluid isotonic fluid
„pure“ water „pure“ water
„pure“ sodium „pure“ sodium
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For more details see physiology

27. The loss of isotonic fluid = isotonic dehydration
Typical situation
Normal The loss of
isotonic fluid vomiting, diarrhea, bleeding, burns,
status
(ascites)
ECF
Consequence
↓ ECF volume (hypovolemia)
ICF
activation of RAAS
27

28. The loss of pure (solute-free) water/hypotonic fluid
= hypertonic dehydration
Typical situations
Normal hyperventilation
The loss of pure water
status no drinking (older people)
osmotic diuresis
ECF ADH deficit (diabetes insipidus)
Consequences
H2O
hypovolemia
ICF ECF becomes hypertonic (hypernatremia)
water osmotically moves from ICF to ECF
cellular dehydration (cells shrink)*
ADH production ↑ (water retention)
* The shrinking of brain neurons disturbs brain functions (confusion, delirium, convulsions, coma)
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29. The loss of pure sodium (salt)
Typical situations
Normal
The loss of salt aldosterone deficit
status diuretics
(also vomiting, sweating, diarrhea)
. .
ECF Consequences
. . . .
ECF becomes hypotonic (hyponatremia)
H2O
water osmotically moves from ECF to ICF →
ICF hypovolemia + intracellular edemas
expansion of ICF → increase of intracranial pressure -
imminent danger of cerebral edemas
ADH production ↓ (water excretion) + RAAS activation
29

30. The overload by isotonic fluid
= isotonic hyperhydration
Typical situations
Normal The overload by excessive infusions by isotonic saline
status isotonic fluid
cardial insufficiency
renal diseases
ECF (secondary hyperaldosteronism)
Consequences
expansion of ECF volume
ICF
ECF edema
30

31. The overload by pure (solute-free) water/hypotonic fluid
= hypotonic hyperhydration (water intoxication)
Typical situations
Normal The overload by water
excessive drinking simple water
status
SIADH*, also stress, trauma, infections
. . . . gastric lavage
ECF
. . . . . excessive infusion of glucose solution
H2O Consequences
ICF ECF volume expansion
ECF becomes hypotonic (hyponatremia)
water moves osmotically to ICF
ICF + ECF edemas
ADH production ↓ (water diuresis)
* syndrome of inappropriate ADH 31

32. The overload by pure sodium/salt
Typical situations
excessive intake of salt / mineral waters
Normal
The overload by sodium drinking sea water (ship wreck)
status
excessive infusions of Na-salts (ATB ...)
aldosterone hyperproduction
ECF Consequences
H2O
ECF becomes hypertonic (hypernatremia) →
hypervolemia
ICF water moves osmotically from ICF to ECF
ECF (pulmonary) edemas + cellular dehydration
↑ ADH (to retain water)
↑ ANP / urodilatin (to excrete sodium)
RAAS inhibited
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33. Water and osmolality control
Antidiuretic hormone (ADH, Arg-vasopressin, AVP)
released from the nerve terminals in posterior pituitary
Aldosterone
secreted from the zona glomerulosa of adrenal cortex
after activation of the renin-angiotensin system
Natriuretic peptides (ANP, BNP)
secreted from some kinds of cardiomyocytes in heart atria and chambers
33

34. Example
Water and osmolality control is closely inter-related
INTAKE OF Na+ WATER LOSS
Osmolality increase Decrease in ECF volume
Filling of heart Filling of arteries
upper chambers
OSMOSENSORS VOLUMOSENSORS
hypothalamus liver heart juxtaglomerular cells
Secretion of renin
Increase
of HMV
Sensation Secretion of Secretion of
of thirst ADH (vasopressin) aldosterone
Better filling
of arteries
Renal
WATER INTAKE RETENTION OF WATER RETENTION OF Na+
Antagonistic action of
34
natriuretic peptides ANP and BNP
For details see physiology

35. Antidiuretic hormone (ADH, Arg-vasopressin, AVP)
is a nine amino acid cyclic peptide:
Cys–Tyr–Phe–Gln–Asn–Cys–Phe–Arg–Gly
S S
Vasopressin receptors V2 are in the basolateral membranes of renal collecting
ducts.
Vasopressin receptors V1 are responsible for the vasoconstriction.
35

36. The renin-angiotensin aldosterone system (RAAS)
Fluid volume decrease
ANGIOTENSINOGEN Blood pressure decrease
(blood plasma α2-globulin, >400 AA)
Blood osmolality decrease
renin THE JUXTAGLOMERULAR CELLS
Protein (a proteinase) of the renal afferent arterioles
release into the blood
ANGIOTENSIN I
(a decapeptide)
angiotensin-converting enzyme
His-Leu (ACE, a glycoprotein in the lung,
endothelial cells, blood plasma)
ANGIOTENSIN II
(an octapeptide)
Stimulation of ALDOSTERONE production
ANGIOTENSIN III in zona glomerulosa cells of adrenal cortex
Vasoconstriction of arterioles
Rapid inactivation
by angiotensinases
36

37. Angiotensin II and III
N-Terminal sequence of the plasma α2-globulin angiotensinogen:
NH2–Asp–Arg–Val–Tyr–Ile–His-Pro–Phe–His–Leu–Leu–Val–Tyr
renin
Decapeptide angiotensin I:
NH2–Asp–Arg–Val–Tyr–Ile–His-Pro–Phe–His–Leu
angiotensin converting enzyme (ACE)
Octapeptide angiotensin II:
NH2–Asp–Arg–Val–Tyr–Ile–His-Pro–Phe
aminopeptidase
Heptapeptide angiotensin III:
Arg–Val–Tyr–Ile–His-Pro–Phe
inactivating angiotensinases
37

38. Aldosterone acts on gene expression level
Induces the synthesis of Na / K channels, and Na/K-ATPase
Tubular lumen Blood plasma
Aldosterone
3 Na+
2 K+
Natriuretic Na+ Na+
peptides ATP
K+ K+ blood
aquaporin 3 H2O
H2O aquaporin 2
0
receptors V2
for ADH
urine
38

39. Natriuretic peptides
(more types are known)
atrial natriuretic peptide (ANP) brain natriuretic peptide (BNP)
(mainly of cardiac ventricular origin)
Both peptides have a cyclic sequence (17 amino acyl residues) closed
by a disulfide bond; ANP consists of 28 residues, BNP of 32.
They originate from C-ends of their precursors by hydrolytic splitting and have
short biological half-lives. Released N-terminal sequences are inactive, but
because they are long-lived, their determination is useful.
39

40. Natriuretic peptides are antagonists of aldosterone
Both ANP and BNP have been shown
- to have diuretic and natriuretic effects,
- to induce peripheral vasodilatation, and
- to inhibit release of renin from kidneys and aldosterone from adrenal cortex
These peptides are viewed as protectors against volume overload and
as inhibitors of vasoconstriction (e.g. during a high dietary sodium intake).
Membrane receptors for natriuretic peptides are of unique kind –
they exhibit intrinsic guanylate cyclase activity;
binding of natriuretic peptides onto receptors increases intracellular
concentration of cGMP.
40