Electrolyte and Fluid Balance

7/29/2019 Electrolyte and Fluid Balance

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ELECTROLYTEAND FLUIDBALANCEIons capable of carrying an electric charge

Classified as cations and anions


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WATER

Content varies 40-755 of the total body weight

Women have lower content than men

Water is the solvent for all processes in the human

body It transports nutrients to cells, determines cell

volume by its transport into and out of cells,

removes waste products by way of urine, and acts

as the bodys coolant by way of sweating


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WATER

Intracellular fluid (ICF) is the fluid inside the cells

and accounts for about two thirds of total body

water.

Extracellular fluid (ECF) accounts for the other

one third of total body water and can be subdivided

into :

- intravascular extracellular fluid (plasma)

- interstitial cell fluid that surrounds the cells in the

tissue.


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MAINTENANCEOF IONINTHEPLASMA

Active transport is a mechanism that requiresenergy to move ions across cellular

membranes.

For example, maintaining a high intracellular concentration of K

and a high extracellular (plasma) concentration of Na requires

use of energy from ATP in ATPase-dependent ion pumps.

Diffusion is the passive movement of ions across a

membrane. It depends on the size and charge of

the ion being transported and on the nature of the

membrane through which it is passing. The rate ofdiffusion of various ions also may be altered by

physiologic and hormonal processes.


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CLINICAL SIGNIFICANCEOF OSMOLALITY

Osmolality in plasma is important because it is theparameter to which the hypothalamus responds

Regulation of osmolality also affects the Na

concentration in plasma, largely because Na and its

associated anions account for approximately 90% of the

osmotic activity in plasma.

normal plasma osmolality (275 295 mOsm/kg of

plasma H2O)


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CLINICAL SIGNIFICANCEOF OSMOLALITY

osmoreceptors in the hypothalamus respond quickly tosmall changes in osmolality.

1%2% increase in osmolality causes a fourfold

increase in the circulating concentration of AVP

AVP acts by increasing the reabsorption of water in thecortical and medullary collecting tubules.

AVP has a half lifein the circulation of only 15 to 20

minutes.

Renal water excretion is more important in controllingwater excess, whereas thirst is more important in

preventing water deficit or dehydration.


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REGULATIONOF BLOOD VOLUME

Changes in blood pressure are detected in this areas --

Cardiopulmonary circulation, carotid sinus, aortic arch, andglomerular arterioles.

Adequate blood volume is essential to maintain bloodpressure and ensure good perfusion to all tissue and organs.

Regulation of both Na and water are interrelated in controllingblood volume.

The renin-angiotensinaldosterone system responds primarilyto a decreased blood volume.

Renin is secreted near the renal glomeruli in response todecreased renal blood flow (decreased blood volume or bloodpressure).

Renin converts angiotensinogen to angiotensin I, which thenbecomes angiotensin II.

Angiotensin II causes vasoconstriction, which quicklyincreases blood pressure, and secretion of aldosterone, whichincreases retention of Na and the water that accompanies theNa.


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FOUROTHERFACTORSAFFECTBLOODVOLUME:

(1) atrial natriuretic peptide (ANP), released from themyocardial atria in response to volume expansion,promotes Na excretion in the kidney (B-type natriureticpeptide [BNP] and ANP act together in regulating bloodpressure and fluid balance)

(2) volume receptors independent of osmolality stimulate

the release of AVP, which conserves water by renalreabsorption;

(3) glomerular filtration rate (GFR) increases with volumeexpansion and decreases with volume depletion; and

(4) all other things equal, an increased plasma Na will

increase urinary Na excretion and vice versa. Thenormal reabsorption of 98% to 99% of filtered Na by thetubules conserves nearly all of the 150 L of glomerularfiltrate produced daily.


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URINEOSMOLALITY

decreased in diabetes insipidus (inadequate AVP)

and polydipsia (excessive H2O intake) and

increased in conditions such as the syndrome of

inappropriate ADH (AVP) secretion (SIADH) and

hypovolemia (although urinary Na is usually

decreased).


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DETERMINATIONOF OSMOLALITY

Specimen: serum or urine

Methods: determining osmolality are based on

properties of a solution that are related to the

number of molecules of solute per kilogram of

solvent.- An increase in osmolality decreases the freezing

point temperature and the vapor pressure.

Measurement of freezing point depression and

vapor pressure decrease (actually, the dew point)are the two most frequently used methods of

analysis.


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DETERMINATIONOF OSMOLALITY

Osmometers :- that operate by freezing point depression are

standardized using sodium chloride reference solutions.

- After calibration, the appropriate amount of sample ispipetted into the required cuvet or sample cup and

placed in the analyzer.- The sample is then supercooled to 7C and seeded to

initiate the freezing process. When temperatureequilibrium has been reached, the freezing point ismeasured, with results for serum and urine osmolalityreported as milliosmoles per kilogram.

- Calculation of osmolality has some usefulness either asan estimate of the true osmolality or to determine theosmolal gap, which is the difference between themeasuredosmolality and the calculated osmolality.


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OSMOLAL GAP

indirectly indicates the presence of osmotically

active substances other than Na, urea, or glucose,

such as ethanol, methanol, ethylene glycol, lactate,

orhydroxybutyrate


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THE ELECTROLYTES


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SODIUM

Na is the most abundant cation in the ECF,

representing

largely determines the osmolality of the plasma.

Na concentration in the ECF is much larger than

inside the cells.

Because a small amount of Na can diffuse through

the cell membrane, the two sides would eventually

reach equilibrium.

Active transport systems, such as ATPase ion

pumps, are present in all cells

The Na,K-ATPase ion pump moves three Na ions

out of the cell in exchange for two K ions moving

into the cell as ATP is converted to ADP.


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SODIUM

The plasma Na concentration depends greatly on

the intake and excretion of water and, to asomewhat lesser degree, the renal regulation of Na.

Three processes are of primary importance:

(1) the intake of water in response to thirst, as

stimulated or suppressed by plasma osmolality;

(2) the excretion of water, largely affected by AVP

release in response to changes in either blood

volume or osmolality;

(3) the blood volume status, which affects Na

excretion through aldosterone, angiotensin II, and

ANP (atrial natriuretic peptide).


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CLINICAL APPLICATIONS

Hyponatremia

- a serum/plasma level less than 135 mmol/L.4

- most common electrolyte disorders in hospitalized

and nonhospitalized patients.

- Levels below 130 mmol/L are clinically significant.

- assessed by the cause for the decrease or with the

osmolality level.

- Decreased levels may be caused by increased Na

loss, increased water retention, or water imbalance


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Hyponatremia

- Increased Na loss in the urine can occurwith decreasedaldosterone production, certain diuretics (thiazides), withketonuria (Na lost with ketones), or a salt-losingnephropathy (with some renal tubular disorders).

- K deficiency also causes Na loss because of theinverse relationship of the two ions in the renal tubules.

When serum K levels are low, the tubules will conserveK and excrete Na in exchange

- . Each disorder results in an increased urine Na level(20 mmol per day), which exceeds the amount of waterloss.

- Prolonged vomiting or diarrhea or severe burns canresult in Na loss.

- Urine Na levels are usually less than 20 mmol per day inthese disorders, which can be used to differentiateamong causes for urinary loss.


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Hyponatremia

- Water imbalance can occur as a result of excesswater intake,which must be chronic

- In a normal individual, excess intake will not affect

Na levels. Syndrome of inappropriate AVP secretion

(SIADH) causes an increase in water retentionbecause of increased AVP (ADH) production


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Electrolyte and Fluid Balance