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Body water

Fluid and electrolyte balance

© Department of Biochemistry, 2011 (J.D.)

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Average total body water (TBW) as body weight percentage

465260 y and older

475540 – 59 y

506117 - 39 y

575910 - 16 y

621 - 10 y

651 - 12 months

760 - 1 month

FemalesBothMalesAge

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

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Total body water compartments

interstitial fluid (and lymph)2/31/3

extracellular fluid ECF ICF intracellular fluid

IVFblood plasma

ISF

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Quantification of body fluid compartments (in adult male, 70 kg)

Percentage of

20 %33 %14ECF

5 %8 %3.5IVF (blood plasma)

15 %25 %10.5ISF

40 %67 %28ICF

60 %100 %42 TBW

Body weightBody fluidVolume (Liters)Fluid

The contribution of lymph and transcellular fluid is negligible.

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Transcellular fluidsare 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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Daily water balance (average data, 70 kg male)

2500 mltotal output2500 mltotal input

1500 mlurine

100 mlfeces 300 mlmetabolic water

100 mlsweat1000 mlfood

800 mlinsensible (skin, lungs)1200 mlbeverages

Water outputWater input

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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Metabolic water

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

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

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Average ion concentrations in blood plasma, ISF, and ICF (mmol/l)

6-2Proteins254Organic anions

100.50.5SO42-

65 *11HPO42- + H2PO4

-

102825HCO3-

8115103Cl-

1511.5Mg2+

traces1.32.5Ca2+

15544K+

10142142Na+

ICFISFBlood plasmaIon

* Most of them are organic phosphates (hexose-P, creatine-P, nucleotides, nucleic acids)

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103

25

18

2

1

5

103

25

2

1

0.5

4

Cl-

HCO3-

Protein-

HPO42-

SO42-

OA

142

4

5

3

142

4

2.5

1.5

Na+

K+

Ca2+

Mg2+

ChargeAnionChargeCation

Molarity (mmol/l)Anion

Molarity (mmol/l)Cation

The average molarity of ions in blood plasma

Total buffer bases: 42 ± 3 mmol/l65 – 85 g/l

total positive charge 154

total negative charge 154

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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 × [SO4

2-] = 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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Sodium balance

120-240 mmol/day (urine)[99 % of filtered Na+ is reabsorbed in kidneys]~10 mmol/day (stool), 10-20 mmol/day (sweat)

Output

aldosterone = salt conserving hormoneANP = atrial natriuretic peptide = antagonist of aldosterone

Regulation

130-145 mmol/lgradient between ECF and ICF is created and maintained by Na+, K+-ATPase

Blood level

ECF (50 %), bones (40 %), ICF (10 %)the main cation of ECF, responsible for osmolality and volume of ECF

Distribution

150-280 mmol/daytable salt (NaCl), salty foods (sausages etc.), some mineral waters

Input

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Hormones regulating sodium

excretion of Na+ and water resorption of Na+ excretion of K+

Main effects (in kidneys)

peptidesteroidChemical type

heart atriumadrenal cortexProduced in

Atrial natriuretic peptide AldosteroneFeature

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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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Potassium balance

45-90 mmol/day (urine)5-10 mmol/day (stool)

Output

the secretion of K+ into urine (in distal tubule) depends on many factors: K intake, aldosterone production, alkalosis/acidosis, anions in urine

Regulation

3.8-5.2 mmol/l, the gradient between ECF and ICF is maintained by Na+, K+-ATPaseblood level of K+ depends on pH

Blood level

ICF (98 %), ECF (2 %)the main cation of ICF, associated with proteins (polyanions) and phosphates

Distribution

40-120 mmol/daymainly from plant food: potatoes, legumes, fresh and dried fruits, nuts etc.

Input

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pH

K+

pH = 6.8 ~ 7.0 mmol K+ / l

pH = 7.4 ~ 4.4 mmol K+ / l

pH = 7.7 ~ 2.5 mmol K+ / l

Potassium blood level depends on acid-base status

H+ H+

K+K+

cell

H+ H+

K+ K+

cell

cation exchangeacidosis →

hyperkalemia

alkalosis →

hypokalemia

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The hydration of Na+ and K+ cation

• 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+

Hydrated Free

0,460,27K+

0,520,19Na+

Ion diameter (nm) Ion

K+

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The main species determining the osmolality of blood plasma

proteins

Na+

glucose

urea

water

IVF ICFISF

×

××

×barrier:

blood vessel wallspore-lined

barrier:cell membranehighly selective

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The movement of ions and polar neutral molecules across cell membranesis 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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In membranes, some of aquaporin types exist as homotetramers, or formregular 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.

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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]

Osmolality of blood plasma males 290 ± 10 mmol/kg H2Ofemales 285 ± 10 mmol/kg H2O

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 of biological fluids is measured by osmometers based mostly on the cryoscopic principle.

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Osmolarity = i c = i × molarity (mmol/l)

Osmolality = i cm = i × molality (mmol/kg H2O)

Distinguish

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Oncotic pressure – colloid osmotic pressure

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.

The capillary wall, which separates plasma from the interstitial fluid, is freely permeable to water and electrolytes, but restricts the flow of proteins.

Osmotic pressure of blood plasma: 780 - 795 kPaOncotic 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

Within the extracellular fluid, the distribution of water between bloodplasma and interstitial fluid depends on the plasma protein concentration.

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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.

The significance of oncotic pressure

Blood capillary

Endothelial cells

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The movement of fluid between plasma and interstitial fluid

Oncotic pressure can be measured by means of colloid osmometers.

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Six cases of water/sodium imbalance

„pure“ sodium„pure“ sodium

„pure“ water„pure“ water

isotonic fluidisotonic fluid

The overload byThe loss of

For more details see physiology

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ICF

ECF

The loss of isotonic fluid

Normalstatus

The loss of isotonic fluid = isotonic dehydration

↓ ECF volume (hypovolemia)

activation of RAAS

Consequence

vomiting, diarrhea, bleeding, burns,(ascites)

Typical situation

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ICF

ECF

The loss of pure waterNormalstatus

H2O

The loss of pure (solute-free) water/hypotonic fluid = hypertonic dehydration

hypovolemiaECF becomes hypertonic (hypernatremia)water osmotically moves from ICF to ECFcellular dehydration (cells shrink)*ADH production ↑ (water retention)

Consequences

hyperventilationno drinking (older people) osmotic diuresisADH deficit (diabetes insipidus)

Typical situations

* The shrinking of brain neurons disturbs brain functions (confusion, delirium, convulsions, coma)

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ICF

. .. . . .

ECF

The loss of salt Normalstatus

H2O

The loss of pure sodium (salt)

ECF becomes hypotonic (hyponatremia)water osmotically moves from ECF to ICF →hypovolemia + intracellular edemas

expansion of ICF → increase of intracranial pressure - imminent danger of cerebral edemas

ADH production ↓ (water excretion) + RAAS activation

Consequences

aldosterone deficitdiuretics(also vomiting, sweating, diarrhea)

Typical situations

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ICF

ECF

The overload by isotonic fluid

Normalstatus

The overload by isotonic fluid = isotonic hyperhydration

expansion of ECF volumeECF edema

Consequences

excessive infusions by isotonic salinecardial insufficiencyrenal diseases (secondary hyperaldosteronism)

Typical situations

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ICF

. . . . . . . . .

ECF

The overload by water Normalstatus

The overload by pure (solute-free) water/hypotonic fluid = hypotonic hyperhydration (water intoxication)

ECF volume expansionECF becomes hypotonic (hyponatremia) water moves osmotically to ICFICF + ECF edemasADH production ↓ (water diuresis)

Consequences

excessive drinking simple waterSIADH*, also stress, trauma, infectionsgastric lavageexcessive infusion of glucose solution

Typical situations

H2O

* syndrome of inappropriate ADH

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ICF

ECF

The overload by sodiumNormalstatus

The overload by pure sodium/salt

ECF becomes hypertonic (hypernatremia) → hypervolemiawater moves osmotically from ICF to ECFECF (pulmonary) edemas + cellular dehydration↑ ADH (to retain water)↑ ANP / urodilatin (to excrete sodium)RAAS inhibited

Consequences

excessive intake of salt / mineral watersdrinking sea water (ship wreck)excessive infusions of Na-salts (ATB ...)aldosterone hyperproduction

Typical situations

H2O

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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

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OSMOSENSORShypothalamus liver

VOLUMOSENSORS heart juxtaglomerular cells

WATER INTAKE

Sensation of thirst

RenalRETENTION OF WATER RETENTION OF Na+

INTAKE OF Na+ WATER LOSS

Osmolality increase Decrease in ECF volume

Water and osmolality control is closely inter-related

Filling of heart upper chambers Filling of arteries

Secretion of ADH (vasopressin)

Increaseof HMV

Secretion of renin

Better fillingof arteries

Secretion ofaldosterone

Antagonistic action ofnatriuretic peptides ANP and BNP

For details see physiology

Example

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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.

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The renin-angiotensin aldosterone system (RAAS)

THE JUXTAGLOMERULAR CELLSof the renal afferent arterioles

Fluid volume decreaseBlood pressure decreaseBlood osmolality decrease

Stimulation of ALDOSTERONE production in zona glomerulosa cells of adrenal cortex

Vasoconstriction of arterioles

angiotensin-converting enzyme(ACE, a glycoprotein in the lung,endothelial cells, blood plasma)

release into the blood

ANGIOTENSINOGEN(blood plasma α2-globulin, >400 AA)

ANGIOTENSIN II

ANGIOTENSIN I

ANGIOTENSIN III

Rapid inactivationby angiotensinases

Protein

His-Leu

(a decapeptide)

(an octapeptide)

renin(a proteinase)

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Angiotensin II and III

NH2–Asp–Arg–Val–Tyr–Ile–His-Pro–Phe–His–Leu–Leu–Val–TyrN-Terminal sequence of the plasma α2-globulin angiotensinogen:

reninDecapeptide angiotensin I:

NH2–Asp–Arg–Val–Tyr–Ile–His-Pro–Phe–His–Leuangiotensin 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

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Aldosterone acts on gene expression level Induces the synthesis of Na / K channels, and Na/K-ATPase

Natriuretic peptides

K+K+

Na+Na+

3 Na+

2 K+

H2Oaquaporin 3

ATP

H2O0aquaporin 2

Tubular lumen

receptors V2

Aldosterone

for ADH

Blood plasma

urine

blood

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Natriuretic peptides (more types are known)

brain natriuretic peptide (BNP)(mainly of cardiac ventricular origin)

atrial natriuretic peptide (ANP)

Both peptides have a cyclic sequence (17 amino acyl residues) closedby 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.

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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.

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).

Natriuretic peptides are antagonists of aldosterone