Fluid and electrolyte balance is a dynamic process that is crucial for life.

Potential and actual disorders of fluid and electrolyte balance occur in every setting, with every disorder, and with a varietyof changes that affect well people.

The nurse also must use effective teaching and communication skills to help prevent and treat various fluid and electrolyte disturbances.

Approximately 60% of a typical adult’s weight consists of fluid (water and electrolytes).

In general, younger people have a higher percentage

of body fluid than older people and men have proportionately more body

fluid than women. Obese people have less fluid than thin

people because fat cells contain little water.

Factors that influence the amount of body fluid are: age

genderbody fat

Body fluid is located in two fluid compartments: the intracellular space (fluid in the cells) the extracellular space (fluid outside the cells)

Approximately two thirds (2/3) of body fluid is in the intracellular fluid (ICF) compartment (located primarily in the skeletal muscle mass)

The extracellular fluid (ECF) divided into theintravascular, interstitial, and transcellular fluid spaces.

Intravascular--(the fluid within the blood vessels) contains plasma

Approximately 3 L of the average 6 L of blood volume is made up of plasma.

The remaining 3 L is made up of erythrocytes, leukocytes, and thrombocytes.

The interstitial space contains the fluid that surrounds the cell and totals about 11 to 12 L in an adult. (ex. Lymph)

The transcellular space is the smallest division of the ECF compartment, contains approximately 1 L of fluid at any given time. (ex.cerebrospinal, pericardial, synovial, intraocular, and pleural fluids; sweat; and digestive secretions)

Body fluid normally shifts between the two major compartments or spaces in an effort to maintain an equilibrium between the spaces.

Loss of fluid from the body can disrupt this equilibrium.

Sometimes fluid is not lost from the body but is unavailable for use by either the ICF or ECF.

Loss of ECF into a space that does not contribute to equilibrium between the ICF and the ECF is referred to as a third-space fluid shift, or “third spacing”

An early clue of a third-space fluid shift is a decrease in urine output despite adequate fluid intake.

Urine output decreases because fluid shifts out of the intravascular space; the kidneys then receive less blood and attempt to compensate by decreasing urine output.

Other signs and symptoms of third spacing that indicate an intravascular fluid volume deficit include increased heart rate

decreased blood pressuredecreased central venous pressure

edemaincreased body weightimbalances in fluid intake and output

(I&O).

Third-space shifts occur in ascites, burns, peritonitis, bowel obstruction, and massive bleeding into a joint or body cavity.

Body Fluid

ICF 2/3 (most skeletal mass)

Intravascular

6L (bld. Vol.)3L Plasma3L RBC,WBC,Platelets

ECF 1/3 Interstitial 11-12L Surround cells, Lymph

Transcellular 1LCSF, pericardial, synovial, intraocular, pleural fld.,

sweat, digestive secretion

ElectrolytesElectrolytes in body fluids are active chemicals

(cations, which carry positive charges, and anions, which carry negative charges).

The major cations in body fluid are sodium, potassium, calcium, magnesium, and hydrogen ions.

The major anions are chloride, bicarbonate, phosphate, sulfate, and proteinate ions.

electrolyte concentration in the body is expressed in terms of milliequivalents (mEq) per liter, a measure of chemical activity,

More specifically, a milliequivalent is defined as being equivalent to the electrochemicalactivity of 1 mg of hydrogen.

In a solution, cations and anions are equal in mEq/L.

Electrolyte concentrations in the ICF differ from those in the ECF

Because special techniques are required to measure electrolyte concentrations in the ICF

It is customary to measure the electrolytes in the most accessible portion of the ECF, namely the plasma.

Extracellular Fluid (Plasma) Intracellular Fluid

CationsSodium (Na) 142Potassium (K) 5Calcium (Ca++) 5Magnesium (Mg++) 2Total cations 154

AnionsChloride (Cl−) 103Bicarbonate (HCO3−) 26Phosphate (HPO4−−) 2Sulfate (SO4−−) 1Organic acids 5Proteinate 17Total anions 154

CationsPotassium (K+) 150Magnesium (Mg++) 40Sodium (Na+) 10Total cations 200

AnionsPhosphates and sulfates 150Bicarbonate (HCO3−) 10Proteinate 40Total anions 200

Approximate Major Electrolyte Contentin Body Fluid (ELECTROLYTES MEQ/L)

Sodium ions

positively chargedfar outnumber the other cations in the ECF (because it affects the overall concentration of the ECF) regulating the volume of body fluidretention of sodium is associated with fluid retention, and excessive loss of sodium is usually associated with decreased volume of body fluid

The major electrolytes in the ICF are potassium and phosphate

The ECF has a low concentration of potassium and can tolerate only small changes in potassium concentrations

Release of large stores of intracellularpotassium, typically caused by trauma to the cells and tissues, can be extremely dangerous

The body expends a great deal of energy maintaining the high extracellular concentration of sodium and the high intracellular concentration of potassium. It does so by means of cell membrane pumps that exchange sodium and potassium ions.

PISO = Potassium, Inside; Sodium, Outside

Normal movement of fluids through the capillary wall into the tissues depends on hydrostatic pressure* (at both the arterial and the

venous ends of the vessel) and the osmotic pressure exerted by the protein of plasma.

The direction of fluid movement depends on the differences in these two opposing forces (hydrostatic versus osmotic pressure).

*(the pressure exerted by the fluid on the walls of the blood vessel)

In addition to electrolytes,

the ECF transports other substances(enzymes and hormones)

It also carries blood components(red and white blood cells)

REGULATION OF BODY FLUID COMPARTMENTSOsmosis and Osmolality

When two different solutions are separated by a membrane that is impermeable to the dissolved substances

Fluid shifts through the membrane from the region of low solute concentration to the region of high solute concentration until the solutions are of equal concentration;

This diffusion of water caused by a fluid concentration gradient is known as osmosis

The magnitude of this force depends on the number of particles dissolved in the solutions, not on their weights.

The number of dissolved particles contained in a unit of fluid determines the osmolality of a solution, which influences the movement of fluid between the fluid compartments.

Tonicity is the ability of all the solutes tocause an osmotic driving force that promotes water movement from one compartment to another.

The control of tonicity determines the normal state of cellular hydration and cell size.

Effective osmoles (capable of affecting water movement).

Sodium GlucoseSorbitol Mannitol

Three other terms are associated with osmosis:

osmotic pressure oncotic pressureosmotic diuresis

• Osmotic pressure is the amount of hydrostatic pressure needed to stop the flow of water by osmosis. It is primarily determined by the concentration of solutes.• Oncotic pressure is the osmotic pressure exerted by proteins (eg, albumin).• Osmotic diuresis occurs when the urine output increases due to the excretion of substances such as glucose, mannitol, or contrast agents in the urine.

Diffusion

Diffusion is the natural tendency of a substance to move from an area of higher concentration to one of lower concentration . It occurs through the random movement of ions and molecules.Examples of diffusion

>exchange of oxygen and carbon dioxide between the pulmonary capillaries and alveoli

>tendency of sodium to move from the ECF compartment,

Filtration

Hydrostatic pressure in the capillaries tends to filter fluid out of the vascular compartment into the interstitial fluid.

Movement of water and solutes occurs from an area of high hydrostatic pressure to an area of low hydrostatic pressure.

Filtration allows the kidneys to filter 180 L of plasma per day.

Another example of filtration is the passage of water and electrolytes from the arterial capillary bed to the interstitial fluid;in this instance, the hydrostatic pressure is furnished by the pumping action of the heart.

Sodium–Potassium Pump

Sodium concentration is greater in the ECF than in the ICF, and because of this, sodium tends to enter the cell by diffusion.

This tendency is offset by the sodium–potassium pump, which is located in the cell membrane and actively moves sodium from the cell into the ECF.

Conversely, the high intracellular potassium concentration is maintained by pumping potassium into the cell.

By definition, active transport implies that energy must be expended for the movement to occur against a concentration gradient.

ROUTES OF GAINS AND LOSSES

>Water and electrolytes are gained in various ways.

>A healthy person gains fluids by drinking and eating.

In patients with some disorders, fluids may be provided by the parenteral route (intravenously or

subcutaneously) or by means of an enteral feeding tube in the stomach or intestine

Kidneys

Usual daily urine volume in the adult is 1 to 2 L.

A general rule is that the output is approximately 1 mL of urine per kilogramof body weight per hour (1 mL/kg/h) in all age groups.

Skin

Sensible perspiration refers to visible water and electrolyte loss through the skin (sweating).

The chief solutes in sweat are sodium, chloride, and potassium.

Actual sweat losses vary from 0 to 1,000 mL or more every hour (depending on the environmental temperature)

Continuous water loss by evaporation (approximately 600 mL/day) occurs through the skin as insensible perspiration, a nonvisible form of water loss.

Fever greatly increases insensible water loss through the lungs and the skin, as does loss of the natural skin barrier (through major burns, for example).

Lungs

The lungs normally eliminate water vapor (insensible loss) at a rate of approximately 400 mL every day. The loss is much greaterwith increased respiratory rate or depth, or in a dry climate.

GI Tract

The usual loss through the GI tract is only 100 to 200 mL daily, (approximately 8 L every 24 hours in GI circulation).

The bulk of fluid is reabsorbed in the small intestine (diarrhea and fistulas cause large losses)

In healthy people, the daily average intake and output of water are approximately equal.

Average Daily Intake and Output in an Adult

INTAKE OUTPUTOral liquids 1,300 mL Urine 1,500 mLWater in food 1,000 mL Stool 200 mLWater produced 300 mL Insensible (by metabolism ) Lungs 300 mL

Skin 600 mL

Total gain* 2,600 mL Total lo 2,600 mL

LABORATORY TESTS FOR EVALUATINGFLUID STATUS

Osmolality reflects the concentration of fluid that affects the movement of water between fluid compartments by osmosis.

Osmolality measures the solute concentration per kilogram in blood and urine.

It is also a measure of a solution’s ability to create osmotic pressure and affect the movement of water.

Serum osmolality primarily reflects the concentration of sodium. Urine osmolality is determined by urea, creatinine, and uric acid.

When measured with serum osmolality, urine osmolality is the most reliable indicator of urine concentration.

Osmolality is reported as milliosmoles per kilogram of water (mOsm/kg).

Osmolarity, another term that describes the concentration of solutions (mOsm/L).

The term “osmolality,” is used more often in clinical practice.Normal serum osmolality is 280 to 300 mOsm/kgNormal urine osmolality is 250 to 900 mOsm/kg.

Sodium predominates in ECF osmolality and holds water in this compartment.

Serum osmolality may be measured directly through laboratory tests or estimated at the bedside by doubling the serum sodium level or by using the following formula:

Na⁺x2=Glucose + BUN = Approx. value18 3 of serum osmolality

The calculated value usually is within 10 mOsm of the measured Osmolality.

Comparison of Serum and Urine Osmolality

FACTORS FACTORSINCREASING DECREASING

FLUID OSMOLALITY OSMOLALITY

Serum Free water loss SIADH(275–300 mOsm/kg) Diabetes insipidus Renal failure

Sodium overload Diuretic useHyperglycemia AdrenalUremia insufficiency

Urine Fluid volume deficit Fluid volume excess(250–900 mOsm/kg) SIADH Diabetes insipidus

HF Acidosis

Urine specific gravity measures the kidneys’ ability to excrete or conserve water.

The specific gravity of urine is compared to the weight of distilled water, which has a specific gravity of 1.000.

The normal range of specific gravity is 1.010 to 1.025.

Urine specific gravity can be measured at the bedside by placing a calibrated hydrometer or urinometer in a cylinder of approximately 20 mL of urine.

Specific gravity can also be assessed with a refractometer or dipstick with a reagent.

Specific gravity varies inversely with urine volume; normally, the larger the volume ofurine, the lower the specific gravity.

Specific gravity is a less reliable indicator of concentration than urine osmolality; increased glucose or protein in urine can cause a falsely high specific gravity.

Factors that increase or decrease urine osmolality are the same for urine specific gravity.

Blood urea nitrogen (BUN) is made up of urea, an end product of metabolism of protein (from both muscle and dietary intake) by the liver.

Amino acid breakdown produces large amounts of ammonia molecules, which are absorbed into the bloodstream.

Ammonia molecules are converted to urea and excreted in the urine. The normal BUN is 10 to 20 mg/dL (3.5–7 mmol/L).

The BUN level varies with urine output. Factors that increase BUN include decreased renal function, GI bleeding, dehydration,increased protein intake, fever, and sepsis. Those that decrease BUN include end-stage liver disease, a low-protein diet, starvation, and any condition that results in expanded fluid volume(eg, pregnancy).

Creatinine is the end product of muscle metabolism. It is a better indicator of renal function than BUN (does not vary with protein intake and metabolic state)

The normal serum creatinine is approximately 0.7 to 1.5 mg/dL (SI: 60–130 mmol/L)

Its concentration depends on lean body mass and varies from person to person.

Serum creatinine levels increase when renal function decreases.

Hematocrit measures the volume percentage of red blood cells (erythrocytes) in whole blood.

normal ranges 44% to 52% for males 39% to 47% for females

Conditions:increase the hematocrit value

dehydration and polycythemia

decrease hematocrit overhydration and anemia

Urine sodium values change with sodium intake and the status of fluid volume (as sodium intake↑ , excretion ↑; as the circulating fluid volume ↓, sodium is conserved)

Normal urine sodium levels range from 50 to 220 mEq/24 h (50–220 mmol/24 h)

A random specimen usually contains more than 40 mEq/L of sodium

Urine sodium levels are used to assess volume status and are useful in the diagnosis of hyponatremia and acute renal failure.

HOMEOSTATIC MECHANISMS

The body is equipped with remarkable homeostatic mechanisms to keep the composition and volume of body fluid within narrow limits of normal.

Organs involved in homeostasis: kidneys lungs heart adrenal glands parathyroid glands pituitary gland

Kidney Functions

Vital to the regulation of fluid and electrolyte balance

Filter 170 L of plasma every day in the adult, while excreting only 1.5 L of urine

Act both autonomously and in response to blood-borne messengers (aldosterone and antidiuretic hormone (ADH)

Major functions of the kidneys in maintaining normal fluid balance include the following:

• Regulation of ECF volume and osmolality by selective retention and excretion of body fluids• Regulation of electrolyte levels in the ECF by selective retention of needed substances and excretion of unneeded substances• Regulation of pH of the ECF by retention of hydrogen ions• Excretion of metabolic wastes and toxic substances

Renal failure will result in multiple fluid and electrolyte problemsRenal function declines with advanced age (as do muscle mass and daily exogenous creatinine production)

High-normal and minimally elevated serum creatinine values may indicate substantially reduced renal function in the elderly

Heart and Blood Vessel Functions

The pumping action of the heart circulates blood through the kidneys under sufficient pressure to allow for urine formation.

Failure of this pumping action interferes with renal perfusion and thus with water and electrolyte regulation.

Lung Functions

Lungs are also vital in maintaining homeostasis exhalation- remove approximately 300 mL of water daily in the normal adult

Abnormal conditions, such as hyperpnea (abnormally deep respiration) or continuous coughing, increase this loss

Mechanical ventilation with excessive moisture decreases it.

The lungs also have a major role in maintaining acid–base balance

Changes from normal aging result in decreased respiratory function, causing increased difficulty in pH regulation in older adults with major illness or trauma.

Pituitary Functions

Hypothalamus manufactures ADH, which is stored in the posterior pituitary gland

(released as needed)ADH is sometimes called the water-conserving hormone (because it causes the body to retain water)

Functions of ADH >maintaining the osmotic pressure of

the cells by controlling the retention or excretion of water by the kidneys

>regulating blood volume

Adrenal Functions

Aldosterone, a mineralocorticoid secreted by the zona glomerulosa (outer zone) of the adrenal cortex (profound effect on fluid balance)

Increased secretion of aldosterone causes sodium retention (and thus water retention) and potassium loss

decreased secretion of aldosterone causes sodium and water loss and potassium retention.

↑aldosterone =sodium retention (thus water

retention) and potassium loss

↓aldosterone = sodium and water loss and potassium retention.

Cortisol, another adrenocortical hormone, has only a fraction of the mineralocorticoid potency of aldosterone

When secreted in large quantities, it can also produce sodium and fluid retention and potassium deficit

↑Cortisol=retain Na⁺, Fluid K⁺ deficit

Parathyroid Functions

The parathyroid glands,(embedded in the thyroid gland) regulate calcium and phosphate

balance by means of parathyroid hormone (PTH)

PTH influences bone resorptioncalcium absorption from the intestinescalcium reabsorption from the renal

tubules

Other Mechanisms

Changes in the volume of the interstitial compartment within the ECF can occur without affecting body function

The vascular compartment, cannot tolerate change as readily and must be carefully maintained to ensure that tissues receive adequate nutrients.

BARORECEPTORS

The baroreceptors are small nerve receptors

Detect changes in pressure within blood vessels and transmit this information to CNS

Responsible for monitoring the circulating volume

Regulate sympathetic and parasympathetic neural activity as well as endocrine activities

Categorized as low-pressure and high-pressure baroreceptor systems

Low-pressure baroreceptors-located in the cardiac atria (left atrium)High-pressure baroreceptors- nerve endings in the aortic arch and in the cardiac sinus

Another high-pressure baroreceptor is located in the afferent arteriole of the juxtaglomerular apparatus of the nephron

As arterial pressure decreases, baroreceptors transmit fewer impulses from the carotid sinuses and the aortic arch to the vasomotorcenter. A decrease in impulses stimulates the sympathetic nervous system and inhibits the parasympathetic nervous system.

The outcome is an increase in cardiac rate, conduction, and contractility and in circulating blood volume.

↓ arterial pressure ↓

baroreceptors transmit ↓

fewer impulses ↓

from the carotid sinuses and the aortic arch

↓ to the vasomotor center ↓

↓ impulses ↓ stimulates SNS and inhibits PNS

⇩ ↑ cardiac rate, conduction

↑ contractility, circulating blood volume

Sympathetic stimulation constrictsrenal arterioles; this increases the release of aldosterone, decreases glomerular filtration, and increases sodium and water reabsorption.

Sympathetic stimulation constricts renal arterioles:

↑ the release of aldosterone

↓glomerular filtrationand ↑sodium and water reabsorption.

RENIN–ANGIOTENSIN–ALDOSTERONE SYSTEM

Renin-an enzyme that converts angiotensinogen, (inactive substance formed by the liver), into angiotensin I

Angiotensin-converting enzyme (ACE) convertsangiotensin I to angiotensin II. Angiotensin II, with its vasoconstrictor properties, increases arterial perfusion pressure and stimulates thirst.

Renin is released by the juxtaglomerular cells of the kidneys in response to decreased renal perfusion.

As the sympathetic nervous system is stimulated, aldosterone is released in response to an increased release of renin

Aldosterone is a volume regulator and is also released as serum potassium increases, serum sodium decreases, or adrenocorticotropic hormone increases

Kidney (Juxtaglomerular cell)

↓ ↓ renal perfusion

↓ Renin

↓ converts into Angiotensin I

↓ Angiotensin Converting Enzyme (ACE)

↓Angiotensin II

↑ release Renin

↓Stimulates SNS

↓Aldosterone Release

↓ ↑ serum K

↓ serum Na-vasoconstriction-↑ arterial perfusion perfusion-stimulate thirst

ADH AND THIRST

ADH and the thirst mechanism have important roles in maintaining sodium concentration and oral intake of fluids

Oral intake is controlled by the thirst center located in the hypothalamus

As serum concentration or osmolality increases or blood volume decreases, neurons in the hypothalamus are stimulated by intracellular dehydration; thirst then occurs, and the person increases oral intake of fluids

Water excretion is controlled by ADH, aldosterone, and baroreceptors

The presence or absence of ADH is the most significant factor in determining whether the urine that is excreted

is concentrated or diluted.

↑ serum conc⁻;smolality

↓ bld. Volume

⇣ cellulardehydration

⇣hypothalamus (nueron)

↓ Thirst occurs

OSMORECEPTORS

Located on the surface of the hypothalamus, osmoreceptors sense changes in sodium concentration. As osmotic pressure increases, the neurons become dehydrated and quickly release impulses to the posterior pituitary, which increases the release of ADH. ADH travels in the blood to the kidneys, where it alters permeability to water, causing increased reabsorption of water and decreased urine output.

Na⁺ conc⁻ ↡ Hypothalamus (osmoreceptor) ↓ ↑ osmotic pressure

↓ Neuron become dhn ↓ Quick impulse to PPG ↓ ↑ ADH

Travel in blood to kidney↓

Alter permeability of water↓

↑ reabsorption of water↓

↓ urine output

The retained water dilutes the ECF and returns its concentration to normal

Restoration of normal osmotic pressure provides feedback to the osmoreceptors to inhibit further ADH release

RELEASE OF ATRIAL NATRIURETIC PEPTIDE

Atrial natriuretic peptide (ANP) is released by cardiac cells in the atria of the heart in response to increased atrial pressure

Any disorder that results in volume expansion or increased cardiac filling pressures (eg, high sodium intake, heart failure, chronic renal failure, atrial

tachycardia, or use of vasoconstrictor agents) will increase the release of ANP.

Atria of Heart

↓Atrial Natriuretic Peptide

↑ pressure

Therefore:

↑ cardiac filling pressure = ⇧ ANP Volume expansion

- ↑ Na⁺ intake- HF- CRF- Atrial tachycardia- Vasoconstrictor agent

The action of ANP is the direct opposite of the renin–angiotensin–aldosterone system and decreases blood pressure and volume

ANP in plasma is normally 20 to 77 pg/mL (20—77 ng/L).

⇧ANPacute heart failure paroxysmal atrial tachycardia hyperthyroidism subarachnoid hemorrhagesmall cell lung cancer

⇩ANP chronic heart failure medications urea (Ureaphil)

prazosin (Minipress)

“A wise man makes his own decisions, an ignorant man follows public opinion…”

Sunset is as good as a sunrise