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Chapter 17
Fluid and Electrolyte Balance
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Homeostasis
• Homeostasis
– Dynamic process through which the body maintains
balance by constantly adjusting to internal and
external stimuli.
– Fluids move into and out of cells, bringing with them
enzymes, hormones, and nutrients, as well as
removing waste products.
– Continual movement of fluids is necessary to
maintain homeostasis.
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Homeostasis and Feedback
• Negative feedback
– When the body reverses an original stimulus to
regain homeostasis
• Positive feedback
– When the body enhances or intensifies an original
stimulus
• The major systems involved in feedback are the nervous
and endocrine systems.
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Body Fluids
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Fluids and Body Weight
• Fluids constitute 50% to 60% of total body weight.
• They are composed of water and electrolytes.
• Intracellular fluid constitutes about one-half to two-thirds
of the total body fluid in an adult.
• Extracellular fluid constitutes about one-third of the total
body fluid in an adult.
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Question
Is the following statement true or false?
Extracellular fluid is the most important fluid in fluid
balance.
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Answer
True
Without adequate ECF, the body cannot maintain normal
blood pressure. A significant loss of ECF volume can drop
blood pressure to a life-threatening point where cells can
no longer function, due to a lack of oxygen and nutrients.
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Regulation of ECF
• Primary mechanisms
– Actions of the thirst center in the hypothalamus
– Release of antidiuretic hormone (vasopressin—ADH)
from the pituitary gland
– Effects of the renin–angiotensin–aldosterone (RAA)
system
– Release of the atrial natriuretic peptide (ANP)
hormone by the heart
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Overhydration and Edema
• Daily output should approximately equal intake.
• Overhydration
– Excess of water in the body
• Edema
– Excess accumulation of fluid in interstitial (tissue)
spaces also called third-space fluid
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Overhydration and Edema (cont’d)
• Causes of edema
– Increased capillary pressure
– Decreased colloidal osmotic pressure
– Increased capillary permeability
– Obstruction of lymphatic flow
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Prevention of Edema
• Monitor the client’s sodium intake and blood pressure.
• Encourage intake of foods with potassium.
• Administer medications as ordered.
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Dehydration
• Fluid volume deficit
• Water output is greater than intake.
• Causes include:
– Disturbance of electrolytes
– Prolonged sun exposure, excessive exercise,
diarrhea, vomiting, and burns
– Inappropriate use of diuretics, malnutrition,
excessive fasting, anorexia, or bulimia
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Functions of Water
• Primary solvent within the body
• Primary compound in all body fluids
• Suspension agent
• Helps regulate body temperature, body pH, and fluid
pressures inside and outside cells
• Assists or participates in chemical reactions
• May be end product of chemical reactions
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Electrolytes and Ions
• An electrolyte is a substance that dissociates into ions
when dissolved in water.
• An ion is an atom that has acquired an electrical charge
and bonding ability.
• Cation is a positively charged ion.
– Sodium, potassium, calcium, magnesium
• Anion is a negatively charged ion.
– Chlorine, bicarbonate
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Question
Is the following statement true or false?
Common table salt is a compound.
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Answer
True
Common table salt is a compound. A compound combines
elements in exact proportions, which are the same each
time.
Saline solution (salt and water) is a mixture because it can
be separated without a chemical reaction.
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Fluid and Electrolyte Transport
• Permeability
– Ability of a membrane to allow molecules to pass
• Freely permeable membranes
– Allow easy transfer from intravascular fluid to
interstitial fluid
• Selectively permeable membranes
– Allow only specific substances to pass through
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Factors Affecting Permeability
• Size of pores in the membrane
• Osmotic pressure
• Hydrostatic pressure
• Electrical charges of the molecule, the body fluid, or the
plasma membrane
• Solubility of the molecules
• Size of the molecules
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Transportation of Fluids and Molecules
• Passive transport processes
– Diffusion
– Osmosis
– Filtration
• Active transport processes
– Move solutes “uphill,” against the normal rules of
concentration and pressure.
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Passive Transport: Diffusion
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Passive Transport: Osmosis
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Passive Transport: Filtration
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Question
Which of these is the diffusion of a pure solvent across a
semipermeable membrane?
a. diffusion
b. osmosis
c. filtration
d. active transport
e. crenation
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Answer
b. osmosis
Osmosis is the diffusion of a pure solvent, such as water,
across a semipermeable membrane in response to a
concentration gradient, in situations where the
molecules of a higher concentration are nondiffusible.
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Fluid and Electrolyte Balance
• Dependent on
– Cell membrane permeability
– Osmolarity—the property of particles in a solution to
dissociate into ions
– Electroneutrality—the balance of positive and
negative charges
• Monitor of fluid balance
– Intake and output records and daily weights
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Acid–Base Balance
• Major mechanisms to control acid–base balance
– The kidneys excrete ammonia (NH3) in the form of
ammonium (NH4) in an effort to balance hydrogen
ions
– pH buffers
• bicarbonate–base
• carbon dioxide–acid
– Excretion of CO2 (constantly produced by the cells)
– Hydrogen ions are making constant chemical changes
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Significance of Arterial Blood Gas Values
• Arterial blood gases (ABGs) are measured in a laboratory
test to determine the extent of compensation by the
buffer system.
• The pH level and amounts of specific gases in the blood
indicate if there is more acid or base and their associated
values.
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Risk for Fluid and Electrolyte Imbalances
• Infants
– More body fluid than adults
– ECF is lost more easily than ICF
– Immature kidney function
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Risk for Fluid and Electrolyte Imbalances
(cont’d)
• Elders
– ICF levels decrease; thirst sensation declines.
– Muscle tissue turns to fat.
– Many medications cause loss of fluid and electrolytes.
– Circulatory and renal disorders may cause fluid
retention.
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End of Presentation
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