17
Fluid, Electrolyte, and Acid-Base
Imbalances
Margaret R. Rateau
http://evolve.elsevier.com/Lewis/medsurg/
CONCEPTUAL FOCUS
Acid-Base Balance Safety
Fluid and Electrolytes Self-Management
Perfusion
LEARNING OUTCOMES
1. Describe the composition of the major body fluid e. Calcium imbalances: hypercalcemia and hypocalcemia
compartments. f. Phosphate imbalances: hyperphosphatemia and
2. Define processes involved in regulating water hypophosphatemia
and electrolyte movement between the body fluid 4. Identify the processes involved in maintaining acid-base
compartments. balance.
3. Describe the etiology, manifestations, and interprofessional 5. Discuss the etiology, manifestations, and nursing and
management of: interprofessional management of acid-base imbalances:
a. Extracellular fluid volume imbalances: fluid volume metabolic acidosis, metabolic alkalosis, respiratory acidosis,
deficit and fluid volume excess and respiratory alkalosis.
b. Sodium imbalances: hypernatremia and hyponatremia 6. Describe the composition of and indications for common
c. Potassium imbalances: hyperkalemia and hypokalemia IV fluid solutions.
d. Magnesium imbalances: hypermagnesemia and 7. Discuss the types and nursing management of commonly
hypomagnesemia used central venous access devices.
KEY TERMS
acidosis hypernatremia
alkalosis hypertonic
buffers hypocalcemia
central venous access devices (CVADs) hypokalemia
electrolytes hyponatremia
fluid spacing hypotonic
hydrostatic pressure isotonic
hypercalcemia osmolality
hyperkalemia
Body fluids and electrolytes play an important role in maintain- Fluid and electrolyte imbalances occur in most patients with
ing homeostasis, the body’s stable internal environment. Body a major illness or injury because illness disrupts normal homeo-
fluids are in constant motion transporting nutrients, electro- static mechanisms. Illness or disease directly causes some fluid
lytes, and oxygen to cells and carrying waste products away and electrolyte imbalances (e.g., burns, heart failure). Other
from cells. The body uses many adaptive responses to keep the times, therapeutic measures (e.g., colonoscopy preparation,
composition and volume of fluids and electrolytes within nar- diuretics) cause or contribute to imbalances. Perioperative
row limits to maintain homeostasis and promote health. patients are at risk for developing fluid and electrolyte
302
, CHAPTER 17 Fluid, Electrolyte, and Acid-Base Imbalances 303
imbalances because of fluid restrictions, blood or fluid loss, and has a higher percentage of water. Fat tissue has a lesser percent-
the stress of surgery. age of water. So, the more fat present in the body, the less the
Conceptually, these imbalances are often reflected by total water content. Women generally have a lower percentage
changes in perfusion, gas exchange, mobility, and cogni- of body water because they tend to have less lean body mass
tion. For example, a patient with metastatic colon cancer may than men. Older adults also tend to have less lean body mass.
develop hypercalcemia because of bone destruction from tumor This results in a lower percentage of body water when compared
invasion. They have severe muscle weakness and confusion. to younger adults. In older adults, body water content averages
Chemotherapy used to treat the cancer may result in nausea and 45% to 50% of body weight. This places them at a higher risk for
vomiting, causing dehydration and hypotension. When correct- fluid-related problems than young adults.
ing dehydration with IV fluids, the patient needs close monitor-
ing to prevent fluid overload. Body Fluid Compartments
We classify fluid and electrolyte imbalances as deficits or The 2 fluid compartments in the body are the intracellular space
excesses. Fluid volume deficit (hypovolemia) and volume excess (inside the cells) and the extracellular space (outside the cells)
(hypervolemia) are common clinical conditions. Volume imbal- (Fig. 17.2). About two-thirds of body water is found within cells.
ances are usually accompanied by one or more electrolyte It is the intracellular fluid (ICF). ICF makes up about 40% of body
imbalances, especially changes in the serum sodium level. weight of an adult. This means a 70-kg person would have about
Although each imbalance is discussed separately in this 42 L of water, with about 28 L of that water within their cells.
chapter, it is common for more than one imbalance to occur in The fluid in the extracellular space is extracellular fluid (ECF).
the same patient. For example, a patient with prolonged nasoga- The 2 main compartments containing ECF are the interstitial
stric (NG) suction will lose sodium, potassium, hydrogen, and fluid, the fluid in the spaces between cells, and the intravascu-
chloride. This may result in low sodium and potassium levels, lar fluid or plasma, the liquid part of blood. Other ECF com-
fluid volume deficit, and metabolic alkalosis caused by the loss partments include lymph and transcellular fluids. Transcellular
of HCl acid. fluids include cerebrospinal fluid; fluid in the gastrointestinal
It is important to anticipate the potential for fluid and elec- (GI) tract and joint spaces; and pleural, peritoneal, intraoc-
trolyte imbalances associated with certain disorders and medical ular, and pericardial fluid. ECF makes up about one-third of
therapies, recognize the signs and symptoms of imbalances, and the body water. This amounts to about 14 L in a 70-kg person.
intervene with the appropriate action. This chapter describes the About one-third of ECF is in the intravascular space as plasma
(1) normal control of fluids, electrolytes, and acid-base balance; (2) (3 L in a 70-kg person). Two-thirds are in the interstitial space
conditions that disrupt homeostasis and resulting manifestations; or about 10 L in a 70-kg person. The fluid in the transcellular
and (3) actions that the HCP and you can take to manage fluid, spaces totals about 1 L at any given time.
electrolyte, and acid-base imbalances and restore homeostasis.
Calculation of Fluid Gain or Loss
One liter of water weighs 2.2 lb (1 kg). Bodyweight change,
WATER CONTENT OF THE BODY especially a sudden change, is an excellent indicator of overall
The body is mainly composed of water. It accounts for about fluid volume loss or gain. For example, if a patient drinks 240
50% to 60% of body weight in the adult. Water content varies mL (8 oz) of fluid, weight gain will be 0.5 lb (0.23 kg). A patient
with body mass, gender, and age (Fig. 17.1). Lean body mass receiving diuretic therapy who loses 4.4 lb (2 kg) in 24 hours
has a fluid loss of about 2 L. An adult patient who is fasting may
lose 1 to 2 lb/day. A weight loss exceeding this is likely due to
100
loss of body fluid.
90
Percent of body weight that is water
80
70
ICF
60
50
Plasma: (3 L)
40
30 Interstitial fluid Plasma
(IF): (10 L) IF
20
Intracellular fluid
10 (ICF): (28 L)
ICF
0
Preterm Neonate Child Adult Older adult
Fig. 17.2 Relative volumes of 3 body fluids. Values represent fluid dis-
Fig. 17.1 Body water over the life span. tribution in a young male adult.
,304 SECTION 3 Problems Related to Homeostasis and Protection
These include simple diffusion, facilitated diffusion, and active
ELECTROLYTES transport. Water moves as driven by two forces: hydrostatic
Electrolytes are substances whose molecules dissociate, or split, pressure and osmotic pressure.
into ions when placed in water. Ions are electrically charged
particles. Cations are positively charged ions. Examples include Diffusion
sodium (Na+), potassium (K+), calcium (Ca2+), and magnesium Diffusion is the movement of molecules from an area of high
(Mg2+) ions. Anions are negatively charged ions. Examples include concentration to low concentration (Fig. 17.4). Net movement
bicarbonate (HCO3−), chloride (Cl−), and phosphate (PO3− 4 ) ions. of molecules stops when the concentrations are equal in both
Most proteins bear a negative charge and thus are anions. areas. It occurs in liquids, gases, and solids. Simple diffusion
requires no external energy.
Measurement of Electrolytes
We express electrolyte concentration in body fluids as milliequiv- Facilitated Diffusion
alents (mEq) per liter. Since electrolytes are active chemicals, we Facilitated diffusion involves the use of a protein carrier in the
express their concentration according to their chemical activity, cell membrane. The protein carrier combines with a molecule,
or the number of electrolytes able to combine chemically. Ions especially one too large to pass easily through the cell membrane
combine milliequivalent for milliequivalent. For example, 1 mEq and helps move the molecule across the membrane from an area
(1 mmol) of sodium combines with 1 mEq (1 mmol) of chloride. of high to low concentration. Like simple diffusion, facilitated
diffusion is passive and requires no energy. An example of facil-
Electrolyte Composition of Fluid Compartments itated diffusion is glucose transport into the cell. The large glu-
Electrolyte composition varies between ECF and ICF. The overall cose molecule must combine with a carrier molecule to be able
concentration of electrolytes is nearly the same in the ECF and ICF. to cross the cell membrane and enter most cells.
However, concentrations of specific ions differ greatly (Fig. 17.3). In
ECF, the main cation is sodium, with small amounts of potassium, Active Transport
calcium, and magnesium. The primary ECF anion is chloride, with Active transport is a process in which molecules move against
small amounts of bicarbonate, sulfate, and phosphate anions. the concentration gradient. External energy is needed for this
In ICF, the main cation is potassium, with small amounts of process. An example is the sodium-potassium pump. The con-
magnesium and sodium. The main ICF anion is phosphate, with centrations of sodium and potassium differ between the ICF and
some protein and a small amount of bicarbonate. Table 17.1 lists ECF (see Fig. 17.3). To maintain this concentration difference,
normal serum electrolyte values.
TABLE 17.1 Normal Serum Electrolyte
MECHANISMS CONTROLLING FLUID AND Values
ELECTROLYTE MOVEMENT Electrolyte Reference Interval
The movement of electrolytes and water between ICF and ECF Anions
to maintain homeostasis involves many different processes. Bicarbonate (HCO3 − ) 22–26 mEq/L (22–26 mmol/L)
Chloride (Cl−) 98–106 mEq/L (98–106 mmol/L)
Phosphate (PO34−) 3.0–4.5 mg/dL (0.97–1.45 mmol/L)
PLASMA INTRACELLULAR
Cations Anions Cations Anions Cations
Calcium (Ca2+) (total) 9.0–10.5 mg/dL (2.25–2.62 mmol/L)
Calcium (ionized) 4.5–5.6 mg/dL (1.05–1.3 mmol/L)
Magnesium (Mg2+) 1.3–2.1 mEq/L (0.65–1.05 mmol/L)
Potassium (K+) 3.5–5.0 mEq/L (3.5–5.0 mmol/L)
Na+ Cl– K+ PO43– Sodium (Na+) 136–145 mEq/L (136–145 mmol/L)
Lump
of sugar
HCO3–
Protein–
Protein–
+
K Na+
2+
HCO3–
Ca Other Time
Mg2+ PO43– Mg2+ Cl–
Fig. 17.4 Diffusion is the movement of molecules from an area of high
Fig. 17.3 The relative concentrations of the major cations and anions in concentration to an area of low concentration. Eventually, the sugar
the intracellular space and the plasma. molecules are evenly distributed.
, CHAPTER 17 Fluid, Electrolyte, and Acid-Base Imbalances 305
greater a solution’s pull or osmotic pressure. Osmotic pressure is
Na measured in milliosmoles (mOsm). We express it as either fluid
Extracellular
Na
osmolarity or fluid osmolality. Although you will often see the
K terms osmolarity and osmolality used interchangeably, they are
K
ATP different measurements. Osmolarity measures the total millios-
Na moles per liter of solution or the concentration of molecules per
volume of solution (mOsm/L). Osmolality measures the num-
Na Na K
ber of milliosmoles per kilogram of water or the concentration
of molecules per weight of water. Osmolality is the preferred
Intracellular
Na
Na Na measure to evaluate the concentration of plasma, urine, and
other body fluids. Changes in the ECF osmolality can cause sig-
K
Cell ATP nificant changes in the ICF osmolality. These changes can affect
membrane K normal cell function.1
Measurement of Osmolality
Fig. 17.5 Sodium-potassium pump. As sodium (Na+) diffuses into the
Osmolality is nearly the same in the various body fluid spaces.
cell and potassium (K+) diffuses out of the cell, an active transport sys-
tem supplied with energy delivers Na+ back to the extracellular com- Therefore, measuring or estimating plasma osmolality is a good
partment and K+ to the intracellular compartment. ATP, Adenosine way to assess the state of the body’s water balance. Calculate the
triphosphate. plasma osmolality using the following formula.2
Plasma osmolality = ( 2 × Na) + ( BUN/2.8) + ( Glucose/18)
Membrane
5% 10%
(permeable
7.5%
Normal plasma osmolality is between 280 and 295 mOsm/
to H2O,
albumin albumin not albumin) 7.5% albumin kg. A value greater than 295 mOsm/kg means that the concen-
albumin tration of solute is too great, or the water content is too little. This
condition is termed water deficit. A value less than 275 mOsm/
H2O H2O kg means there is too little solute for the amount of water or
too much water for the amount of solute. This is termed water
excess. Both conditions are clinically significant.
Net osmosis Equilibrium Urine osmolality can range from 100 to 1300 mOsm/kg. It
depends on fluid intake, the amount of antidiuretic hormone
TIME
(ADH) in circulation, and the renal response to ADH.
Fig. 17.6 Osmosis is the process of water movement through a semi-
permeable membrane from an area of low solute concentration to an
Osmotic Movement of Fluids
area of high solute concentration.
The osmolality of the fluid surrounding cells affects them.
the cell uses active transport to move sodium out of the cell and Fluids with the same osmolality as the cell interior are isotonic.
potassium into the cell (Fig. 17.5). The energy source for this Normally, ECF and ICF are isotonic to one another, so no net
movement is adenosine triphosphate (ATP). ATP is made in the movement of water occurs.
cell mitochondria. Changes in the osmolality of ECF change the volume of
cells. Solutions in which the solutes are less concentrated
Osmosis than in the cells are hypotonic (hypoosmolar). If a cell is sur-
Osmosis is the movement of water “down” a concentration gra- rounded by hypotonic fluid, water moves into the cell, causing
dient, that is, from a region of low solute concentration to one of it to swell and possibly burst. Fluids with solutes more concen-
high solute concentration, across a semipermeable membrane. trated than in cells, or an increased osmolality, are hypertonic
Osmosis requires no outside energy sources. It stops when (hyperosmolar). If hypertonic fluid surrounds a cell, water
the concentration differences disappear or when hydrostatic leaves the cell to dilute ECF. The cell shrinks and may eventu-
pressure builds and opposes any further movement of water. ally die (Fig. 17.7).
Imagine a chamber with two compartments separated by a
semipermeable membrane, one that allows only the movement Hydrostatic Pressure
of water (Fig. 17.6). If you add albumin to one side, water will Hydrostatic pressure is the force of fluid in a compartment
move from the less concentrated side (has more water) to the pushing against a cell membrane or vessel wall. In the blood
more concentrated side of the chamber (has less water) until the vessels, hydrostatic pressure is the BP generated by the heart’s
concentrations are equal. contraction. Hydrostatic pressure in the vascular system grad-
Whenever dissolved substances are contained in a space with ually decreases as the blood moves through the arteries until it
a semipermeable membrane, they can pull water into the space is about 30 mm Hg in the capillary bed. At the capillary level,
by osmosis. The concentration of the solution determines the hydrostatic pressure is the major force that pushes water out of
strength of the osmotic pull. The higher the concentration, the the vascular system and into the interstitial space.
Fluid, Electrolyte, and Acid-Base
Imbalances
Margaret R. Rateau
http://evolve.elsevier.com/Lewis/medsurg/
CONCEPTUAL FOCUS
Acid-Base Balance Safety
Fluid and Electrolytes Self-Management
Perfusion
LEARNING OUTCOMES
1. Describe the composition of the major body fluid e. Calcium imbalances: hypercalcemia and hypocalcemia
compartments. f. Phosphate imbalances: hyperphosphatemia and
2. Define processes involved in regulating water hypophosphatemia
and electrolyte movement between the body fluid 4. Identify the processes involved in maintaining acid-base
compartments. balance.
3. Describe the etiology, manifestations, and interprofessional 5. Discuss the etiology, manifestations, and nursing and
management of: interprofessional management of acid-base imbalances:
a. Extracellular fluid volume imbalances: fluid volume metabolic acidosis, metabolic alkalosis, respiratory acidosis,
deficit and fluid volume excess and respiratory alkalosis.
b. Sodium imbalances: hypernatremia and hyponatremia 6. Describe the composition of and indications for common
c. Potassium imbalances: hyperkalemia and hypokalemia IV fluid solutions.
d. Magnesium imbalances: hypermagnesemia and 7. Discuss the types and nursing management of commonly
hypomagnesemia used central venous access devices.
KEY TERMS
acidosis hypernatremia
alkalosis hypertonic
buffers hypocalcemia
central venous access devices (CVADs) hypokalemia
electrolytes hyponatremia
fluid spacing hypotonic
hydrostatic pressure isotonic
hypercalcemia osmolality
hyperkalemia
Body fluids and electrolytes play an important role in maintain- Fluid and electrolyte imbalances occur in most patients with
ing homeostasis, the body’s stable internal environment. Body a major illness or injury because illness disrupts normal homeo-
fluids are in constant motion transporting nutrients, electro- static mechanisms. Illness or disease directly causes some fluid
lytes, and oxygen to cells and carrying waste products away and electrolyte imbalances (e.g., burns, heart failure). Other
from cells. The body uses many adaptive responses to keep the times, therapeutic measures (e.g., colonoscopy preparation,
composition and volume of fluids and electrolytes within nar- diuretics) cause or contribute to imbalances. Perioperative
row limits to maintain homeostasis and promote health. patients are at risk for developing fluid and electrolyte
302
, CHAPTER 17 Fluid, Electrolyte, and Acid-Base Imbalances 303
imbalances because of fluid restrictions, blood or fluid loss, and has a higher percentage of water. Fat tissue has a lesser percent-
the stress of surgery. age of water. So, the more fat present in the body, the less the
Conceptually, these imbalances are often reflected by total water content. Women generally have a lower percentage
changes in perfusion, gas exchange, mobility, and cogni- of body water because they tend to have less lean body mass
tion. For example, a patient with metastatic colon cancer may than men. Older adults also tend to have less lean body mass.
develop hypercalcemia because of bone destruction from tumor This results in a lower percentage of body water when compared
invasion. They have severe muscle weakness and confusion. to younger adults. In older adults, body water content averages
Chemotherapy used to treat the cancer may result in nausea and 45% to 50% of body weight. This places them at a higher risk for
vomiting, causing dehydration and hypotension. When correct- fluid-related problems than young adults.
ing dehydration with IV fluids, the patient needs close monitor-
ing to prevent fluid overload. Body Fluid Compartments
We classify fluid and electrolyte imbalances as deficits or The 2 fluid compartments in the body are the intracellular space
excesses. Fluid volume deficit (hypovolemia) and volume excess (inside the cells) and the extracellular space (outside the cells)
(hypervolemia) are common clinical conditions. Volume imbal- (Fig. 17.2). About two-thirds of body water is found within cells.
ances are usually accompanied by one or more electrolyte It is the intracellular fluid (ICF). ICF makes up about 40% of body
imbalances, especially changes in the serum sodium level. weight of an adult. This means a 70-kg person would have about
Although each imbalance is discussed separately in this 42 L of water, with about 28 L of that water within their cells.
chapter, it is common for more than one imbalance to occur in The fluid in the extracellular space is extracellular fluid (ECF).
the same patient. For example, a patient with prolonged nasoga- The 2 main compartments containing ECF are the interstitial
stric (NG) suction will lose sodium, potassium, hydrogen, and fluid, the fluid in the spaces between cells, and the intravascu-
chloride. This may result in low sodium and potassium levels, lar fluid or plasma, the liquid part of blood. Other ECF com-
fluid volume deficit, and metabolic alkalosis caused by the loss partments include lymph and transcellular fluids. Transcellular
of HCl acid. fluids include cerebrospinal fluid; fluid in the gastrointestinal
It is important to anticipate the potential for fluid and elec- (GI) tract and joint spaces; and pleural, peritoneal, intraoc-
trolyte imbalances associated with certain disorders and medical ular, and pericardial fluid. ECF makes up about one-third of
therapies, recognize the signs and symptoms of imbalances, and the body water. This amounts to about 14 L in a 70-kg person.
intervene with the appropriate action. This chapter describes the About one-third of ECF is in the intravascular space as plasma
(1) normal control of fluids, electrolytes, and acid-base balance; (2) (3 L in a 70-kg person). Two-thirds are in the interstitial space
conditions that disrupt homeostasis and resulting manifestations; or about 10 L in a 70-kg person. The fluid in the transcellular
and (3) actions that the HCP and you can take to manage fluid, spaces totals about 1 L at any given time.
electrolyte, and acid-base imbalances and restore homeostasis.
Calculation of Fluid Gain or Loss
One liter of water weighs 2.2 lb (1 kg). Bodyweight change,
WATER CONTENT OF THE BODY especially a sudden change, is an excellent indicator of overall
The body is mainly composed of water. It accounts for about fluid volume loss or gain. For example, if a patient drinks 240
50% to 60% of body weight in the adult. Water content varies mL (8 oz) of fluid, weight gain will be 0.5 lb (0.23 kg). A patient
with body mass, gender, and age (Fig. 17.1). Lean body mass receiving diuretic therapy who loses 4.4 lb (2 kg) in 24 hours
has a fluid loss of about 2 L. An adult patient who is fasting may
lose 1 to 2 lb/day. A weight loss exceeding this is likely due to
100
loss of body fluid.
90
Percent of body weight that is water
80
70
ICF
60
50
Plasma: (3 L)
40
30 Interstitial fluid Plasma
(IF): (10 L) IF
20
Intracellular fluid
10 (ICF): (28 L)
ICF
0
Preterm Neonate Child Adult Older adult
Fig. 17.2 Relative volumes of 3 body fluids. Values represent fluid dis-
Fig. 17.1 Body water over the life span. tribution in a young male adult.
,304 SECTION 3 Problems Related to Homeostasis and Protection
These include simple diffusion, facilitated diffusion, and active
ELECTROLYTES transport. Water moves as driven by two forces: hydrostatic
Electrolytes are substances whose molecules dissociate, or split, pressure and osmotic pressure.
into ions when placed in water. Ions are electrically charged
particles. Cations are positively charged ions. Examples include Diffusion
sodium (Na+), potassium (K+), calcium (Ca2+), and magnesium Diffusion is the movement of molecules from an area of high
(Mg2+) ions. Anions are negatively charged ions. Examples include concentration to low concentration (Fig. 17.4). Net movement
bicarbonate (HCO3−), chloride (Cl−), and phosphate (PO3− 4 ) ions. of molecules stops when the concentrations are equal in both
Most proteins bear a negative charge and thus are anions. areas. It occurs in liquids, gases, and solids. Simple diffusion
requires no external energy.
Measurement of Electrolytes
We express electrolyte concentration in body fluids as milliequiv- Facilitated Diffusion
alents (mEq) per liter. Since electrolytes are active chemicals, we Facilitated diffusion involves the use of a protein carrier in the
express their concentration according to their chemical activity, cell membrane. The protein carrier combines with a molecule,
or the number of electrolytes able to combine chemically. Ions especially one too large to pass easily through the cell membrane
combine milliequivalent for milliequivalent. For example, 1 mEq and helps move the molecule across the membrane from an area
(1 mmol) of sodium combines with 1 mEq (1 mmol) of chloride. of high to low concentration. Like simple diffusion, facilitated
diffusion is passive and requires no energy. An example of facil-
Electrolyte Composition of Fluid Compartments itated diffusion is glucose transport into the cell. The large glu-
Electrolyte composition varies between ECF and ICF. The overall cose molecule must combine with a carrier molecule to be able
concentration of electrolytes is nearly the same in the ECF and ICF. to cross the cell membrane and enter most cells.
However, concentrations of specific ions differ greatly (Fig. 17.3). In
ECF, the main cation is sodium, with small amounts of potassium, Active Transport
calcium, and magnesium. The primary ECF anion is chloride, with Active transport is a process in which molecules move against
small amounts of bicarbonate, sulfate, and phosphate anions. the concentration gradient. External energy is needed for this
In ICF, the main cation is potassium, with small amounts of process. An example is the sodium-potassium pump. The con-
magnesium and sodium. The main ICF anion is phosphate, with centrations of sodium and potassium differ between the ICF and
some protein and a small amount of bicarbonate. Table 17.1 lists ECF (see Fig. 17.3). To maintain this concentration difference,
normal serum electrolyte values.
TABLE 17.1 Normal Serum Electrolyte
MECHANISMS CONTROLLING FLUID AND Values
ELECTROLYTE MOVEMENT Electrolyte Reference Interval
The movement of electrolytes and water between ICF and ECF Anions
to maintain homeostasis involves many different processes. Bicarbonate (HCO3 − ) 22–26 mEq/L (22–26 mmol/L)
Chloride (Cl−) 98–106 mEq/L (98–106 mmol/L)
Phosphate (PO34−) 3.0–4.5 mg/dL (0.97–1.45 mmol/L)
PLASMA INTRACELLULAR
Cations Anions Cations Anions Cations
Calcium (Ca2+) (total) 9.0–10.5 mg/dL (2.25–2.62 mmol/L)
Calcium (ionized) 4.5–5.6 mg/dL (1.05–1.3 mmol/L)
Magnesium (Mg2+) 1.3–2.1 mEq/L (0.65–1.05 mmol/L)
Potassium (K+) 3.5–5.0 mEq/L (3.5–5.0 mmol/L)
Na+ Cl– K+ PO43– Sodium (Na+) 136–145 mEq/L (136–145 mmol/L)
Lump
of sugar
HCO3–
Protein–
Protein–
+
K Na+
2+
HCO3–
Ca Other Time
Mg2+ PO43– Mg2+ Cl–
Fig. 17.4 Diffusion is the movement of molecules from an area of high
Fig. 17.3 The relative concentrations of the major cations and anions in concentration to an area of low concentration. Eventually, the sugar
the intracellular space and the plasma. molecules are evenly distributed.
, CHAPTER 17 Fluid, Electrolyte, and Acid-Base Imbalances 305
greater a solution’s pull or osmotic pressure. Osmotic pressure is
Na measured in milliosmoles (mOsm). We express it as either fluid
Extracellular
Na
osmolarity or fluid osmolality. Although you will often see the
K terms osmolarity and osmolality used interchangeably, they are
K
ATP different measurements. Osmolarity measures the total millios-
Na moles per liter of solution or the concentration of molecules per
volume of solution (mOsm/L). Osmolality measures the num-
Na Na K
ber of milliosmoles per kilogram of water or the concentration
of molecules per weight of water. Osmolality is the preferred
Intracellular
Na
Na Na measure to evaluate the concentration of plasma, urine, and
other body fluids. Changes in the ECF osmolality can cause sig-
K
Cell ATP nificant changes in the ICF osmolality. These changes can affect
membrane K normal cell function.1
Measurement of Osmolality
Fig. 17.5 Sodium-potassium pump. As sodium (Na+) diffuses into the
Osmolality is nearly the same in the various body fluid spaces.
cell and potassium (K+) diffuses out of the cell, an active transport sys-
tem supplied with energy delivers Na+ back to the extracellular com- Therefore, measuring or estimating plasma osmolality is a good
partment and K+ to the intracellular compartment. ATP, Adenosine way to assess the state of the body’s water balance. Calculate the
triphosphate. plasma osmolality using the following formula.2
Plasma osmolality = ( 2 × Na) + ( BUN/2.8) + ( Glucose/18)
Membrane
5% 10%
(permeable
7.5%
Normal plasma osmolality is between 280 and 295 mOsm/
to H2O,
albumin albumin not albumin) 7.5% albumin kg. A value greater than 295 mOsm/kg means that the concen-
albumin tration of solute is too great, or the water content is too little. This
condition is termed water deficit. A value less than 275 mOsm/
H2O H2O kg means there is too little solute for the amount of water or
too much water for the amount of solute. This is termed water
excess. Both conditions are clinically significant.
Net osmosis Equilibrium Urine osmolality can range from 100 to 1300 mOsm/kg. It
depends on fluid intake, the amount of antidiuretic hormone
TIME
(ADH) in circulation, and the renal response to ADH.
Fig. 17.6 Osmosis is the process of water movement through a semi-
permeable membrane from an area of low solute concentration to an
Osmotic Movement of Fluids
area of high solute concentration.
The osmolality of the fluid surrounding cells affects them.
the cell uses active transport to move sodium out of the cell and Fluids with the same osmolality as the cell interior are isotonic.
potassium into the cell (Fig. 17.5). The energy source for this Normally, ECF and ICF are isotonic to one another, so no net
movement is adenosine triphosphate (ATP). ATP is made in the movement of water occurs.
cell mitochondria. Changes in the osmolality of ECF change the volume of
cells. Solutions in which the solutes are less concentrated
Osmosis than in the cells are hypotonic (hypoosmolar). If a cell is sur-
Osmosis is the movement of water “down” a concentration gra- rounded by hypotonic fluid, water moves into the cell, causing
dient, that is, from a region of low solute concentration to one of it to swell and possibly burst. Fluids with solutes more concen-
high solute concentration, across a semipermeable membrane. trated than in cells, or an increased osmolality, are hypertonic
Osmosis requires no outside energy sources. It stops when (hyperosmolar). If hypertonic fluid surrounds a cell, water
the concentration differences disappear or when hydrostatic leaves the cell to dilute ECF. The cell shrinks and may eventu-
pressure builds and opposes any further movement of water. ally die (Fig. 17.7).
Imagine a chamber with two compartments separated by a
semipermeable membrane, one that allows only the movement Hydrostatic Pressure
of water (Fig. 17.6). If you add albumin to one side, water will Hydrostatic pressure is the force of fluid in a compartment
move from the less concentrated side (has more water) to the pushing against a cell membrane or vessel wall. In the blood
more concentrated side of the chamber (has less water) until the vessels, hydrostatic pressure is the BP generated by the heart’s
concentrations are equal. contraction. Hydrostatic pressure in the vascular system grad-
Whenever dissolved substances are contained in a space with ually decreases as the blood moves through the arteries until it
a semipermeable membrane, they can pull water into the space is about 30 mm Hg in the capillary bed. At the capillary level,
by osmosis. The concentration of the solution determines the hydrostatic pressure is the major force that pushes water out of
strength of the osmotic pull. The higher the concentration, the the vascular system and into the interstitial space.
