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Fluid, Electrolyte and AcidBase Homeostasis


Principles of Human Anatomy and Physiology, 11e. 1. Chapter 27 ... and peritoneal fluids between serous membranes; and glomerular filtrate in the kidneys. ... – PowerPoint PPT presentation

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Title: Fluid, Electrolyte and AcidBase Homeostasis

Chapter 27
  • Fluid, Electrolyte and Acid-Base Homeostasis
  • Lecture Outline

Chapter 27Fluid, Electrolyte and Acid-Base
  • Body fluid
  • all the water and dissolved solutes in the bodys
    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.

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

  • 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 bodys fluid is located
    in cells and is called intracellular fluid (ICF).
  • The other third is called extracellular fluid
  • 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.

  • 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.

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
  • 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.

Body Water Gain and Loss (Figure 27.2)
  • 45-75 body weight
  • declines with age since fat contains almost no
  • Gain from ingestion and metabolic water formed
    during aerobic respiration dehydration
    synthesis reactions (2500 mL/day)
  • Normally loss gain
  • urine, feces, sweat, breathe

Dehydration Stimulates Thirst
  • Regulation of fluid gain is by regulation of

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
  • Drinking occurs ? body water levels return to

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.

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

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

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
  • slows release of renin formation of angiotensin
  • increases filtration rate reduces water Na
  • 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

Hormone Regulation of Water Balance
  • Antidiuretic hormone (ADH) from the posterior
  • 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

Movement of Water
  • Intracellular and interstitial fluidsnormally
    have the same osmolarity,so cells neither swell
    nor shrink
  • Swollen cells of water intoxicationbecause Na
    concentration of plasmafalls below normal
  • drink plain water faster than kidneys canexcrete
  • replace water lost from diarrhea or vomitingwith
    plain water
  • may cause convulsions, coma death unless oral
    rehydration includes small amount salt in water

  • Electrolytes serve four general functions in the
  • 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

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

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.

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)

Sodium (Na) is the most abundant extracellular
  • 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

Edema, Hypovolemia and Na Imbalance
  • Sodium retention causes water retention
  • edema is abnormal accumulation of interstitial
  • 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

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.

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
  • It plays a role in forming HCl in the stomach.

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
  • abnormal plasma K levels adversely affect
    cardiac and neuromuscular function

Bicarbonate (HCO3-) is a prominent ion in the
  • It is a significant plasma anion in electrolyte
  • 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 if levels are too
  • excrete excess in the urine

Calcium (Ca2), 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
  • increases production of calcitriol (Ca2
    absorption from GI tract and reabsorption from
    glomerular filtrate)

Magnesium (Mg2) is primarily an intracellular
  • 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.

  • 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

  • Table 27.2 describes the imbalances that result
    from the deficiency or excess of several

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

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)

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

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

Carbonic Acid-Bicarbonate Buffer System
  • Acts as extracellular intracellular buffer
  • 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

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

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.

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

Kidney Excretion of H
  • Metabolic reactions produce 1mEq/liter of
    nonvolatile acid for every kilogram of body
  • 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

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
  • Table 27.3 summarizes the mechanism that
    maintains pH of body fluids.

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
  • Alkalosis is a blood pH above 7.45. Its principal
    effect is overexcitability of the central nervous
    system through facilitation of synaptic

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

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.

  • Diagnosis of acid-base imbalances employs a
    general four-step 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
  • Look at the noncausative value and determine if
    it is compensating for the problem.

Summary of Causes
  • Respiratory acidosis alkalosis are disorders
    involving changes in partial pressure of CO2 in
  • Metabolic acidosis alkalosis are disorders due
    to changes in bicarbonate ion concentration in

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

Respiratory Alkalosis
  • Arterial blood pCO2 is too low
  • Hyperventilation caused by high altitude,
    pulmonary disease, stroke, anxiety, aspirin
  • Renal compensation involves decrease in excretion
    of H and increase reabsorption of bicarbonate
  • Treatment
  • breathe into a paper bag

Metabolic Acidosis
  • Blood bicarbonate ion concentration too low
  • loss of ion through diarrhea or kidney
  • accumulation of acid (ketosis with
  • kidney failing to remove H from protein
  • Respiratory compensation by hyperventilation
  • Treatment
  • IV administration of sodium bicarbonate
  • correct the cause

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

Diagnosis of Acid-Base Imbalances
  • Evaluate
  • systemic arterial blood pH
  • concentration of bicarbonate (too low or too
  • PCO2 (too low or too high)
  • Solutions
  • if problem is respiratory, the pCO2 will not be
  • if problem is metabolic, the bicarbonate level
    will not be normal

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
  • 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
  • Higher K and Cl- concentrations than adults

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 NH4

  • end
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