Fluid, Electrolyte, & Acid-Base Balance

1
Chapter 27

• Fluid, Electrolyte, Acid-Base Balance

2
Body Water Content

• Infants have low body fat, low bone mass, and are
  73 or more water
• Total water content declines throughout life
• Healthy males are about 60 water healthy
  females are around 50
• This difference reflects females
• Higher body fat
• Smaller amount of skeletal muscle
• In old age, only about 45 of body weight is water

3
Fluid Compartments

• Water occupies two main fluid compartments
• Intracellular fluid (ICF) about 2/3 by volume,
  contained in cells
• Extracellular fluid (ECF) consists of two major
  subdivisions
• Plasma the fluid portion of the blood
• Interstitial fluid (IF) fluid in spaces between
  cells
• lymph, cerebrospinal fluid, eye humors, synovial
  fluid, serous fluid, and gastrointestinal
  secretions

4
Composition of body fluids

• Water is the universal solvent
• Solutes are either electrolytes or
  nonelectrolytes
• Electrolytes ions w/electrical charge
  inorganic salts, inorganic/organic acids bases,
  some proteins
• Nonelectrolytes have bonds that prevent
  dissociation in solution (no electrical charge)
  glucose, lipids, creatinine, urea

5
Osmosis

• Water moves from areas of lesser osmolality to
  areas of greater osmolality
• electrolytes have a greater influence on
  movement of H2O b/c they dissociate into more
  particles than nonelectrolytes
• ie. NaCl?Na Cl-
• glucose?glucose

6
Extracellular and Intracellular Fluids

• Each fluid compartment of the body has a
  distinctive pattern of electrolytes
• Extracellular fluids are similar (except for the
  high protein content of plasma)
• Sodium is the chief cation
• Chloride is the major anion
• Intracellular fluids have low sodium and chloride
• Potassium is the chief cation
• Phosphate is the chief anion

7
Water balance- see figure 27-1

• Water input
• 60 ingested liquids
• 30 ingested solids
• 10 metabolic water (water of oxidation)
  produced via cellular metabolism
• Water output
• 28 vaporized thru lungs/skin (insensible water
  loss)
• 8 perspiration
• 4 feces
• 60 thru kidneys as urine

8
Thirst mechanism

• Located in hypothalamus is poorly understood
• Triggered by a decrease in plasma volume by gt10
  or increase in plasma osmolality by 1-2

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3 Primary Regulatory Hormones

• Affect fluid and electrolyte balance
• antidiuretic hormone
• aldosterone
• natriuretic peptides

10
Antidiuretic Hormone (ADH)

• Stimulates water conservation at kidneys
• reducing urinary water loss concentrating urine
• Stimulates thirst center promoting fluid intake

11
Aldosterone

• Is secreted by adrenal cortex in response to
• rising K or falling Na levels in blood
• activation of reninangiotensin system
• Determines rate of Na absorption and K loss
  along DCT and collecting system

12
Water Follows Salt

• High aldosterone plasma concentration
• causes kidneys to conserve salt
• Conservation of Na by aldosterone
• also stimulates water retention
• Obligatory water loss insensible (lung, skin,
  feces, etc.) minimum sensible (500 ml in
  urine) loss

13
Natriuretic Peptide

• ANP is released by cardiac muscle cells
• In response to abnormal stretching of heart walls
  caused by
• elevated blood pressure
• an increase in blood volume
• Reduce thirst
• Block release of ADH and aldosterone
• Cause diuresis
• Lower blood pressure and plasma volume

14
Excess water intake

• Normal function
• 30 after ingestion, kidneys start to eliminate
  excess water (time needed to (-) ADH release)
• Diuresis reaches peak in 1 hour
• Urine output declines to its lowest levels after
  3 hours

15
Causes of Overhydration

• Ingestion of large volume of fresh water
• Injection into bloodstream of hypotonic solution
• Endocrine disorders
• excessive ADH production
• Inability to eliminate excess water in urine
• chronic renal failure
• heart failure
• cirrhosis

16
Dehydration

• Also called water depletion
• Develops when water loss is greater than gain
• Severe water loss causes
• excessive perspiration
• inadequate water consumption
• repeated vomiting
• diarrhea

17
Electrolyte Balance

• Electrolytes are salts, acids, and bases, but
  electrolyte balance usually refers only to salt
  balance
• Salts are important for
• Neuromuscular excitability
• Secretory activity
• Membrane permeability
• Controlling fluid movements
• Salts enter the body by ingestion and are lost
  via perspiration, feces, and urine

18
Electrolyte Balance

• When the body loses water
• plasma volume decreases electrolyte
  concentrations rise
• When the body loses electrolytes
• water is lost by osmosis

19
Sodium

• Has the primary role in controlling ECF volume
  water distribution in the body
• NaHCO3 NaCl account for 90-95 of all solutes
  in the ECF
• The single most abundant cation in the ECF and
  accounts for virtually all of the osmotic P
• B/C all body fluids are in osmotic equilibrium, a
  change in Na affects plasma volume BP as
  well as ICF IF volumes as well

20
Na balance

• Aldosterone makes DCT CTs in kidneys more
  permeable to Na (65 Na reabsorbed in PCT 25
  reclaimed in Loop of Henle)
• H2O may or may not follow depending on levels of
  ADH (aldosterone usually allows for easier
  excretion of H2O)
• If needed, almost all of the Na may be reabsorbed
  in DCT leading to urine with high H2O content
  little Na excretion
• CV system baroreceptors
• High blood volume?carotid/aortic sinuses? alert
  brain stem ?decreased SNS output to
  kidneys?increased GRF?increased Na H2O
  output?decreased BV BP
• Low blood volume?constriction of afferent
  arterioles?reduced filtrate formation?decreased
  urinary output?increased BV BP

21
Abnormal Na Concentrations in ECF

• Hyponatremia
• body water content rises (overhydration)
• ECF Na concentration lt 130 mEq/L
• Hypernatremia
• body water content declines (dehydration)
• ECF Na concentration gt 150 mEq/L

22
Na balance, cont.

• ADH increases H2O reabsorption
• Atrial natriuretic peptide/factor (ANP/F)
• Released by certain cells of heart atria when
  stretchedreduces BV BP by (-) nearly all
  events that promote vasoconstriction Na/H2O
  retention
• Estrogens chemically similar to aldosterone
• Progesterone blocks effect of aldosterone so it
  has a diuretic effect
• Glucocorticoids tends to have an aldosterone
  like effect promotes edema

23
Potassium

• The chief intracellular cation
• Essential for protein synthesis normal
  neuromuscular functioning
• Levels affect resting membrane potential
  (especially in the heart)
• Increased K levels in ECF decreases membrane
  potential?depolarization? reduced excitability
• Part of the bodys buffer system (ECF K levels
  rise w/acidosis as K leaves the cell H enters
  the cell)

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Regulation of potassium

• Levels maintained mostly by renal mechanisms
• Tubules reabsorb 55 of filtered K
• Thick ascending limb reabsorbs30
• Less than 15 excreted in urine
• K balance falls on cortical collecting ducts by
  changing amount of K secreted in to filtrate
• Generally, K levels are high in the ECF the
  thrust of kidney fnx is to excrete itfailure to
  ingest dietary K results in severe potassium
  deficiency

25
Potassium regulation

• Aldosterone enhances Ksecretion while causing
  Na reabsorption
• There is a one-for-one exchange of Na K in the
  cortical collecting ducts to maintain electrolyte
  balance
• Adrenal cortical cells are extremely sensitive to
  K content of the ECF that baths themK controls
  its own in the ECF via feedback regulation of
  aldosterone release

26
2 Rules of Electrolyte Balance

• Most common problems with electrolyte balance are
  caused by imbalance between gains and losses of
  sodium ions
• Problems with potassium balance are less common,
  but more dangerous than sodium imbalance

27
Calcium balance

• 99 of bodys Ca is in bone in the form of
  calcium phosphate salts (most abundant mineral in
  the body)
• Ionic Ca in ECF important for normal blood
  clotting, cell membrane permeability,
  neuromuscular excitability secretory behavior
• Hypocalcemia?increases excitability causes mm
  tetany
• Hypercalcemia?(-) neurons mm cells and may
  cause life-threatening arrhythmias
• Ca balance is regulated by 2 Hormones PTH
  calcitonin
• 98 of filtered Ca is reabsorbed under normal
  circumstances

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Calcium balance, cont.

• PTH Calcitriol () by decreased plasma Ca
  levels
• Bones activates osteoclasts
• Small intest. enhances intestinal absorption of
  Ca by indirectly () kidneys to activate vit D
• Kidneys increases Ca reabsorption by renal
  tubules while decreasing phosphate ion
  reabsorption
• Calcitonin () by rising plasma Ca levels
• Antagonistic to PTH calcitriol by causing
  deposition of Ca in bone but its effect is
  negligible

29
Anion regulation

• Chloride is major anion in ECF maintains
  osmotic pressure of blood w/ Na
• 99 filtered Cl- is reabsorbed w/ blood pH w/in
  normal limits
• W/acidosis, fewer Cl ions accompany Na b/c
  bicarbonate ion reabsorption is stepped up to
  restore blood pH to normal
• Other anions seem to have a transport maximum
  excesses are spilled over into the urine

30
Acid-base balance

• Arterial blood 7.4
• Venous blood IF 7.35
• More acidic metabolites CO2more acidic
• ICF 7.0
• of H in blood regulated by
• 1. Chemical buffers (rapid/fraction of a second),
  2. Respiratory center of brain stem (1-3 min.),
  
• 3. Renal mechanisms (most potent/require hours to
  a day or more)

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Terms Relating to AcidBase Balance
Table 273
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Strong or Weak

• Strong acids and strong bases
• dissociate completely in solution
• Weak acids or weak bases
• do not dissociate completely in solution
• some molecules remain intact

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Sources of Hydrogen Ions

• Most hydrogen ions originate from cellular
  metabolism
• Breakdown of phosphorus-containing proteins
  releases phosphoric acid into the ECF
• Anaerobic respiration of glucose produces lactic
  acid
• Fat metabolism yields organic acids and ketone
  bodies
• Transporting carbon dioxide as bicarbonate
  releases hydrogen ions

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3 Types of Acids in the Body

• Volatile acids - Can leave solution and enter the
  atmosphere
• Carbonic acid is an important volatile acid in
  body fluid
• Fixed acids - Are acids that do not leave
  solution. Once produced they remain in body
  fluids until eliminated by kidneys
• Sulfuric Acid and Phosphoric Acid are most
  important fixed acids in the body generated
  during catabolism of AAs, phospholipids,
  nucleic acids
• Organic acids
• Produced by aerobic metabolism are metabolized
  rapidly do not accumulate
• Produced by anaerobic metabolism (e.g., lactic
  acid) build up rapidly

35
A Buffer System

• Consists of a combination of
• a weak acid
• and the anion released by its dissociation
• The anion functions as a weak base

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3 major chemical buffers

• 1. Bicarbonate
• The only important ECF buffer
• 2. Phosphate
• Effective in urine ICF (unimportant for
  buffering blood plasma bicarb is more imp.)
• 3. Protein
• Proteins w/in cells in plasma are the bodys
  most powerful plentiful source of buffers
• Major ICF buffer

37
Carbonic Acid

• Is a weak acid
• In ECF at normal pH equilibrium state exists
• Is diagrammed H2CO3 ? H HCO3

38
Protein Buffer Systems

• Depend on ability of amino acids
• Respond to pH changes by accepting or releasing H

39
The Hemoglobin Buffer System

• Is the only intracellular buffer system with an
  immediate effect on ECF pH
• Helps prevent major changes in pH when plasma
  PCO2 is rising or falling
• CO2 diffuses across RBC membrane
• no transport mechanism required
• As carbonic acid dissociates
• bicarbonate ions diffuse into plasma
• in exchange for chloride ions (chloride shift)
• Hydrogen ions are buffered by hemoglobin molecules

40
Problems with Buffer Systems

• Provide only temporary solution to acidbase
  imbalance
• Do not eliminate H ions
• Supply of buffer molecules is limited

41
Maintenance of AcidBase Balance

• For homeostasis to be preserved, captured H
  must
• be permanently tied up in water molecules
• through CO2 removal at lungs
• removed from body fluids
• through secretion at kidney

42
Respiratory regulation of H levels

• Acts slower than chemical buffers but has 1-2 x
  the buffering power than all the chemical buffers
  combined
• CO2 H2O ??H2CO3??H HCO3-
• Alkalosis (rise in pH) causes shift to right
  (more H ) acidosis (drop in pH) causes
  shift to left (more CO2 removed from blood by
  increased ventilation)
• Respiratory alkalosis or acidosis can occur from
  anything that impairs pulmonary fnx

43
Renal regulation of H levels

• Chemical buffers can tie up acids or bases
  temporarily but cannot rid the body of them
• Lungs can dispose of carbonic acid by eliminating
  CO2
• Only the kidneys can rid the body of other acids
  generated by cellular metabolism phosphoric
  acid, uric acid, lactic acid, ketone bodies
• Although the kidneys act slowly, they are the
  ultimate organs of acid-base regulation

44
Renal acid-base regulation

• 1. Conserve (reabsorb) or generation of new
  bicarbonate ions
• To reabsorb bicarb, H must be secreted
• For each H secreted into tubule lumen, one Na is
  reabsorbed from filtrate (maintaining electrolyte
  balance)
• To excrete bicarb, H must be retained
• 2. Excreting bicarbonate ions
• When body is in alkalosis the collecting ducts
  will secrete HCO3 while reclaiming H to acidify
  the blood
• Overall effect in nephrons collecting ducts as
  a whole is to reabsorb more HCO3 than is excreted
  (even in alkalosis)

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Acidosis and Alkalosis

• Affect all body systems
• particularly nervous and cardiovascular systems
• Both are dangerous
• but acidosis is more common
• because normal cellular activities generate acids

46
Acidosis pHlt7.35

• Respiratory acidosis
• Most common cause of acid-base imbalance
• CO2 accumulates in bloodshallow breathing,
  hampered gas exchange pneumonia, emphysema,
  cystic fibrosis
• Metabolic acidosis
• All causes other than respiratory
• Too much alcohol (converts to acetaldehyde?acetic
  acid), excessive loss of HCO3 (diarrhea), lactic
  acidosis (exercise), ketoacidosis (starvation)

47
Alkalosis pHgt7.45

• Respiratory alkalosis
• From hyperventilationrarely result of disease
  process
• Metabolic alkalosis
• Less common than metabolic acidosis
• Caused by excessive vomiting or GI suctioning,
  intake of excessive bases (antacids),
  constipation (more HCO3- is reabsorbed by the
  colon)

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Diagnostic Chart for Acid-Base Disorders
Figure 2715 (1 of 2)
49
Blood Chemistry and AcidBase Disorders
Table 274
50
Limits of acidosis/alkalosis

• pH below 7.0?depression of CNS?coma or death
• pH above 7.8?overexcitation of nervous system?mm
  tetany, extreme nervousness, convulsions?death
  often from respiratory arrest
• Acid-base imbalance due to inadequacy of a
  physiological buffer system is compensated for by
  the other system
• The respiratory system will attempt to correct
  metabolic acid-base imbalances
• The kidneys will work to correct imbalances
  caused by respiratory disease

51
Respiratory Compensation

• In metabolic acidosis
• The rate and depth of breathing are elevated
• Blood pH is below 7.35 and bicarbonate level is
  low
• As carbon dioxide is eliminated by the
  respiratory system, PCO2 falls below normal
• In respiratory acidosis, the respiratory rate is
  often depressed and is the immediate cause of the
  acidosis
• In metabolic alkalosis
• Compensation exhibits slow, shallow breathing,
  allowing carbon dioxide to accumulate in the
  blood
• Correction is revealed by
• High pH (over 7.45) and elevated bicarbonate ion
  levels
• Rising PCO2

52
Renal Compensation

• To correct respiratory acid-base imbalance, renal
  mechanisms are stepped up
• Acidosis has high PCO2 and high bicarbonate
  levels
• The high PCO2 is the cause of acidosis
• The high bicarbonate levels indicate the kidneys
  are retaining bicarbonate to offset the acidosis
• Alkalosis has Low PCO2 and high pH
• The kidneys eliminate bicarbonate from the body
  by failing to reclaim it or by actively secreting
  it

53
Problems with Fluid, Electrolyte, and Acid-Base
Balance

• Occur in the young, reflecting
• Low residual lung volume
• High rate of fluid intake and output
• High metabolic rate yielding more metabolic
  wastes
• High rate of insensible water loss
• Inefficiency of kidneys in infants