Fluid and Electrolytes

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Fluid/Electrolyte Distribution

Body fluid is water containing chemical compounds called

electrolytes plus blood cells and other soluble molecules. Body monitors and

controls two aspects of body fluid balance:

The volume of fluid

in the extracellular space, particularly vascular volume

The concentration

of solutes (osmolality) of all body fluids, which influences volume of ECF

and ICF.

Fluid

Compartments

45% to 80% of body weight is fluid.

Fat contains less fluid than muscle; women have lower fluid

content because they have more adipose tissue than men.

Two main types of body fluid:

Intracellular fluid - located within the cells

Extracellular fluid - comprising all fluid outside the cells.

Primary intracellular

electrolytes are:

potassium, phosphate, and sulfate.

Primary ECF

electrolytes are: sodium,

chloride, and bicarbonate.

ECF includes:

Intravascular

fluid: fluid inside blood and lymphatic vessels

Interstitial

fluid: fluid between cells.

The maintenance of the proportional distribution of ECF

between the vascular and interstitial spaces depend on 3 factors:

Protein content of

the blood (serum proteins, mainly albumin and globulin).

Integrity of the

vascular endothelium (layer of cells lining blood vessels).

Hydrostatic

pressure inside the blood vessels.

Protein content of blood and intact vascular endothelium

function to keep fluids within blood vessels, whereas hydrostatic pressure

tends to force fluid out of the vessels.

Extracellular

Fluid Volume

Volume of ECF, particularly vascular volume, is the most

important and regulated aspect of fluid balance.

Without adequate vascular volume, BP can't be maintained.

Prolonged periods of very low BP can result in shock, potential lethal

condition.

Baroreceptors, respond to

stretching of arteries & monitor vascular volume.

Renin-angiotensin-aldosterone

and natriuretic peptide hormone systems regulate the volume within

narrow limits by adjusting fluid intake and urinary excretion of Na+, Cl-,

and H2O.

Renin-angiotensin

Decreased arterial

blood pressure, decreased renal blood flow, increased renal sympathetic

nerve activity, or low blood sodium levels can stimulate renin

release.

Renin, an enzyme

secreted by cells in the kidney, splits angiotensinogen, produced by liver

and circulating in blood, into angiotensin I.

Converting enzymes

in lungs & other vascular beds converts angiotensin I into angiotensin

II, a potent vasoconstrictor.

Aldosterone

Angiotensin II

stimulates secretion of aldosterone.

Aldosterone,

produced by adrenal cortex, regulates sodium reabsorption in the distal

tubules & collecting ducts of kidney.

Chloride &

water passively accompany the reabsorbed sodium, resulting in reabsorption

of saline, 0.9% solution of sodium chloride, which is same concentration

as ECF.

Natriuretic Peptides

Produced by cardiac

atria (ANP), ventricles (discovered first in brain & called BNP,

referred to as B-type natriuretic peptide), and other body parts in

response to changes in ECF volume. When atrial pressure is increased, ANP

acts on nephron to increase sodium excretion. Low atrial pressures inhibit

release of ANP.

Intracellular

Fluid Volume

Volume in

intracellular space is closely regulated.

Although water

moves freely across cell membrane, many electrolytes move only with active

transport.

Intracellular

organs require appropriate balance of water & electrolytes to function

adequately.

Cells with too

much water in them swell and burst if stretched too far (hypotonic).

Cells with too

little water, shrivel & die (hypertonic).

Osmosis. Water molecules moving through a red blood cell

membrane in three different concentrations of fluid. Left: The normal saline solution has a

concentration nearly the same as that inside the cell, and water molecules move

into and out of the cell at the same rate. Center: The

dilute solution causes the cell to swell and eventually hemolyze (burst)

because of the large number of water molecules moving into the cell. Right: The concentrated solution causes

the water molecules to move out of the cell, leaving it shrunken.

Water Concentration (Osmolality) of Body Fluid

2nd regulated

aspect of body fluid balance.

Osmolality refers to the proportion of dissolved particles

(solute) in a given weight of fluid (e.g., a kilogram).

Expressed in

milliosmoles per kilogram (mOsm/kg).

Osmolarity refers to concentration of dissolved substances in a

given volume of fluid (e.g., a liter).

Expressed in

milliosmoles per liter (mOsm/L).

B/c body fluids are

very dilute, these terms are often used interchangeably.

Normal range for

serum osmolality is 280-300 mOsm/kg (Craven).

Whereas body fluids

are measured in osmolality, other fluids, such as IV fluids, are measured

in osmolarity.

Cells in

hypothalamus monitor changes in body fluid osmolality.

Respond by varying

the secretion of antidiuretic hormone (ADH) from posterior pituitary.

ADH produced by

hypothalamus, passes down axons into posterior pituitary, where it's

stored.

When plasma

osmolality increases & activates hypothalamic osmoreceptors, ADH is

released into systemic circulation.

It maintains

blood's osmolality within normal limits by adjusting amount of water

excreted into urine.

Increased

permeability in distal & collecting tubules in kidney allows more

water reabsorption & serves to conserve water.

When ADH is

increased, urine becomes more concentrated.

If plasma

osmolality is decreased due to excess of water in relation to solute,

ADH release is inhibited.

When vascular

volume is normal, changes in plasma osmolality control ADH release.

But when

arterial BP is noticeably

decreased (e.g., HF, shock), ADH is released regardless of

plasma osmolality in response to input from vascular baroreceptors.

Electrolytes

Chemical compounds

that partially separate in solution.

Carry electrical

charges known as ions.

Cations: Positively charged ions; they include: sodium (Na+), potassium (K+), calcium (Ca++), and magnesium (Mg++).

Anions: Negatively charged ions that include: chloride (Cl-), phosphate (HPO4- and H2PO4-), sulfate (SO4-), and bicarbonate (HCO3-).

Most common extracellular electrolytes are

Sodium (Na+)

Chloride (Cl-)

Bicarbonate (HCO3-)

Most common intracellular electrolytes are

Potassium (K+)

Phosphate (HPO4-)

Sulfate (SO4-)

NORMAL SERUM ELECTROLYTE VALUES

Electrolytes are expressed in terms of their combining power,

or ability of cations to combine with anions.

Milliequivalent (the unit used to give concentration of an electrolyte

in solution) is the measure of this chemical activity.

Maintaining

electrolyte balance refers to keeping the concentration of each

electrolyte in the serum within normal limits.

Electrolyte

imbalance refers to increase or decrease of concentration of ions within

serum.

Because concentration of cellular electrolytes (ICF)

can't be measured clinically, the nurse relies on changes in the serum

levels (ECF) to reflect body electrolyte imbalance.

Sodium

Most abundant

cation in ECF.

Normal serum Na+ levels range from

135-145 mEq/L.

If sodium is high,

that mean that there isn't enough water to follow, if it's low, there's

too much water.

Regulated by

renin-angiotensin-aldosterone system and natriuretic peptides.

Found in table

salt, dairy products, meat, eggs, and certain veggies, food processing

also tends to add salt in the preserving process.

Potassium

Essential for

normal cardiac, neural, and muscle function and contractility of all

muscles.

Normal serum K+ ranges from 3.5 -

5.3 mEq/L.

Two hormones exert

major control over extracellular concentration: insulin & aldosterone.

Insulin,

pancreatic hormone, promotes transfer of potassium (and glucose) from ECF

into skeletal muscle & liver cells.

Aldosterone

enhances renal excretion of potassium.

Increase in serum

potassium stimulates release of aldosterone to lower concentration of

the ion, whereas a decrease in serum potassium inhibits release of

aldosterone to reduce excretion of the ion.

Calcium

Normal total serum

Calcium levels range from 8.2-10.2 mg/dL.

~ 99% of body's

calcium is found within bones & teeth. Remainder is in the serum.

Calcium is present

in blood in two states: ionized & bound to protein.

50% is ionized,

with remainder bound to proteins, mainly albumin.

Only changes in

ionized calcium levels can cause signs and symptoms associated with

calcium imbalances.

b/c large portion

of calcium is bound to albumin, serum albumin levels should be checked

when lab data are evaluated.

If serum albumin levels are decreased which may

happen with liver disease or wasting, total calcium level probably also

will be decreased, but the ionized calcium level may be within normal

limits.

Measurement of ionized calcium level, normally 4.12 - 4.92 mg/dL, is required to determine whether calcium replacement

is required.

Calcium has a

reciprocal relationship with phosphorus.

Calcium is

indispensable for healthy functioning:

Cell membrane

structure depends on calcium, which promotes cell-to-cell adhesion.

Important in

wound healing,

synaptic

transmission in nervous tissue

Membrane

excitability

Muscle

contractility

Teeth and bone

structure

Essential for

blood clotting & is critical in metabolic reactions involved in

energy production (glycolysis).

Parathyroid hormone

(PTH), vitamin D, and to some extent, calcitonin regulate calcium and

phosphate balance.

PTH causes serum

calcium levels to increase by increasing intestinal & renal

reabsorption of calcium & releasing calcium from bone.

PTH increases

calcium levels but decreases serum phosphate levels.

Opposite happens

with decreased secretion of PTH, serum calcium levels are lowered and

serum phosphate concentration is increased.

Calcium is found

in dairy products, sardines, whole grains, leafy green veggies.

Magnesium

Normal serum

magnesium level ranges from 1.6 - 2.6 mg/dL.

50-60% of magnesium

is in bone, much of rest is in soft tissue and body fluids.

Primarily an

intracellular ion, with only 2% in ECF.

Important in

regulating neuromuscular function & cardiac activity.

Magnesium

deficiency often is accompanied by hypocalcemia (Spiegel).

Kidney regulates

magnesium levels by reabsorbing the ion when serum levels are low &

excreting it when they're high.

Sources are green

leafy veggies, legumes, citrus fruit, peanut butter, & chocolate.

Phosphorus

Normal serum

phosphorus level ranges from 2.5

4.5 mg/dL.

Approximately 85%

of phosphorus is in bone, 14% in ICF, and only 0.1% in the ECF

(popovtzer).

Levels are

controlled by regulation of renal excretion under the influence of

vitamin-D and PTH.

Important in energy

metabolism, structure of bones & membranes, and synthesis of nucleic

acids (RNA & DNA).

There's often a

reciprocal relation between phosphorus & calcium levels.

Sources are dairy

products, meats, veggies, fruits, & cereals.

Fluid and Electrolyte Distribution

Fluid movement

occurs primarily by osmosis & filtration.

Electrolytes &

other dissolved particles move by means of diffusion, filtration, and

active transport.

Processes of

Fluid & Electrolyte Movement

Diffusion

Movement of a

solvent or solutes (molecules) from area of higher solvent or solute

concentration to area of lower solvent or solute concentration.

If drop of highly

concentrated solution is placed in one side of container of fluid,

molecules will bounce off each other until they're equally distributed

throughout the solution.

Osmosis

Movement of fluid

through a semipermeable membrane.

Semipermeable

membrane allows some substances to cross it but not others.

Osmosis can occur

if a semipermeable membrane separating two fluid compartments is permeable

to water and if one compartment contains a greater

concentration of a dissolved substance (hyperosmolar)than the other

compartment (hypoosmolar) lesser

concentration than other compartment.

Capillary vessel

walls & cell walls are semipermeable membranes.

Active Transport

Process by which

ions & other molecules are moved across membranes from an area of

lesser concentration to area of greater concentration.

Energy is required

to move ions against a concentration gradient.

Sodium–potassium pump. Energy provided by ATP causes sodium

to move to the outside of the cell and potassium to move to the inside of the

cell. For every molecule of ATP, three molecules of sodium are transported out

and two molecules of potassium move into the cell.

Higher

concentration of sodium ions is present in ECF, whereas concentration of

sodium in ICF is lower.

Pump prevents

sodium from accumulating inside cells by actively transporting sodium out

of cells.

Filtration

Involves transfer

of water & dissolved substances through a permeable membrane from a

region of high pressure to a region of low pressure.

Hydrostatic

pressure, pressure exerted by fluid against the walls of its container,

promotes flow of fluid out of the capillaries.

Filtration occurs

within kidney's glomerular capillaries & in blood capillaries.

Pressures

Affecting Fluid & Electrolyte Movement

Osmotic Pressure

& Tonicity

Osmotic pressure, force of attraction for water by undissolved

particles, helps to keep fluid within blood vessels, opposing net flow

outward.

Plasma proteins

contribute to osmotic pressure because they attract water.

Term for osmotic

pressure that's produced by plasma proteins is called colloid oncotic pressure.

Major extracellular

substances that contribute to movement of water between ECF& cell

cytoplasm are sodium, chloride, and glucose.

Solution that has

the same osmotic pressure or osmolality as blood plasma is called

iso-osmotic.

Tonicity, refers to fluid's effect on cell size.

When an isotonic solution enters circulation, there's no net movement

of water across membranes, so cells retain their normal size.

Normal saline

(0.9% NaCl) is an isotonic solution.

Hypotonic solution has a concentration of solute that's less

than that of blood plasma

When hypotonic

solution (water) surrounds cells, water crosses membrane into cells,

causing them to swell, which is why water is never used as an IV fluid.

Hypertonic solution, the effective concentration of solute is

greater than that of blood plasma.

When a hypertonic

solution such as 3% sodium chloride (hypertonic saline) is infused,

water leaves cells, causing them to decrease in size.

Hydrostatic

Pressure

Causes filtration

of fluid from area of higher pressure to area of lower pressure, such as

blood vessels into interstitial fluid compartment.

Factors that

influence include arterial BP (force with which the heart pumps blood),

the rate of blood flow, and venous pressure.

Filtration

Pressure

Hydrostatic

pressure minus osmotic pressure equals filtration pressure.

In normal

arterioles, the hydrostatic pressure is about 32 mm Hg, and the colloid

oncotic pressure is about 22 mm Hg.

The filtration

difference btwn them is about +10 mm Hg.

B/c this is a

positive pressure, fluid enters out of the vessel into the interstitial

fluid.

In the venule,

hydrostatic pressure is about 12 mm Hg, whereas colloid oncotic pressure

remains 22 mm Hg, resulting in filtration pressure of about -10 mm Hg

B/c this pressure is negative, fluid

filters from the interstitial fluid back into the venous capillaries

& venules.

Life Span Considerations

The very young and

older adults are at greater risk for fluid & electrolyte imbalances.

Adult &

Older Adult

After age 25, the

number of nephrons in the kidney begins to decrease.

Drug clearance is

reduced, placing elderly at risk for adverse drug events.

Older adults often

experience alterations in fluid & electrolyte status secondary to

chronic disease (renal failure, HF).

Older adults are at

increased risk for electrolyte imbalances during & after bowel

preparation for procedures such as a colonoscopy or barium enema.

Normal Acid-Base Balance

Important component

of cellular environment is the hydrogen ion concentrations [H+], which is

regulated within extremely narrow limits.

Vital functions

such as nerve conduction, hormonal activity, and cardiac rhythm depend on

a stable acid-base environment.

Significant

deviations are life threatening.

Acids, Bases, and pH.

Any substance that

can donate free H+ ions to a solution is called an acid.

Any substance that

can decrease H+ in a solution is a base (alkali).

The pH scale measures the degree of acidity or alkalinity

Acids and Bases in the Blood

pH of ECF is

normally 7.35-7.45.

This range is

maintained by buffers that limit pH changes in body fluids and by

elimination of acids from body through the lungs & kidneys.

Very important acid

in the body is carbonic acid.

Together, carbonic

acid & bicarbonate ion form buffer pair.

Most important in

clinical practice for 2 reasons:

B/c all buffer

systems are in equilibrium, changes in bicarbonate-carbonic acid system

mirror changes in all other systems.

The body

regulates the carbon dioxide level by changes in the respiratory rate

(ventilation) and bicarbonate level by adjustments in the amount of

bicarbonate lost in urine or amount regenerated by kidneys.

Acid-Base Balance Regulation

Hydrogen ion

balance is closely regulated.

Death usually

results if blood pH falls below 6.8 or increases above 7.8.

B/c metabolism

continually produces acids, maintenance of pH depends on 2 processes:

buffering & compensation.

Buffers allow

acids/bases to be transported from where they're produced to where

they're excreted w/o causing large swings in pH.

With compensation,

the body either excretes or retain acids or bases to compensate for

losses.

Buffering

Substances that

help to prevent large changes in pH by absorbing or releasing H+ ions.

Compensation

The respiratory

& renal systems excrete acids & bases from the body and are able

to compensate for any imbalances.

TABLE

28-4 NORMAL ARTERIAL BLOOD GAS VALUES

Abbreviation

Normal Range

Definition

pH

7.35–7.45

Reflects the hydrogen ion concentration of arterial blood

Acidosis: <7.35

Alkalosis: >7.45

PaCO2

35–45 mm Hg

Reflects partial pressure of carbon dioxide in arterial blood

Hypocapnia: Low

partial pressure of carbon dioxide in arterial blood, <35 mm Hg

Hypercapnia: High

partial pressure of carbon dioxide in arterial blood, >45 mm Hg

PaO2

80–100 mm Hg

Partial pressure of oxygen in arterial blood

HCO3−

22–26 mEq/L

Amount of bicarbonate in arterial blood

Respiratory

Compensation

The lungs, under

control of chemoreceptor area in the brainstem respiratory center, are

responsible for controlling amount of carbon dioxide in blood.

Renal

Compensation

Kidneys influence

maintenance of normal acid-base balance by changing rate of excretion or

retention of H+ and HCO3- ions.

Factors Affecting Fluid, Electrolyte, and Acid-Base

Balance

Homeostasis

requires balanced fluid, electrolyte, and acid-base status.

Hormonal controls

& kidney function regulate this process

Fluid and Food Intake

Water is usually

taken in orally, about 2/3 in the form of water or other beverages and the

remainder from foods.

Average daily

intake for an adult is 1.300 mL of water (~6 glasses).

Additional 1.000

mL of water is obtained from food (fruits & veggies are 80-90%

water).

About 300 mL of

water is obtained through food oxidation.

The thirst

mechanism helps regulate fluid intake.

Thirst center, is

usually stimulated by an increase in plasma osmolality;

In cases of

prolonged fluid intake deficit, it can also be stimulated by decrease in

blood volume (ECF deficit).

Food provides body

with electrolytes.

Calcium in dairy

products

Sodium in salt,

processed meat & foods, bread & dairy

Potassium in

bananas, melons, oranges, apricots, broccoli, raisins, dates

Fluid & Electrolyte Output

Water & electrolytes can be lost in 4 ways:

From kidneys as

urine

From skin as

perspiration

From

gastrointestinal tract in stool or vomit

From lungs as

insensible water loss

Kidney is main organ regulating fluid balance.

Glomerular

filtration & tubular reabsorption permit kidneys to conserve or

excrete water & electrolytes to maintain homeostasis.

Normal urine output for 24 hr is about 1,500 mL if intake is

normal.

Loss of fluid through skin as sweat accounts for 100-200 mL

of fluid.

Insensible fluid loss through the skin amounts to 300-400

mL/day.

Insensible fluid

loss occurs when water molecules move from high concentration (body) to

low concentration (atmosphere).

Vomiting

Sufficient gastric

juice (ECF w/ acid) is lost from stomach, H+, Na+, & Cl- ions are depleted,

increasing risk of ECF volume deficit and/or metabolic acidosis.

Excessive loss of K+ may lead to

hypokalemia.

Diarrhea

Intestinal

secretions contain bicarbonate so diarrhea may result in metabolic

acidosis due to depletion of base.

Diaphoresis

Sweat is a

hypotonic fluid containing sodium, potassium, and chloride.

Sweating can

increase the loss of fluid & electrolytes.

Use of Diuretics

Prescribed to

increase excretion of Na+, Cl- and H2O in patients with

high BP or w/ chronic heart, renal, or liver problems.

Diuretics, except

for potassium-sparing diuretics, also promote excretion of potassium &

magnesium from body; increasing risk of electrolyte deficits.

Stress

When stress occurs,

aldosterone production is increased, causing ECF retention. S

It also increases

ADH production, resulting in decreased renal excretion of water.

Chronic Illness

Imbalances commonly

accompany chronic renal, heart, liver, & respiratory problems.

Renal Failure

As kidney function

decreases due to damaged or dying nephrons, there may be an abnormal loss

or accumulation of sodium, chloride, potassium, and fluid in the body,

resulting in ECF and water excesses or even deficits. The kidneys are less

able to regulate electrolyte excretion, so abnormal levels may

occur.

Heart Failure

As the heart fails

to pump effectively, blood pressure falls. The secretion of aldosterone

and ADH is stimulated, often resulting in ECF volume and water excesses.

This fluid collects in the lungs, increasing the risk of pulmonary edema,

and in the rest of the body, where it appears as pitting or dependent

edema.

LIVER FAILURE

Liver failure influences ECF and water balance. People with liver

failure frequently present with a water excess thought to be related to

increased plasma levels of ADH.

RESPIRATORY FAILURE

Chronic respiratory problems disrupt acid–base balance.

Progressive destruction of alveoli limits the lungs’ functional ability to

excrete carbon dioxide (carbonic acid). The pH of the blood falls, and

chronic respiratory acidosis occurs.

SURGERY

Many preoperative and postoperative

factors influence the surgical patient’s fluid, electrolyte, and acid–base

status. Before surgery, the patient may not be allowed food or fluids by

mouth (NPO) and may receive enemas. During surgery, increased

insensible water loss occurs as the internal body structures are exposed

to air, and blood also is lost.

ALTERED FLUID,

ELECTROLYTE, AND ACID–BASE BALANCE

Disruptions in homeostasis and disease states can affect fluid,

electrolyte, or acid–base balance.

Fluid imbalances

include ECF volume excess or deficit and water excess or deficit.

Electrolyte imbalances include excesses or deficits of potassium, calcium,

magnesium, or phosphate.

Fluid Imbalances

A state of fluid

imbalance can occur if too much (excess) or too little (deficit) fluid is

in any fluid compartment. The two major categories of fluid balance

problems are ECF volume balance problems and water or osmolality balance

problems.

Extracellular

Fluid Volume Imbalance

ECF is a 0.9%

solution of sodium, chloride, and water. Because cell membranes are

relatively impermeable to the sodium ion and also because sodium is

actively pumped out of cells (with chloride and water following

passively), changes in

ECF volume

influence only the vascular and interstitial compartments of body

fluid.

The most critical ECF volume imbalance is an ECF deficit

because it can lead to low blood pressure, shock, and death.

Extracellular

Fluid Volume Deficit

ECF

volume deficit involves the loss of ECF, which contains primarily

sodium, chloride, bicarbonate, and water in the same concentrations as

plasma. Loss of this isotonic fluid results in a decrease in the volume of

the ECF compartment. There are two subdivisions of the ECF: the vascular

volume (fluid inside the blood and lymphatic vessels) and the interstitial

volume (fluid between the cells). Other terms that are used for ECF volume

deficit include hypovolemia, saline

deficit, and isotonic

dehydration.

A special type of

ECF volume balance problem is sometimes called third

spacing.

It occurs when fluid leaves the vascular volume and is trapped within the

interstitial fluid in a given body area . A “third space” is any area

in which fluid accumulates that is physiologically unavailable to return

to its appropriate compartment.

Extra Cellular

Fluid Volume Excess

ECF

volume excess, an increase in interstitial or vascular volume, often

occurs in persons with cardiac failure, renal failure, or liver disease.

When excess fluid cannot be eliminated, hydrostatic pressure forces some

of it into the interstitial space, where it is observable as edema.

Medical management

of ECF volume excess typically involves restriction of sodium and saline

intake (low-sodium diet) and administration of diuretics. The underlying

pathology is identified and treated.

Water or

Osmolality Imbalance

Water or osmolality

balance problems occur when water intake is increased or decreased

markedly or when water is retained or excreted excessively.

Water moves down

its concentration gradient from areas of higher concentration to areas of

lower concentration.

For example, if a patient is given a

hyperosmotic IV solution (e.g., 2× normal saline), the osmolality of the

ECF will increase, and water will move from the cells into the ECF until

the osmolality of both compartments is the same. Conversely, if a patient

takes in too much water, the osmolality of the ECF will decrease, and

water will move from the ECF into the cells.

Water Deficit or

Hyperosmolality

A water deficit or

serum hyperosmolality occurs when there is a decrease in water intake, an

increase in water loss, or an excess intake of solute.

The estimated serum

osmolality is greater than 300 mOsm/kg, and the serum sodium may be

greater than 145 mEq/L, depending on the glucose level.

As serum

osmolality increases, water is drawn from the ICF compartment, causing

cellular shrinking.

As fluid is pulled

from the cells of the brain, confusion, agitation, convulsions, coma, and

death may result.

If a water deficit

develops slowly, brain cells have some capacity to adapt by creating

so-called idiogenic osmoles that prevent excessive cell shrinkage.

The medical

management of a water deficit involves giving water either orally, if

feasible, or intravenously in the form of 5% dextrose in water.

Because of the

brain cell adaptation that can occur with a water deficit, it is

recommended that the rate at which the serum sodium is returned toward

normal should be no greater than 2 mEq/L/h in adults and 0.5 mEq/L/h in

children.

Other terms used

for water deficit include hypernatremia, hypertonic

dehydration, and hypertonicity.

Water Excess or

Hypoosmolality

A water excess or

body fluid hypoosmolality occurs when there is an increase in water

intake, abnormal secretion of ADH, or decreased urinary output of water.

The estimated

serum osmolality is less than 275 mOsm/L, and the serum sodium is less

than 135 mEq/L, depending on the glucose level.

The major impact is

on the cells of the central nervous system because the swollen cells are

encased with the rigid skull.

The resulting

symptoms include lethargy, irritability, confusion, personality changes,

seizures, coma, and eventually death if there is no treatment or

treatment is ineffective.

If water excess

develops quickly, there is risk of permanent brain damage;

if it develops slowly, the brain cells

are able to adapt by extruding osmolytes by a process that is not yet

well understood.

Other terms used for water excess include hypotonic

disorder, hyponatremia, and hypotonicity.

Acid-Base Imbalances

Arterial pH between

7.35-7.45 is necessary for efficient cellular metabolism.

Person is said to

be acidotic when arterial pH is less than 7.35 and alkalotic when pH is

greater than 7.45.

RESPIRATORY

ACIDOSIS

Respiratory acidosis is indicated by a

low pH accompanied by an increased arterial concentration of carbon

dioxide, which often is clinically defined as a PaCO2 of greater

than 45 mm Hg.

When lung disease

(e.g., asthma, emphysema) or depressed neural or muscular function (as

with opioid overdose, head trauma, or motor neuron disease) compromises

breathing ability, carbon dioxide accumulates in the blood.

As the carbonic

acid level increases, free [H+] increases,

causing pH to drop.

With time, the

kidneys compensate for this acid buildup by increasing the excretion of H+ ion into the

urine and the return of HCO3− ion to the

blood.

METABOLIC

ACIDOSIS

Metabolic acidosis occurs either when

excess acid is ingested or created (diabetic ketoacidosis) or when the

kidneys are unable to retain enough bicarbonate ions to buffer free

hydrogen ions in the blood.

Metabolic acidosis is characterized by

a pH lower than 7.35 and a plasma HCO3− concentration

lower than 22 mEq/L.

It can occur with loss of bicarbonate,

as may happen with severe diarrhea, or with acid accumulation (e.g.,

ketoacids formed in uncontrolled diabetes mellitus or lactic acids

produced by oxygen deprivation).

The respiratory system compensates for metabolic acidosis by

increasing ventilation, thus increasing the rate of carbonic acid

excretion, resulting in a fall in PaCO2.

This respiratory compensation occurs relatively rapidly but

cannot alone return the acid–base balance to normal limits.

Renal compensation with the excretion of H+ ion and

retention of HCO3− takes from hours to days to occur and can, in

conjunction with respiratory compensation, return the pH to within normal

limits.

RESPIRATORY

ALKALOSIS

Hyperventilation

causes respiratory alkalosis, which is present when a high pH is

accompanied by a blood carbon dioxide concentration lower than 35 mm Hg.

Hyperventilation,

commonly caused by anxiety, fever, pain, high altitude, or asthma,

increases carbon dioxide (carbonic acid) excretion, leading to a relative

excess of blood base and an increase in pH.

To

compensate, the kidneys increase the excretion of HCO3− to the

urine, and pH returns toward normal.

Intervention,

mainly treatment for the causative underlying disorder, may be required

to reduce the hyperventilation.

METABOLIC

ALKALOSIS

Metabolic

alkalosis occurs when there is excessive loss of body acids or with

unusual intake of alkaline substances.

It can also occur in conjunction with an

ECF deficit or potassium deficit (known as contraction alkalosis).

Vomiting

or vigorous nasogastric suction frequently causes metabolic

alkalosis. Endocrine disorders and ingestion of large amounts of

antacids are other causes. The loss of stomach acid or taking in of

base causes H+ shifts in the blood, and pH increases.

Compensation

for metabolic alkalosis includes a decrease in ventilation, which allows

the blood carbon dioxide concentration to rise. The kidneys respond to

metabolic alkalosis by retaining acid and excreting HCO3−.