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
Tortora & Grabowski 9/e 2000 JWS
27-1
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
Tortora
& Grabowski
9/e 2000 JWS fluids
27-2
Body Water Gain and Loss
• 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
Tortora & Grabowski 9/e 2000 JWS
27-3
Regulation of Water Gain
• Formation of metabolic water is not regulated
– function of the need for ATP
• Main regulator of water gain is intake
regulation
• Stimulators of thirst center in hypothalamus
– dry mouth, osmoreceptors in hypothalamus,
decreased blood volume causes drop in BP &
angiotensin II
• Drinking occurs
– body water levels return to normal
Tortora & Grabowski 9/e 2000 JWS
27-4
Dehydration Stimulates Thirst
• Regulation of
fluid gain is by
regulation of
thirst.
Tortora & Grabowski 9/e 2000 JWS
27-5
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
Tortora & Grabowski 9/e 2000 JWS
27-6
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
Tortora & Grabowski 9/e 2000 JWS
27-7
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
Tortora & Grabowski 9/e 2000 JWS
27-8
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
Tortora & Grabowski 9/e 2000 JWS
27-9
Enemas and Fluid Balance
• Introduction of a solution into the bowel to
stimulate activity and evacuate feces
• Increase risk of fluid & electrolyte
imbalance unless isotonic solution is used
Tortora & Grabowski 9/e 2000 JWS
27-10
Concentrations of Electrolytes
• Functions of electrolytes
–
–
–
–
control osmosis between fluid compartments
help maintain acid-base balance
carry electric current
cofactors needed for enzymatic activity
• Concentration expressed in mEq/liter or
milliequivalents per liter for plasma,
interstitial fluid and intracellular fluid
Tortora & Grabowski 9/e 2000 JWS
27-11
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)
Tortora & Grabowski 9/e 2000 JWS
27-12
Sodium
• 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
Tortora & Grabowski 9/e 2000 JWS
27-13
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
Tortora & Grabowski 9/e 2000 JWS
27-14
Chloride
• Most prevalent extracellular anion
• Moves easily between compartments due to Clleakage 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 & hydrochloric acid of gastric juice
Tortora & Grabowski 9/e 2000 JWS
27-15
Potassium
• Most abundant cation in intracellular fluid
• Helps establish resting membrane potential &
repolarize nerve & muscle tissue
• Exchanged for H+ to help regulate pH in
intracellular fluid
• 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
Tortora & Grabowski 9/e 2000 JWS
27-16
Bicarbonate
• Common extracellular anion
• Major buffer in plasma
• 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 if levels are too low
– excrete excess in the urine
Tortora & Grabowski 9/e 2000 JWS
27-17
Calcium
• Most abundant mineral in body (skeleton &
teeth)
• Abundant extracellular cation in body fluids
• 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)
Tortora
& Grabowski 9/e 2000 JWS
27-18
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
Tortora & Grabowski 9/e 2000 JWS
27-19
Magnesium
• Found in bone matrix and as ions in body
fluids
– intracellular cofactor for metabolic enzymes,
heart, muscle & nerve function
• Urinary excretion increased in
hypercalcemia, hypermagnesemia, increased
extracellular fluid volume, decreases in
parathyroid hormone and acidosis
Tortora & Grabowski 9/e 2000 JWS
27-20
Acid-Base Balance
• 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)
Tortora & Grabowski 9/e 2000 JWS
27-21
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
Tortora & Grabowski 9/e 2000 JWS
27-22
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+
Tortora & Grabowski 9/e 2000 JWS
27-23
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
Tortora & Grabowski 9/e 2000 JWS
27-24
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
Tortora & Grabowski 9/e 2000 JWS
27-25
Exhalation of Carbon Dioxide
• Breathing plays a role in the
homeostasis of pH
• 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
Tortora & Grabowski 9/e 2000 JWS
27-26
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
Tortora & Grabowski 9/e 2000 JWS
27-27
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
Tortora & Grabowski 9/e 2000 JWS
27-28
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
Tortora & Grabowski 9/e 2000 JWS
27-29
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
Tortora & Grabowski 9/e 2000 JWS
27-30
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
Tortora & Grabowski 9/e 2000 JWS
27-31
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
Tortora & Grabowski 9/e 2000 JWS
27-32
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
Tortora & Grabowski 9/e 2000 JWS
27-33
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
Tortora & Grabowski 9/e 2000 JWS
27-34
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
Tortora & Grabowski 9/e 2000 JWS
27-35
Impaired Homeostasis in the Elderly
• Decreased volume of intracellular fluid
– inadequate fluid intake
• Decreased total body K+ due to loss of muscle tissue or
potassium-depleting 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
Tortora & Grabowski 9/e 2000 JWS
27-36