Transcript Chapter 3

Chapter 27
Fluid, Electrolyte and Acid-Base Homeostasis
Lecture Outline
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Chapter 27
Fluid, Electrolyte and Acid-Base Homeostasis
• Body fluid
– all the water and dissolved solutes in
the body’s fluid compartments
• Mechanisms regulate
– total volume
– distribution
– concentration of solutes and pH
• Regulatory mechanisms insure
homeostasis of body fluids since their
malfunction may seriously endanger
nervous system and organ functioning.
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FLUID COMPARTMENTS AND FLUID BALANCE
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Balance Between Fluid Compartments
Volume of fluid in each is
kept constant. Since water
follows electrolytes, they
must be in balance as well
• Only 2 places for exchange between compartments:
– cell membranes separate intracellular from interstitial fluid.
– only in capillaries are walls thin enough for exchange between plasma
and interstitial fluids
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Introduction
• In lean adults body fluids comprise about 55-60% (Figure
27.1) of total body weight.
– Water is the main component of all body fluids.
– About two-thirds of the body’s fluid is located in cells and
is called intracellular fluid (ICF).
– The other third is called extracellular fluid (ECF).
– About 80% of the ECF is interstitial fluid and 20% is
blood plasma.
• Some of the interstitial fluid is localized in specific places, such as
lymph; cerebrospinal fluid; gastrointestinal tract fluids; synovial fluid;
fluids of the eyes (aqueous humor and vitreous body) and ears
(endolymph and perilymph); pleural, pericardial, and peritoneal fluids
between serous membranes; and glomerular filtrate in the kidneys.
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Membranes
• Selectively permeable membranes separate body fluids into
distinct compartments.
– Plasma membranes of individual cells separate
intracellular fluid from interstitial fluid.
– Blood vessel walls divide interstitial fluid from blood
plasma.
• Although fluids are in constant motion from one
compartment to another, the volume of fluid in each
compartment remains fairly stable – another example of
homeostasis.
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Fluid and Solute Balance
• Fluid balance means that the various body compartments
contain the required amount of water, proportioned
according to their needs.
– Fluid balance, then, means water balance, but also
implies electrolyte balance; the two are inseparable.
• Osmosis is the primary way in which water moves in and out
of body compartments. The concentrations of solutes in the
fluids is therefore a major determinant of fluid balance.
• Most solutes in body fluids are electrolytes, compounds that
dissociate into ions.
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Body Water Gain and Loss (Figure 27.2)
• 45-75% body weight
– declines with age since fat
contains almost no water
• Gain from ingestion and
metabolic water formed during
aerobic respiration & dehydration
synthesis reactions (2500
mL/day)
• Normally loss = gain
– urine, feces, sweat, breathe
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Dehydration
Stimulates Thirst
• Regulation of fluid
gain is by regulation
of thirst.
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Regulation of Water Gain
• Metabolic water volume depends mostly on the level of
aerobic cellular respiration, which reflects the demand for
ATP in body cells.
• The main way to regulate body water balance is by
adjusting the volume of water intake.
• When water loss is greater than water gain, dehydration
occurs (Figure 27.3).
• The stimulus for fluid intake (gain) is dehydration resulting in
thirst sensations; one mechanism for stimulating the thirst
center in the hypothalamus is the renin-angiotensin II
pathway, which responds to decreased blood volume
(therefore, decreased blood pressure) (Figure 27.3).
• Drinking occurs  body water levels return to normal
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Regulation of Water and Solute Loss
• Although increased amounts of water and solutes are lost
through sweating and exhalation during exercise, loss of
body water or excess solutes depends mainly on regulating
how much is lost in the urine (Figure 27.4).
• Under normal conditions, fluid output (loss) is adjusted by
– antidiuretic hormone (ADH)
– atrial natriuretic peptide (ANP)
– aldosterone
all of which regulate urine production.
• Table 27.1 summarizes the factors that maintain body water
balance.
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Regulation of Water
and Solute Loss
• Elimination of excess water or
solutes occurs through
urination
• Consumption of very salty meal
demonstrates function of three
hormones
• Demonstrates how
– “water follows salt”
– excrete Na+ and water will
follow and decrease blood
volume
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Movement of Water Between Body Fluid
Compartments
• A fluid imbalance between the intracellular and interstitial
fluids can be caused by a change in their osmolarity.
• Most often a change in osmolarity is due to a change in the
concentration of Na+.
– When water is consumed faster than the kidneys can
excrete it, water intoxication may result (Figure 27.5).
– Repeated use of enemas can increase the risk of fluid
and electrolyte imbalances. (Clinical Application)
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Hormone Effects on Solutes
• Angiotensin II and aldosterone promote reabsorption of Na+
and Cl- and an increase in fluid volume
– stretches atrial volume and promotes release of ANP
– slows release of renin & formation of angiotensin II
• increases filtration rate & reduces water & Na+
reabsorption
• decreases secretion of aldosterone slowing
reabsorption of Na+ and Cl- in collecting ducts
• ANP promotes natriuresis or the increased excretion of Na+
and Cl- which decreases blood volume
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Hormone Regulation of Water Balance
• Antidiuretic hormone (ADH) from the posterior pituitary
– stimulates thirst
– increases permeability of principal cells of collecting ducts
to assist in water reabsorption
– very concentrated urine is formed
• ADH secretion shuts off after the intake of water
• ADH secretion is increased
– large decrease in blood volume
– severe dehydration and drop in blood pressure
– vomiting, diarrhea, heavy sweating or burns
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Movement of Water
• Intracellular and interstitial fluids
normally have the same osmolarity,
so cells neither swell nor shrink
• Swollen cells of water intoxication
because Na+ concentration of plasma
falls below normal
– drink plain water faster than kidneys
can
excrete it
– replace water lost from diarrhea or
vomiting
with plain water
– may cause convulsions, coma & death
unless oral rehydration includes small
amount salt in water intake
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ELECTROLYTES IN BODY FLUIDS
• Electrolytes serve four general functions in the body.
– Because they are more numerous than nonelectrolytes,
electrolytes control the osmosis of water between body
compartments.
– maintain the acid-base balance required for normal
cellular activities.
– carry electrical current, which allows production of action
potentials and graded potentials and controls secretion of
some hormones and neurotransmitters. Electrical
currents are also important during development.
– cofactors needed for optimal activity of enzymes.
• Concentration expressed in mEq/liter or milliequivalents per
liter for plasma, interstitial fluid and intracellular fluid
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Concentrations of Electrolytes in Body Fluids
• To compare the charge carried by ions in different solutions,
the concentration is typically expressed in
milliequivalents/liter (mEg/Liter), which gives the
concentration of cations or anions in a solution.
• The chief difference between plasma and interstitial fluid
– plasma contains quite a few protein anions
– interstitial fluid has hardly any since plasma proteins
generally cannot move out of impermeable blood vessel
walls
– plasma also contains slightly more sodium ions but fewer
chloride ions than the interstitial fluid. In other respects,
the two fluids are similar.
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Concentrations of Electrolytes in Body Fluids
• Intracellular fluid (ICF) differs considerably from extracellular
fluid (ECF), however.
• Figure 27.6 compares the concentrations of the main
electrolytes and protein anions in plasma, interstitial fluid,
and intracellular fluid.
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Comparison Between Fluid Components
• Plasma contains many proteins, but interstitial fluid does not
– producing blood colloid osmotic pressure
• Extracellular fluid contains Na+ and Cl• Intracellular fluid contains K+ and phosphates (HPO4 -2)
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Sodium (Na+) is the most abundant extracellular ion.
• Most abundant extracellular ion
– accounts for 1/2 of osmolarity of ECF
• Average daily intake exceeds normal requirements
• Hormonal controls
– aldosterone causes increased reabsorption Na+
– ADH release ceases if Na+ levels too low--dilute
urine lost until Na+ levels rise
– ANP increases Na+ and water excretion if Na+
levels too high
• Excess Na+ in the body can result in edema. Excess loss of Na+
causes excessive loss of water, which results in hypovolemia,
an abnormally low blood volume. (Clinical Application)
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Edema, Hypovolemia and Na+ Imbalance
• Sodium retention causes water retention
– edema is abnormal accumulation of interstitial fluid
• Causes of sodium retention
– renal failure
– hyperaldosterone
• Excessive loss of sodium causes excessive loss of
water (low blood volume)
– due to inadequate secretion of aldosterone
– too many diuretics
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Chloride (Cl-) is the major extracellular anion.
• Regulation of Cl- balance in body fluids is indirectly
controlled by aldosterone. Aldosterone regulate sodium
reabsorption; the negatively charged chloride follows the
positively charged sodium passively by electrical attraction.
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Chloride (Cl-) is the major extracellular anion.
• Most prevalent extracellular anion
• Moves easily between compartments due to Cl- leakage
channels
• Helps balance anions in different compartments
• Regulation
– passively follows Na+ so it is regulated indirectly by
aldosterone levels
– ADH helps regulate Cl- in body fluids because it controls
water loss in urine
• Chloride shift across red blood cells with buffer movement
• It plays a role in forming HCl in the stomach.
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Potassium (K+) is the most abundant cation in
intracellular fluid.
• It is involved in maintaining fluid volume, impulse
conduction, muscle contraction.
• Exchanged for H+ to help regulate pH in intracellular fluid
• The plasma level of K+ is under the control of
mineralocorticoids, mainly aldosterone.
• Helps establish resting membrane potential & repolarize
nerve & muscle tissue
• Control is mainly by aldosterone which stimulates principal
cells to increase K+ secretion into the urine
• abnormal plasma K+ levels adversely affect cardiac and
neuromuscular function
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Bicarbonate (HCO3-) is a prominent ion in the plasma.
• It is a significant plasma anion in electrolyte balance.
• It is a major component of the plasma acid-base buffer
system.
– Concentration increases as blood flows through systemic
capillaries due to CO2 released from metabolically active
cells
– Concentration decreases as blood flows through
pulmonary capillaries and CO2 is exhaled
• Kidneys are main regulator of plasma levels
– intercalated cells form more HCO3- if levels are too low
– excrete excess in the urine
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Calcium (Ca+2), the most abundant ion in the body, is
principally an extracellular ion.
• It is a structural component of bones and teeth.
• Important role in blood clotting, neurotransmitter release,
muscle tone & nerve and muscle function
• Regulated by parathyroid hormone
– stimulates osteoclasts to release calcium from bone
– increases production of calcitriol (Ca+2 absorption from
GI tract and reabsorption from glomerular filtrate)
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Magnesium (Mg+2) is primarily an intracellular cation.
• It activates several enzyme systems involved in the
metabolism of carbohydrates and proteins and is needed for
operation of the sodium pump.
• It is also important in neuromuscular activity, neural
transmission within the central nervous system, and
myocardial functioning.
• Several factors regulate magnesium ion concentration in
plasma. They include hypo- or hypercalcemia, hypo- or
hypermagnesemia, an increase or decrease in extracellular
fluid volume, an increase or decrease in parathyroid
hormone, and acidosis or alkalosis.
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Phosphate
• Present as calcium phosphate in bones and teeth, and in
phospholipids, ATP, DNA and RNA
• HPO4 -2 is important intracellular anion and acts as buffer of
H+ in body fluids and in urine
– mono and dihydrogen phosphate act as buffers in the
blood
• Plasma levels are regulated by parathyroid hormone &
calcitriol
– resorption of bone releases phosphate
– in the kidney, PTH increase phosphate excretion
– calcitriol increases GI absorption of phosphate
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Review
• Table 27.2 describes the imbalances that result from the
deficiency or excess of several electrolytes.
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Clinical Application
• Individuals at risk for fluid and electrolyte imbalances
include those dependent on others for fluid and food needs;
those undergoing medical treatment involving intravenous
infusions, drainage or suction, and urinary catheters, those
receiving diuretics, and post-operative individuals, burned
individuals, individuals with chronic disease, and those with
altered states of consciousness.
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Acid-Base Balance
• The overall acid-base balance of the body is
maintained by controlling the H+ concentration of
body fluids, especially extracellular fluid.
• Homeostasis of H+ concentration is vital
– proteins 3-D structure sensitive to pH changes
– normal plasma pH must be maintained between
7.35 - 7.45
– diet high in proteins tends to acidify the blood
• 3 major mechanisms to regulate pH
– buffer system
– exhalation of CO2 (respiratory system)
– kidney excretion of H+ (urinary system)
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Actions of Buffer Systems
•
•
•
•
•
Prevent rapid, drastic changes in pH
Change either strong acid or base into weaker one
Work in fractions of a second
Found in fluids of the body
3 principal buffer systems
– protein buffer system
– carbonic acid-bicarbonate buffer system
– phosphate buffer system
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Protein Buffer System
• Abundant in intracellular fluids & in plasma
– hemoglobin very good at buffering H+ in RBCs
– albumin is main plasma protein buffer
• Amino acids contains at least one carboxyl group (COOH) and at least one amino group (-NH2)
– carboxyl group acts like an acid & releases H+
– amino group acts like a base & combines with H+
– some side chains can buffer H+
• Hemoglobin acts as a buffer in blood by picking up CO2 or
H+
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Carbonic Acid-Bicarbonate Buffer System
• Acts as extracellular & intracellular buffer system
– bicarbonate ion (HCO3-) can act as a weak base
• holds excess H+
– carbonic acid (H2CO3) can act as weak acid
• dissociates into H+ ions
• At a pH of 7.4, bicarbonate ion concentration is about 20
times that of carbonic acid
• Can not protect against pH changes due to respiratory
problems
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Phosphate Buffer System
• Most important intracellularly, but also acts to buffer acids in
the urine
• Dihydrogen phosphate ion acts as a weak acid that can
buffer a strong base
• Monohydrogen phosphate acts a weak base by buffering
the H+ released by a strong acid
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Exhalation of Carbon Dioxide
• The pH of body fluids may be adjusted by a change in the
rate and depth of respirations, which usually takes from 1 to
3 minutes.
• An increase in the rate and depth of breathing causes more
carbon dioxide to be exhaled, thereby increasing pH.
• A decrease in respiration rate and depth means that less
carbon dioxide is exhaled, causing the blood pH to fall.
• The pH of body fluids, in turn, affects the rate of breathing
(Figure 27.7).
• The kidneys excrete H+ and reabsorb HCO3- to aid in
maintaining pH.
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Exhalation of Carbon Dioxide
• pH modified by changing rate &
depth of breathing
– faster breathing rate, blood
pH rises
– slow breathing rate, blood pH
drops
• H+ detected by chemoreceptors
in medulla oblongata, carotid &
aortic bodies
• Respiratory centers inhibited or
stimulated by changes is pH
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Kidney Excretion of H+
• Metabolic reactions produce 1mEq/liter
of nonvolatile acid for every kilogram of
body weight
• Excretion of H+ in the urine is only way
to eliminate huge excess
• Kidneys synthesize new bicarbonate
and save filtered bicarbonate
• Renal failure can cause death rapidly
due to its role in pH balance
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Regulation of Acid-Base Balance
• Cells in the PCT and collecting ducts secrete hydrogen ions into the
tubular fluid.
• In the PCT Na+/H+ antiporters secrete H+ and reabsorb Na+ (Figure
26.13).
• The apical surfaces of some intercalated cells include proton pumps (H+
ATPases) that secrete H+ into the tubular fluid and HCO3– antiporters in
their basolateral membranes to reabsorb HCO3– (Figure 27.8).
• Other intercalated cells have proton pumps in their basolateral
membranes and Cl–/HCO3– antiporters in their apical membranes.
• These two types of cells help maintain body fluid pH by excreting excess
H+ when pH is too low or by excreting excess HCO3– when the pH is too
high.
• Table 27.3 summarizes the mechanism that maintains pH of body fluids.
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Acid-Base Imbalances
• The normal pH range of systemic arterial blood is between
7.35-7.45.
• Acidosis is a blood pH below 7.35. Its principal effect is
depression of the central nervous system through
depression of synaptic transmission.
• Alkalosis is a blood pH above 7.45. Its principal effect is
overexcitability of the central nervous system through
facilitation of synaptic transmission.
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Acid-Base
Imbalances
Acidosis---blood pH below 7.35
Alkalosis---blood pH above 7.45
• Compensation is an attempt to correct the
problem
– respiratory compensation
– renal compensation
• Acidosis causes depression of CNS---coma
• Alkalosis causes excitability of nervous tissue--spasms, convulsions & death
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Acid-Base Imbalances
• Compensation refers to the physiological response to an
acid-base imbalance.
• Respiratory acidosis and respiratory alkalosis are primary
disorders of blood PCO2.
• metabolic acidosis and metabolic alkalosis are primary
disorders of bicarbonate concentration.
• A summary of acidosis and alkalosis is presented in Table
27.4.
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Diagnosis
• Diagnosis of acid-base imbalances employs a general fourstep process.
– Note whether the pH is high or low relative to the normal
range.
– Decide which value of PCO2 or HCO3- could cause the
abnormality.
– Specify the problem source as respiratory or metabolic.
– Look at the noncausative value and determine if it is
compensating for the problem.
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Summary of Causes
• Respiratory acidosis & alkalosis are disorders involving
changes in partial pressure of CO2 in blood
• Metabolic acidosis & alkalosis are disorders due to changes
in bicarbonate ion concentration in blood
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Respiratory Acidosis
• Cause is elevation of pCO2 of blood
• Due to lack of removal of CO2 from blood
– emphysema, pulmonary edema, injury to the
brainstem & respiratory centers
• Treatment
– IV administration of bicarbonate (HCO3-)
– ventilation therapy to increase exhalation of CO2
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Respiratory Alkalosis
• Arterial blood pCO2 is too low
• Hyperventilation caused by high altitude, pulmonary
disease, stroke, anxiety, aspirin overdose
• Renal compensation involves decrease in excretion of H+
and increase reabsorption of bicarbonate
• Treatment
– breathe into a paper bag
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Metabolic Acidosis
• Blood bicarbonate ion concentration too low
– loss of ion through diarrhea or kidney dysfunction
– accumulation of acid (ketosis with dieting/diabetes)
– kidney failing to remove H+ from protein metabolism
• Respiratory compensation by hyperventilation
• Treatment
– IV administration of sodium bicarbonate
– correct the cause
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Metabolic Alkalosis
• Blood bicarbonate levels are too high
• Cause is nonrespiratory loss of acid
– vomiting, gastric suctioning, use of diuretics,
dehydration, excessive intake of alkaline drugs
• Respiratory compensation is hypoventilation
• Treatment
– fluid and electrolyte therapy
– correct the cause
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Diagnosis of Acid-Base Imbalances
• Evaluate
– systemic arterial blood pH
– concentration of bicarbonate (too low or too high)
– PCO2 (too low or too high)
• Solutions
– if problem is respiratory, the pCO2 will not be normal
– if problem is metabolic, the bicarbonate level will not be
normal
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Homeostasis in Infants
•
•
•
•
•
•
•
More body water in ECF so more easily disrupted
Rate of fluid intake/output is 7X higher
Higher metabolic rate produces more metabolic wastes
Kidneys can not concentrate urine nor remove excess H+
Surface area to volume ratio is greater so lose more water through skin
Higher breathing rate increase water loss from lungs
Higher K+ and Cl- concentrations than adults
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Impaired Homeostasis in the Elderly
• Decreased volume of intracellular fluid
– inadequate fluid intake
• Decreased total body K+ due to loss of muscle tissue or potassiumdepleting diuretics for treatment of hypertension or heart disease
• Decreased respiratory & renal function
– slowing of exhalation of CO2
– decreased blood flow & glomerular filtration rate
– reduced sensitivity to ADH & impaired ability to produce dilute urine
– renal tubule cells produce less ammonia to combine with H+ and
excrete as NH+4
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Questions?
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end
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