Chpt. 27: Fluid, Electrolyte, Acid-Base Balance
Human Body consists of:
water, proteins, lipids, minerals, carbohydrates, miscellaneous
Body Fluids
water accounts for 50-60% of body weight
total body water depends on:
skeletal muscle vs. adipose tissue
Fluid Balance: amount of water gained = amount of water lost to environment
Digestive system
Urinary system
Electrolyte Balance:
balancing absorption in digestive tract with loss at sweat glands & urine
Acid-Base Balance: – pH
Kidney important for eliminating excess H+ and HCO3- ions
Importance of Water: 99% of Extracellular Fluid
Essential component of intracellular fluid
Optimal heating/cooling
Prevent mucus membranes from drying out
Diffusion medium for gasses, nutrients, wastes
Fluid Compartments: 2 main compartments to store water
1. Intracellular Fluid Compartment (ICF): cytosol
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2. Extracellular Fluid Compartment (ECF):
*1.) plasma
2.) interstitial fluid (IF)
3.) other – CSF, synovial
exchange of water between plasma and IF due to HP and OP
Composition of Body Fluids:
water:
solutes: electrolytes & nonelectrolytes
1. nonelectrolytes: do not dissociate
2. electrolytes: dissociate into ions in water
-ions are charged particles,
electrolytes more important for osmolarity
Each fluid compartment has specific electrolytes
ECF:
chief cation:
chief anions:
chief cation:
chief anion:
ICF:
Basic Concepts in the Regulation of Fluids and Electrolytes
1.) receptors monitoring fluids monitor ECF not ICF
mainly plasma and CSF
2.) no receptors for water or specific electrolytes
osmoreceptors monitor osmolarity of fluid
plasma volume indicates water balance
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3.) cells cannot move water by active transport: water moves via osmosis
4.) balance between intake and excretion determines overall fluid balance
Main Hormones for Water/Electrolyte Balance:
1. ADH: released from?
Released in response to
Concentrates urine
Stimulates thirst
2. Aldosterone:
Released from
Released in response to ↓plasma Na+ levels and/or ↑K+
Renin also triggers its release
Increases reabsorption of Na+
3. ANP
Released from
Released in response to ↑plasma Na+ levels and/or ↓K+
Increases secretion of Na+, H2O follows so it?
Fluid Balance
water circulates freely within ECF (from capillaries due to HP), in lymph, serous fluid, CSF
water moves between ICF and ECF due to osmotic pressure (OP)
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normally ECF and ICF are in osmotic equilibrium
Water Gains:
cellular metabolism
Water Losses: urine, feces
insensible perspiration: water vaporizes with breathing or through skin
sensible perspiration:
other: fever
maintain tonicity of fluids via water shifts
Fluid Shifts - water movement between ECF and ICF in response to osmotic gradient
• if ECF osmotic concentration increases, it becomes hypertonic:
• If ECF osmotic concentration decreases, it becomes hypotonic:
Note ICF volume higher than ECF ICF acts as water reserve
Remember: anything that changes solute concentration effects osmotic pressure
Edema:
anything that increases fluid flow out of bloodstream or decreases its return
factors that increase fluid loss:
1. increased capillary hydrostatic pressure:
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2. increased capillary permeability:
Dehydration: water loss exceeds water intake
-common with hemorrhage, severe burns, vomiting, diarrhea, sweating,
water deprivation, diuretic abuse and endocrine disorders
water loss from ECF so Na ion concentrations increase (hypernatremia)
increased plasma osmolarity causes:
1.
2.
Thirst Mechanism:
• decrease plasma vol. by 10% and/or increased in plasma osmolarity triggers hypothalamic thirst center.
• hypertonic plasma pulls water from salivary glands which creates dry mouth
• when water leaves cells in thirst center (hypothalamus) it depolarizes them = sensation of thirst
• thirst quenched
Overhydration – water excess
Hypotonic Hydration: water intoxication
renal insufficiency, drinking large amounts very quickly, renal failure, heart failure, endocrine disorder
hyponatremia
-causes water to
- s/s: confusion, hallucinations, convulsions, coma, death
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-must be infused with IV of salt (hypertonic) solution
Electrolyte Balance: (salts, acids, bases)
salts important for
most important are:
salts: thru foods & water
salts lost thru
GI disorders:
renal mechanisms extremely important: (table 27-2 page 1010 causes & consequences of electrolyte imbalances.)
Sodium Balance (135 – 145 mEq/L)
Na+ salts 90% of all solutes in
Na+ important for:
cause
mosm of total 300 mosm
Sodium Balance:
Gains via digestive tract vs. losses from urine/perspiration
hypernatremia (>145 mEq/L): dehydration
triggers thirst center in hypothalamus & release of ADH
hyponatremia (<135 mEq/L): water intoxication
inhibits release of ADH
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Regulation of Sodium Balance: (function to restore BV & BP)
1. Aldosterone: secreted by
Controls Na+ reabsorption in DCT
with or without Aldosterone:75-80% Na+
reabsorbed in PCT
w/ aldosterone: remaining 20-25% is
reabsorbed in DCT
Triggers for Aldosterone:
Addison's Disease:
2. ADH: released from
regulates water reabsorption
in DCT &
decreased ADH:
increased ADH:
osmoreceptors in hypothalamus
sense osmolarity of plasma
3. ANF (Atrial Natriuretic Factor)
hormone released from
released in response
diuretic and natriuretic
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promotes excretion
inhibits distal tubule cells ability
inhibits release of
4. Other Hormones:
A. estrogens: similar to aldosterone
B. progesterone:
C. glucocorticoids: cortisol
Potassium (K+) Balance (3.8 – 5.0 mEq/L)
main intracellular cation (98% in ICF)
necessary for: RMP, neuromuscular functioning
balance due to rate of absorption vs losses in urine
3 factors determine the rate of K secretion from tubules:
1. K+ concentration in the ECF: higher ECF concentration increases rate of secretion
2. pH of ECF:
H+ & K+ compete for secretion for every Na+ reabsorbed therefore
if pH decreases: H+ secretion
if pH increases: H+ secretion
and K secretion
and K secretion
3. Aldosterone levels:
stimulates reabsorp. of Na and excretion of K
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1:1 exchange
adrenal cortex cells sense
hyperkalemia (>5 mEq/L): from renal failure, chronic acidosis
causes cardiac arrhythmias, muscle spasms
hypokalemia (<3.5 mEq/L): from low-K diet, diuretics, hypersecretion of aldosterone, chronic
alkalosis
cells hyperpolarize: muscle weakness, paralysis, cardiac arrest
Calcium Balance (4.3 – 5.3 mEq/L)
most abundant mineral in body - 99% of calcium found in
Ca++ important for:
calcium levels regulated by PTH and calcitonin
PTH:
1. increases calcium levels by reabsorbing Ca++ from bones (osteoclasts)
2. increases intestinal absorption of Ca++
3. increases Ca++ reabsorption by kidney tubules
Calcitonin:
1. decreases blood calcium levels by putting calcium into bones (osteoblasts)
hypercalcemia (>11 mEq/L): confusion, muscle pain, cardiac arrhythmias, kidney stones
hypocalcemia (<4 mEq/L): muscle spasms, convulsions, cramps, weak heart beat, cardiac arrhythmias, osteoporosis
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Magnesium (Mg) Balance (1.4 – 2.0 mEq/L)
important for: bone structure, co-factor for enzymes
60% found in the skeleton and higher in ICF then ECF
hypermagnesemia: confusion, lethargy, respiratory depression, hypotension
hypomagnesemia: hypocalcemia, muscle weakness, cramps, cardiac arrhythmia
Phosphate Balance
important for: bone mineralization, metabolism, synthesis of nucleic acids
Chloride Balance (100-108 mEq/L)
major anion in ECF: follows Na+
absorbed across digestive tract with Na+
hyperchloremia causes acidosis
hypochloremia causes alkalosis, muscle cramps
Acid-Base Balance
pH of body fluids is altered by acids & bases
strong vs. weak acids & bases:
strong acids completely dissociate into H+
weak acids do not completely dissociate
strong bases completely dissociate into OH-
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critical for homeostasis
arterial blood pH:
interstitial fluid and venous blood pH:
Acidosis: physiological state resulting from abnormally low plasma pH
blood pH < 7.35: acidosis* or acidemia
*note: 7-7.35: physiological acidosis
Alkalosis: physiological state resulting from abnormally
blood pH > 7.45: alkalosis or alkalemia
Both effect all body systems – especially nervous & cardioavascular
Acidosis more common because
acidosis can be fatal – CNS degenerates – coma
cardiac contractions become weak, irregular
circulatory collapse from vasodilation
Acids classified as:
1. volatile: can leave solution and enter atmosphere
Carbonic acid
2. fixed & organic: from metabolism
phosphoric and sulfuric acid from protein metabolism are fixed acids
Lactic acid, pyruvic acid, ketone bodies: organic acids
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Volatile Acids: Most CO2 in solution converts to carbonic acid which dissociates
PCO2 most important factor affecting pH in body tissues
PCO2 and pH are inversely related: CO2 levels ↑ = ↑H+ = ↓ pH
H+ ion concentration regulated by:
1. Chemical Buffers
2. Respiratory Buffers
3. Renal Mechanisms
Mechanisms of pH control: balance gains and losses of H+
Through buffers: buffers are dissolved compounds that stabilize pH by providing or removing H+
1. Chemical Buffers:
acids: proton donors:
-acidity: due to
Weak acids:
Strong acids:
bases: proton acceptors
-strong bases: hydroxides:
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-weak bases: HCO3 and NH3
Chemical Acid-Base Buffers:
buffer pairs
when pH decreases: weak base binds to/picks up H+ to
when pH increases:
3 major chemical buffers: (figure 27-10 page 1014)
A . Protein
Help regulate pH in ECF and ICF
B. Sodium – Bicarbonate: Carbonic Acid - Bicarbonate
C. Phosphate
Buffers pH of ICF and urine
A. Protein Buffer System: depends on amino acids
Amino acids have carboxyl (acid) groups COOH: which functions as an acid
R - COOH --> R-COO + H+
the same protein molecule also has NH2 which functions as a base
R-NH2 + H+ --> R-NH3
amphoteric molecules: function as

The Hemoglobin Buffer System

Hydrogen ions from CO2 loading are buffered by hemoglobin molecules

CO2 + HHb ↔HbCO2 + H+

O2 + HHb ↔ HbO2 + H+
B. Sodium- Bicarbonate System
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in ECF and ICF
H2CO3 and NaHCO3
HCl + NaHCO3 --> H2CO3 + NaCl
When buffering strong acid – the weak base is used to convert the strong acid into a weak acid
Goal: to replace strong acid with weak acid (and salt by-product)
NaOH + H2CO3 --> NaHCO3 + H2O
When buffering strong base – the weak acid is used to convert the strong base into a weak base
Goal: to replace strong base with weak base (and water by-product)
w/i cells K and Mg bicarb. help w/ bicarb. system
the buffering power is directly proportional to the concentration of the buffers
need base bicarb. (BB) to carbonic acid ratio of 20:1
C. Phosphate Buffer System:
uses phosphate:
weak acid: NaH2PO4
weak base: Na2HPO4
When buffering a strong acid: the weak base is used to convert the strong acid into a weak acid
HCl + Na2HPO4 --> NaH2PO4 + NaCl
and if buffer a strong base:
The weak acid is used to convert the strong base into a weak base
NaOH + NaH2PO4 --> Na2HPO4 + H2O
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Limitations of these buffers:
Provide temporary solutions – do not eliminate H+ ions

Maintenance of Acid–Base Balance
For homeostasis to be preserved, captured H+ must:
1.
Be permanently tied up in water molecules:
– through CO2 removal at lungs
2.
Be removed from body fluids:
– through secretion at kidney
2. Respiratory System Regulation
Respiratory system is a physiological buffer: ties up H+ in H2O
Cannot protect ECF from changes in pH that result from
Functions only when respiratory system and control centers are working normally
Ability to buffer acids is limited by
if pH decreases:
if pH increases:
3. Renal Mechanisms:
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
Renal Compensation

Is a change in rates of H+ and HCO3- secretion or reabsorption by kidneys in response to changes in plasma pH

The body normally generates enough organic and fixed acids each day to add 100 mEq of H+ to ECF

Kidneys assist lungs by eliminating any CO2 that

Enters renal tubules during filtration

Diffuses into tubular fluid en route to renal pelvis
rids body of acids
metabolic acidosis: results from their accumulation
H+ enter filtrate and must be buffered
Regulation of H+ ion Secretion in Urine
tubule cells respond to pH of ECF and changes
H+ secretion accordingly
for every H+ secreted into tubule 1 Na+ reabsorbed
H+ secretion is dependant on CO2 levels
Bicarbonate Ions: alkaline reserve
-must replenish stores of HCO3 in blood
-tubule cells very impermeable to HCO3 in filtrate:
can't reabsorb them from filtrate
-pure H+ ions not excreted directly in urine = decrease urine pH too low
1. First buffered by HCO3 in filtrate
-if HCO3 "used up" must secrete H+ into urine. These H+ are then buffered by carbonic-acid buffers
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2. phosphate buffer system
3. ammonium buffer system
2. Phosphate Buffer System
H + Na2HPO4 --> NaH2PO4 which leaves in urine
3. Ammonium Buffer System
uses NH3 produced from the metabolism of
NH3 diffuses into filtrate and: NH3 + H+ --> NH4 (ammonium)
Renal Response to Acidosis:
↑ secretion of H + (less K+ is secreted)
↓ secretion of HCO3- (more Cl- is secreted)
Renal Response to Alkalosis:
↓ secretion of H + (more K+ is secreted)
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-
-
↑secretion of HCO3 (less Cl is secreted)
Abnormalities of Acid Base Balance
Acute: the initial phase
Compensated: when condition persists
Respiratory: imbalance with CO2
Metabolic: generation of organic or fixed acids
1. Respiratory Acidosis or Alkalosis
caused by failure of
PCO2 most important indicator of respiratory function
normal range: 35 – 45 mmHg
respiratory acidosis:
Acute: cardiac arrest or drowning
Chronic: COPD, pneumonia
respiratory alkalosis:
Acute: pain, anxiety
2. Metabolic Acidosis or Alkalosis
all other abnormalities of acid-base imbalance except those caused by PCO2 levels
HCO3 most important indicator of metabolic acidosis/alkalosis
normal values 22 - 28 mEq/L
metabolic acidosis:
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1.production of large numbers of fixed or organic acids: ketoacidosis, alcohol poisoning, lactic acidosis
2. Impaired H+ excretion at kidneys
3. Severe bicarbonate loss: diarrhea
metabolic alkalosis:
1. Too much HCO3- (constipation, overuse of antacids)
2. Loss of H+ (vomiting)
Compensated Acidosis/Alkalosis: one system fails the other “compensates”
Respiratory Acidosis:
pH=
PCO2=
HCO3- =
Respiratory Alkalosis:
pH=
PCO2=
HCO3- =
Metabolic Acidosis:
pH=
PCO2=
HCO3- =
Metabolic Alkalosis:
pH=
PCO2=
HCO3- =
Anion gap (AG): shows amount of unmeasured anions – a calculation
AG = Na+ - [Cl-] + [HCO3-]
(are usually more unmeasured anions than cations so its usually + value
Normal 3- 11 mmol/L
Every HCO3 lost is replaced by a Cl- anion: diarrhea, Kidney loss of HCO3-
If increases shows loss of HCO3 without increase in Cl
(HCO3 is being used to buffer the acid and the HCO3 negative charge is being replaced with nonmeasured anions (lactate, PO4-, acetoacetate)
Interstitial fluid (IF) is too negative
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anion-gap metabolic acidosis: (ketoacidosis, respiratory failure (cells use anaerobic metabolism),
kidney damage, some poisons, excessive aspirin, antifreeze
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