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Chapter 27
Fluid, Electrolyte,
and Acid-Base
Homeostasis
Body Fluids

The human body requires constant attention to the
methods of regulating body fluid. These processes are
necessary to maintain required proportions of water
and solutes among body compartments
l
Water is by far the largest single component of the
body making up 55–80% of total body mass
(depending on age and sex). Filtration, reabsorption,
diffusion, and osmosis continually exchange water
and solutes among these compartments
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Body Fluids

In addition to its job of filtering and excreting waste
products from the blood, the kidneys are also charged to
take the lead in maintaining the composition of water
and salts in the body’s various fluid compartments
l
This makes the study of fluid
balance in this chapter a
fitting enjoiner to our
recent discussion of
renal function
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Body Fluids
(Interactions Animation)

Body Fluids and Water Flow
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Fluid Compartments

The fluid compartments of the body are all contained in
either the intracellular compartment or the
extracellular compartment
Intracellular fluid is all fluid contained inside cells,
and comprises 2/3 of all body fluids
Extracellular fluid is all fluid outside the confines of
the plasma membranes that contain all intracellular
contents. 1/3 of all body fluid is contained in the
extracellular compartment
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Fluid Compartments

While the intracellular fluids are all contained within a
single space (inside cells), extracellular fluid is found in a
number of sub compartments
Most extracellular fluid (¾) is found between the cells
of the body (interstitial fluid)
The rest is found in the intravascular fluid space (blood
plasma – about ¼), with small amounts existing as
lymph, CSF, synovial fluid, aqueous humor,
endolymph and perilymph, and pericardial fluid
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Fluid Compartments

Babies are more “wet”
than adults, with water
composing about 80%
of total body mass
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Fluid Balance

Normal fluid intake is through:
Ingestion of liquids and moist foods (2300mL/day)
Metabolic synthesis of water during cellular
respiration and dehydration synthesis (200mL/day)

Normal fluid loss is through:
The kidneys (1500mL/day)
Evaporation from the skin (600mL/day)
Exhalation from the lungs (300mL/day)
In the feces (100mL/day)
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Fluid Balance

Fluid intake and output (I & O) are usually balanced on
a daily basis, despite the fact that
intake of water and electrolytes
are rarely proportional
The kidneys excrete
excess water through
dilute urine, or excess
electrolytes through
concentrated urine
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Fluid Balance
(Interactions Animation)

Fluid Balance Animation
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Fluid Balance

A number of feedback
mechanism contribute to
balance of daily fluid inpu
and output
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Fluid Balance

Three hormones regulate renal Na+ and Cl–
reabsorption or excretion
Angiotensin II and aldosterone promote urinary Na+
and Cl– reabsorption (and water by osmosis) when
dehydrated
Atrial natriuretic peptide (ANP) promotes excretion
of Na+ and Cl– followed by water excretion to
decrease blood volume
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Fluid Balance

Na+ and Cl– balance is
regulated by 3 hormones
Aldosterone
Atrial natriuetic peptide
Angiotension II
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Fluid Balance

In addition to the hormones that regulate Na+ and Cl-
homeostasis, antidiuretic hormone (ADH) is a hormone
that plays a major role in directly regulating water loss
in the collecting ducts of the kidneys
Also known as vasopressin, ADH (from the posterior
pituitary) increases permeability of the collecting
ducts to water by promoting insertion of aquaporin-2
into the principal cells – producing a concentrated
urine
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Electrolytes

Ions form when electrolytes dissolve and dissociate.
They have 4 general functions
Control osmosis of water between body fluid
compartments
Help maintain the acid-base balance
Carry electrical current
Serve as cofactors
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Electrolytes

The term “milliequivalent” (mEq) is used to measure
the number of electrical charges (electrolytes) in blood
serum and other solutions. Denoting the number of
mEq per liter of solution gives the concentration of
anions or cations in a given volume of solution
One equivalent is the charge in 1 mole of H+ ions. A
milliequivalent is simply 1/1000 of an equivalent
◦ Sodium - 136-146 mEq/L
◦ Potassium - 3.5-5.0 mEq/L
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Electrolytes

The size of a substance does not determine its osmotic
contribution – that is determined by the number of
milliequivalents. For example:
1 millimole NaCl = 2 mEq (1mEq of Na+ and
1mEq of Cl-
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Starling Forces

As we have seen, osmotic forces have a definite
influence on movement of water between body
compartments. Osmotic pressure exerted by proteins on
either side of the capillary membrane is called oncotic
pressure. It is not, however, the only force in play hydrostatic forces are another major factor to consider
Net movement of fluids is controlled by all forces
favoring filtration minus all
forces opposing filtration
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Starling Forces

The Starling equation compares the forces at the arterial
end of a capillary with those at the venous end
Forces favoring filtration are the capillary hydrostatic
pressure (pressure against the capillary wall) and the
interstitial oncotic pressure
Forces favoring reabsorption are the plasma oncotic
pressure (water-pulling)
and the interstitial
hydrostatic pressure
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Starling Forces

Normal Starling forces favor a small amount of fluid
flowing out of the capillary which is drained by the
lymphatic system
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Starling Forces

Edema occurs when excess interstitial fluid collects,
causing swelling in the tissues. Edema occurs anytime
filtration exceeds reabsorption
The most important causes of edema are:
◦ increased blood pressure (increased blood
hydrostatic pressure)
◦ an increase in the capillary permeability
◦ a decrease in the concentration of plasma proteins
◦ an obstruction in lymphatic drainage
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Acid-Base Balance

A major homeostatic challenge is keeping the H+
concentration (pH) of body fluids at an appropriate level.
Because metabolic reactions often produce a huge excess
of H+, failure of homeostatic mechanisms would cause
the pH of body fluids to quickly fall to a lethal level
l
In a healthy person, chemical buffers, the lungs, and
the kidneys help maintain the pH of systemic arterial
blood between 7.35 and 7.45
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Acid-Base Balance
1. Buffer systems act quickly to temporarily bind excess
H+ or OH -, sequestering (hiding) the highly reactive
ions until they can be permanently excreted
2. By increasing the rate and depth of breathing, CO2 is
exhaled or retained, and blood pH is corrected
3. Kidney excretion/reabsorption of acidic ions (H+ and
NH4+) or basic ions (HCO3 – or OH -) is the slowest
mechanism; but is the only way to eliminate acids
other than carbonic acid
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Acid-Base Balance
(Interactions Animation)

Regulation of pH
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Acid-Base Imbalances

Respiratory acidosis occurs whenever CO2 accumulates
because of hypoventilation

Metabolic acidosis occurs whenever non-respiratory
acids accumulate, as seen in diabetic ketoacidosis or
aspirin overdose

Respiratory alkalosis occurs whenever too much CO2
is lost because of hyperventilation

Metabolic alkalosis occurs whenever non-respiratory
acids are lost, which happens infrequently
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Acid-Base Imbalances

The homeostatic correction for
states of acidosis (which are much
more common and serious than
states of alkalosis) are depicted in
this flowchart
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End of Chapter 27
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