ir-026 fluid and electrolytes -1

8/9/2019 Ir-026 Fluid and Electrolytes -1

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M Shuja Tahir, M Abid Bashir Faisalabad, Pakistan IR-026

127 April to June, 2010 I N D E P E N D E N T R E V I E W S

cation and chloride and bicarbonateTOTAL BODY WATER are the principle anions.It is the amount of water contentpresent in the body.

INTRAVASCULAR FLUIDIt is 75% of total body weight in a newborn infant. It is the part of extracellular fluidIt is 55% of total body weight in adult present in intravascular compartment.males. It is 5% of body weight or 1/4 of ECFIt is 50% of total body weight in adult (2.8 liters in 70 kg adult).females.

Intra-vascular fluid is present as aThe difference in the percentage of component of blood which isbody water is due to excess amount of composed of cells (red cells, whitefat present in the females. cells, platelets) and plasma (fluids with

dissolved protein).The amount of total body water content(60% of total body weight or 42 liters in INTERSTITIAL FLUID70 Kg patient) is an accepted average It is the part of the extracellular fluidin adults for clinical calculations. present in the interstitial space (15% of

body weight or 3/4 of ECF).INTRACELLULAR FLUID (ICF)

The interstitial fluid is further compli-Two third (66%) of total body water cated by having a rapidly equilibrating(40% of total body weight) is present inor functional compartment as well asthe intracellular compartment. Largest

several more slowly equilibrating or proportion of this water is present in relatively non functioning components.skeletal muscle mass. Because of theThese non functioning componentssmaller muscle mass in female, thecomprise connective tissue water aspercentage of intracellular water iswell as transcellular water including;lower than in males.! Cerebrospinal fluid.! Peritoneal fluid.The principle cations of intracellular ! Joint fluids.compartments are potassium and! Secretions of themagnesium and principle anions are

gastrointestinal tract.phosphates and proteins.! Fluid in renal tubules.

EXTRACELLULAR FLUID (ECF)This non functioning componentOne third (33%) of the total body water normally represents only 10% of the(20% of body weight) is present in theinterstitial fluid volume (1-3% of bodyextracellular compartment. It is further weight).subdivided into intravascular and

interstitial fluid compartments.MOVEMENT OF WATERThe fluids keep on shifting between allIn the ECF, sodium is the principle

FLUID AND ELECTROLYTES -1FLUID COMPARTMENTS

Total Body Water Solids

Males55%

45%


Infants

75%

25%


Females

50%

50%


Average

60% - 42 litres

40%

Intracellular fluid Extra cellular fluid

Transcellular fluid0.5 liter (3 %)

Interstitial fluid11.2 liters (15%)

Intra vascular fluid2.8 liters (5%)

Extra cellular fluid66%

34%

Fluid Compartments

Critical Care


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these compartments to keep the Normal plasma osmolality is 280-310electrical and osmolar equilibrium. mosm/L.Main factor controlling this fluid shiftare pressure gradients (hydrostatic and Sodium is the main ion responsible for

osmotic pressure) and permeability of extracellular osmolarity as it is clear the membranes separating these from the equation that 280 out of 300compartments. (Cel l membrane mOsmol of plasma osmolality arebetween the ICF and ECF and capillary contributed by sodium alone.membrane between intravascular andinterstitial compartment). Although the concentration of different

solutes is different in various compart-Osmosis is the movement of solvent ments, the membranes are freelyparticle from an area of lower solute permeable to water. Therefore osmo-con cen tra tion to higher solute lality of these compartments is same.co nc en tr at io n wh en bo th ar e Similarly electrical equilbirium is alsoseparated by a semi-permeable maintained between all these compart-membrane which allows the passage of ments.solvent molecule but not the solutemolecule. TONICITY

Tonicity can be defined as osmolalityThe physiological and chemical activity due to effective solute.of electrolytes depends upon number of particles present per unit volume Effective solutes are those solutes(osmoles per litre) and number of which can not permeate the cellelectrical charges per unit volume membrane and, therefore, are(equivalents per litre). restricted to ICF or ECF. These are

mainly responsible for water transport

It is important to note that number of across the cell membrane.particles and not the amount or weightof solute is important in maintaining Ineffective solutes can freely cross theosmolality and osmotic pressure. cell membrane and are unable to effect

the shift of water across the membrane.Plasma osmolality is calculated Therefore, these solutes contribute toaccording to the following formula. total body osmolality but not to tonicity.

+ +P (mosm/kg) = 2[Na (mmol/L) + K NORMAL CONTROL OF FLUIDSosm(mmol/L)] + [BUN(mg/dl)/2.8] Kidneys play a major role in+[Glucose(mg/dl)/18] maintaining fluid, electrolyte and acid

base balance.Putting average values in the equationwe get; In the glomerulus, almost everything is

filtered out of plasma. Glomerular P = 2 [140+4] + 20/2.8 + 90/18 filtration rate (GFR) is mainlyosm= 300 dependent upon the hydrostatic

pressure in the glomerular capillariesand capillary flow rate which is


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dependent upon the renal blood flow. Atrial natriuretic hormone (ANH) isIn hypovolaemia, renal blood flow secreted from atria in response todecreases, decreasing the GFR. increase ECF volume. It is responsible

for increased GFR and decreased

In the proximal convoluted tubules, sodium re-absorption. The net result issodium is actively re-absorbed. increased excretion of water andChloride passively follows the sodium sodium and decreased ECF volume.to maintain the electrical equilibriumleading to water re-absorption by CLASSIFICATION OF FLUID ANDosmosis. The net result is decrease in ELECTROLYTE CHANGEvolume of the filtrate but osmolality VOLUME CHANGESremains the same. Addition or subtraction of isotonic

fluids from body would lead toIn the loop of Henle, complex counter significant change in ECF volume butcurrent mechanism is responsible for little or no change is noticed in ICF.further re-absorption of water andsalts. In the descending limb, the CONCENTRATION CHANGESvolume is further decreased by water Addition or subtraction of water or re-absorption leading to increased sodium alone would lead to changes inosmolality. In ascending limb, further volume as well as tonicity of the ECF. Itsalts and water are re-absorbed. The will lead to significant shift of water fluid entering the distal convoluted across the cell membrane i.e additiontubules is hypo-osmolar. of water or subtraction of sodium

would lead to hypotonicity of ECF andIn distal convoluted tubules, further re- water will move into the cell to balanceabsoption of water and sodium takes the osmolality and vice versa.place but it is hormone dependent.

COMPOSITION CHANGES Aldosterone is the most important Changes in concentration of ions other hormone which is secreted from the than sodium would not lead to volumerenal medulla in response to renin or concentration change but can haveangiotensin system. It acts on the distal significant changes in the composition

+convoluted tubules to stimulate water of the ECF. These ions includes K ,+2 +2 - + -and sodium re-absorption. It increases Ca , Mg , Cl , H and OH .

the ECF volume.

DISTRIBUTIONAL CHANGES Antidiuretic hormone (ADH) is secreted The volume is redistributed to thirdby the posterior pituitary gland. It space (tissues in trauma and burns, GITincreases the permeability of collecting

in intestinal obstruction) leading toducts to water leading to water re- distributional hypovolemia. Total water absorption and increase in the ECF may be normal or even more, butvolume. effective intravascular volume

significantly falls.


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REFERENCES1. Shires III, GT, Borber AB, Shires GT. Fluid & Fischer JE. (Ed) Mastery of surgery 3rd Ed.

Electrolyte management of the surgical 1997, Little Drown, Co. Boston. PP. 22-49.patient, In Shwortze I, Shires GT, Spencer FC, Doly JM, Fisher JE, Galloway AC. (Ed). 4. Gnerlich JL, Buchman TA. Fluid,Principles of surgery 7th Ed. 1999. Mc Electrolyte, Acid base disorders. InGrath Hills New York PP 53-76. Klingensmith ME, Chen LE, Glasgow SC,

Goer TA, Melby SJ, (Eds) Washington2. Shires GT, Canizoro PC. Fluid, Electrolyte Manual of surgery 5th Ed. 2008. Lippincott

management of surgical patient. In Williams & Wilkins, Philadelphia. PP 71-sabiston DC (Ed). The text book of surgery. 91.The biological bases of modern surgicalpractice 14th Ed. 1991. W.B Sanden Co. 5. Steele RJC, Patients with metabolicPhiladelphia PP 57-76. disorders. In Cuschieni A, Steelw RJC,

Moosa AR (Ed). Essential surgical practice3. Doly JM, Barie PS, Dudnch SI. Preparation 4th Ed. 2000. Butterworth Heinmann

of the patient. In Nylium (Lm), Baker RJ, Oxford PP. 205-14.

SUMMARYFluid and Electrolytes Movement of water across compartmentsBody fluid compartments Normal control of fluids! Intracellular fluid compartments Classification of fluid & electrolyte changes! Extracellular fluid compartments

The author :Muhammad Abid Bashir,FCPSis associate professor indepartment of Surgery atIndependent MedicalCollege Faisalabad and

instructor of ATLS [email protected]

The author :Muhammad Shuja Tahir

FRCS (Ed), FCPS (Hon)is professor and head of thedepartment of Surgery atIn d epen d e n t Med ica lCollege Faisalabad.

[email protected]

ir-026 fluid and electrolytes -1

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