Fluid, Electrolyte, and Acid-Base Imbalances
Water Content of the Body
• Water accounts for 60% of adult body weight.
• Varies with sex, body mass and age.
○ Lean body mass contains more water.
• Infants and the elderly are at a higher risk for fluid-related problems.
○ In older adults body water content averages 45%-55%of body weight.
○ In infants, water makes up about 70-80%of total body weight.
Body Fluid Compartments
• Two major fluid compartmentsin the body:
○ Extracellular (outside the cells - interstitial fluid, lymph, plasma)
○ Intracellular (inside the cells, a.k.a the cytoplasm)
• There is a small third compartment,known as the transcellularspace (about 1L).
Includes CSF, GI tract, and pleural, synovial, and peritoneal spaces.
○ Loss of this fluid (i.e. by vomiting)can produce serious fluid and electrolyte
○ A "third space" syndrome can occur when an increase in transcellular fluid
occurs at the expense of fluid in other compartments.Examples:after intestinal
paralysis, ascites, hydrocephaly or pleural or pericardial effusions.
Calculation of Fluid Gain or Loss
• One litre of water weighs 1kg.
• Body weight change, especially sudden change, is an excellent indicator of overall
volumeloss or gain.
○ Example: A person who drinks 240mLof liquid gains 0.24kg.
○ A person receiving diuretics who loses 2kg in 24 hours has lost about 2L.
• Electrolytesare substances whose moleculesdissociate or split into ions when placed
Measurement of Electrolytes
• Important to the nurse in evaluating electrolytebalance, as well as determining the
ElectrolyteComposition of Fluid Compartments
• Electrolytecompositionvaries between the ECF and the ICF. The overall concentration
of the electrolytesis approximatelythe same in the two compartments.
Mechanisms ControllingFluid and Electrolyte Movement
• Diffusion is the movementof moleculesfrom an area of high concentration to an area
of low concentration.
○ Net movementof molecules stops when the concentrationsare equal in both
• Facilitated diffusion movesmolecules from an area of high concentration to low
concentration,is passive, and requires no energy other than that of the concentration
○ There must be a membrane carrier molecule to facilitate the rate of diffusion.
○ Glucose transport into the cell is an example.
• Activetransport is a process requiring energy in which moleculesmoveagainst the
○ Example: the sodium-potassium pump needs ATP to be powered.
• Osmosis is the movementof water between two compartmentsseparated by a
membrane permeable to water but not to a solute.
○ Water moves through the membrane from an area of low solute concentration
to an area of high solute concentration.
Water moves from the more dilute compartment(has more water) to a more ○ Water moves from the more dilute compartment(has more water) to a more
concentrated compartment(has less water).
○ Osmotic pressure is the amount of pressure required to stop the osmoticflow of
• Hydrostatic pressure is the force within a fluid compartment.In the blood vessels,
hydrostatic pressure is the blood pressure generated by the contraction of the heart.
○ Hydrostatic pressure in the vascular system gradually decreases as the blood
movesthrough the arteries until it reaches about 40mmHgat the arterial end of
• Oncotic pressure (colloidal osmoticpressure) is osmoticpressure exerted by colloids in
○ The major colloid in the vascular system contributing to the total osmotic
pressure is protein. Protein molecules attract water, pulling fluid from the tissue
space into the vascular space.
○ Unlike electrolytes,the size of protein moleculesprevent them from leaving the
Fluid Movement in Capillaries
• There is normal movementof fluid between the capillary and the interstitium. The
amount and direction of movementare determined by the interaction of
○ (1) capillary hydrostatic pressure
○ (2) plasma oncotic pressure
○ (3) interstitial hydrostatic pressure and
○ (4) interstitial oncotic pressure.
• Capillary hydrostatic pressure and interstitial oncotic pressure cause the movementof
water out of the capillaries.
• Plasma oncotic pressure and interstitial hydrostatic pressure cause the movementof
fluid into the capillary.
• At the arterial end of a capillary, capillary hydrostatic pressure exceeds plasma oncotic
pressure, and fluid is moved into the interstitium.
• At the venous end of a capillary, the capillary hydrostatic pressure is lower than the
plasma oncotic pressure and fluid is drawn back into the capillary by the oncotic
pressure created by plasma proteins.
• If capillary or interstitial pressures are altered, fluid may ABNORMALLY shift from one
compartmentto another, resulting in edema or dehydration.
• Shifts of plasma to interstitial fluid (accumulation of fluid in the interstitium - edema):
○ Occurs if:
Venous hydrostatic pressure increases (inhibits movementback into the
capillary). Examples:fluid overload, CHF, liver failure, obstruction of
venous return to heart (e.g. tourniquets, restrictive clothing, venous
thrombosis),and venous insufficiency (e.g. varicoseveins)
Plasma oncotic pressure decreases (fluid remains in the interstitium if
plasma oncotic pressure is too low to draw fluid back into the capillary).
Seen if the plasma protein content is too low. Examples: excessiveprotein
loss (renal disorders), deficient protein synthesis (liver disease) and
deficient protein intake (malnutrition).
Interstitial oncotic pressure rises. Trauma, burns and inflammationcan
damage capillary walls and allow plasma protein to accumulate in the
• Shifts of Interstitial Fluid to Plasma.
○ Fluid is drawn into the plasma space whenever there is an increase in the plasma
osmoticor oncotic pressure.
○ This could happen with the administration of colloids, dextran, mannitol, or
○ Increasing the tissue hydrostaticpressure is another way of causing a shift of
fluid into the plasma.
Example:the wearing of elastic compressionstockings or hose to Example:the wearing of elastic compressionstockings or hose to
decrease peripheral edema.
• First spacing describes the normal distribution of fluid in the ICF and ECF
• Second spacing refers to an abnormal accumulation of interstitial fluid (e.g. edema).
• Third spacing occurs when fluid accumulatesin a portion of the body from which it is
not easily exchanged with the rest of the ECF. It is trapped an essentially unavailable
for functional use. E.g. ascites, sequestrationof fluid in the peritoneal cavity with
peritonitis and edema associated with burns.
Regulation of Water Balance
• Water balance maintained by a balance of intake and excretion.
• A body fluid deficit or increase in plasma osmolalityis sensed by hypothalamic
osmoreceptors,which in turn stimulate thirst and antidiuretic hormone (ADH).
○ ADH, which is stored in the anterior pituitary, stimulates water reabsorption in
the renal distal and collecting tubules.
• The sensitivity of the thirst mechanism decreases in older adults.
• Factors that stimulateADH release include stress, increased plasma osmolality,
nausea, nicotine and morphine.
Adrenal Cortical Regulation
• ADH affects only water reabsorption.
• Hormonesreleased by the adrenal glands help regulate both water and electrolytes.
• Two groups of hormonesreleased by the adrenal cortex:glucocorticoidsand
○ Glucocorticoids(cortisol)have an ant inflammatoryeffect and increase serum
○ Mineralocorticoids(aldosterone)enhance sodium retention and potassium
May be triggered by decreased renal perfusion.
Kidneys respond by secreting renin into the plasma.
Angiotensin produced in the liver and normally found in blood in acted on
by the renin to form angiotensin I, which converts to angiotensin II, which
stimulates the adrenal cortex to secrete aldosterone.
• In addition to the renin-angiotensin mechanism, increased plasma potassium,
decreased plasma sodium and increased release of adrenocorticotropichormone
(ACTH) from the anterior pituitary all act directly on the adrenal cortex to stimulate
the secretion of aldosterone.
• The primary organs for regulating fluid and electrolytebalance are the kidneys. They
regulate water balance through adjustments in fluid volume.
• Kidneys produce about 1.5L of urine per day.
• Atrial natriuretic factor (ANF) is a hormone released by the cardiac atria in response to
increased atrial pressure (increased volume).
• Primary actions of ANF are vasodilation and increased urinary excretion of sodium and
water, which decreases blood volume.
• GI tract accounts for most of the water intake, but a small amount is normally
eliminated from the GI tract in feces.
Insensible Water Loss
• Invisible vaporization from the lungs and skin
• Assists in regulating body temperature.
• Excessivesweating (sensible perspiration) caused by fever or high environmental
temperaturescan lead to large losses of water and electrolytes. temperaturescan lead to large losses of water and electrolytes.
Sodium and Volume Imbalances
• Sodium plays a major role in maintaining the concentrationand the volume of the ECF.
• Sodium affects the water distribution between ECF and ICF.
• Sodium is also important in the generation and transmissionof nerve impulses and the
regulation of acid-base balance.
• The kidneys are the primary regulators of sodium balance. They regulate the ECF
concentrationof sodium by excreting or retaining water under the influence of ADH.
• Changes in the serum sodium level may reflect either a primary water imbalance, a
primary sodium imbalance or a combination of the two.
• Sodium imbalances are typically associated with imbalances in ECF volume.
• An elevated serum sodium may occur with water loss or sodium gain.
• Na+ >145 mmol/L
• Some commoncauses and clinical manifestations:
Water Loss (sodium concentration) Symptoms
Increased insensible water loss or perspiration d/t Intense thirst; dry, swollen
high fever, heatstroke tongue
Diabetes insipidus Restlessness,agitation,
Deficiency in ADH synthesis Seizures, coma
Decreasein kidney responsivenessto ADH Weakness
Uncontrolled diabetes mellitus Postural hypotension
Sodium Gain Symptoms
IV hypertonic NaCl Restlessness,agitation,
IV sodium bicarbonate twitching
IV excessiveisotonic NaCl Seizures, coma
Primary hyperaldosteronism(hypersecretionof Flushed skin
aldosterone) Weight gain
Saltwater near-drowning (ingestion of seawater) Peripheral and pulmonary
Administration of concentrated hyperosmolartube edema
feedings Increased BP
• Because sodium is the major determinant of the ECF osmolality,hypernatremia causes
hyperosmolality.In turn, hyperosmolalitycauses a shift of water out of the cells, which
leads to cellular dehydration.
• The primary protectionagainst hyperosmolalityis thirst.
• Hypernatremia is not a problem in an alert person who has access to water, can sense
thirst, and is able to swallow.
○ Goal is to treat underlying cause
○ In primary water deficit, the continued water loss must be prevented and water
replacementmust be provided.
○ Goal of treatmentfor NaCl excess is to dilute the sodium concentrationwith
salt-free IV fluids, such as 5% dextrose in water and to promoteexcretionof the
excess sodium by diuretics. Sodium intake will also be restricted.
• May result from loss of sodium-containing fluids or from water excess.
• Na+ <135 mmol/L
• Causes hypo-osmolalotywith a shift of water into the cells.
Sodium Loss Symptoms
GI Losses: diarrhea, vomiting, fistulas, NG suctionIrritability, apprehension, confusion
Renal losses: diuretics, adrenal insufficiency, Na+Postural hypotension Renal losses: diuretics, adrenal insufficiency, Na+ Postural hypotension
wasting renal disease Tachycardia
Skin losses: burns, wound drainage Rapid, thready pulse
Decreased jugular venous filling
Dry mucous membranes
Water Gain (Sodium Dilution) Symptoms
CHF Nausea, vomiting
Excessivehypotonic IV fluids Weight gain
Primary poluydipsia Increased BP
Muscle spasms, seizures, coma
Headache, lassitude, apathy,
• Symptomsof hyponatremiaare related to cellular swelling and are first manifested in
the CNS. The excess water lowers plasma osmoalality,shifting fluid into brain cells.
ExtracellularFluid Volume Imbalances
• ECF volume deficit (hypovolemia)
• ECF volume excess (hypervolemia) are commonlyoccurring clinical conditions.
• ECF volume imbalances are typically accompanied by one or more electrolyte
• Major ICF cation, with 98% of body potassium being intracellular.
• Serum (ECF) levels are 3.5-5.0 mmol/L; intracellular concentrationsare 140-150
• Many of the symptomsrelated to potassium imbalance are due to changes in the ratio
of ECF to ICF potassium.
• Potassium is critical for many cellular and metabolic functions.
○ Necessary for the transmission and conduction of nerve impulses
○ Maintenance of normal cardiac rhythms
○ Skeletal and smooth muscle contraction
○ As the major intracellular cation, K+ regulates intracellular osmolalityand
○ Plays a role in acid-base balance.
• Diet is the source of potassium.
• Kidneys are primary route for potassium loss. About 90% of daily potassium intake is
regulated by the kidneys, the remainder is lost in stool and sweat.
○ If kidney function is significantly impaired, toxic levels of potassium may be
○ There is an inverse relationship between sodium and potassium loss. Factors
that promotesodium retention (i.e. increased aldosterone),promotepotassium
○ Large urine volumescan be associated with excess loss of potassium.
• Disruptions between ICF and ECF potassium often cause clinical problems.
• Factors causing K+ to movefrom the ECF to the ICF:
○ B-Andregenic stimulation (catecholaminerelease in stress, coronaryischemia,
delirium tremens, administration of B-andregenic agonist drugs)
○ Rapid cell building (administrationof folic acid or vitamin B12 to someonewith
megaloblasticanemia, resulting in marked production of RBCs)
• Factors that cause K+ to movefrom ICF to ECF include:
○ Trauma to cells
Exercise ○ Exercise
• Hyperkalemiais high serum potassium (>5.0 mmol/L).
○ May be caused by massive intake of potassium, impaired renal excretion, shift of
potassium from ICF to ECF, or a combination.
○ Most commoncause of hyperkalemiais renal failure.
○ Also commonin clients with massive cell destruction (e.g. burn or crush injury,
tumour lysis), rapid transfusion of aged blood and catabolic state (e.g. severe
○ Metabolic acidosis, particularly when chloride is normal, is associated with a
shift of potassium ions from ICF to ECF as hydrogen ions move into the cell.
○ Adrenal insufficiency leads to retention of K+ in the serum because of
aldosteronedeficiency (because aldosterone conservessodium and excretes
○ Certain drugs such as K+-sparing diuretics and ACE inhibitors may contribute to
the developmentof hyperkalemia.