Fluid Electrolyte AcidBase

7/29/2019 Fluid Electrolyte AcidBase

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Chpt. 27: Fluid, Electrolyte, Acid-Base Balance

Human Body consists of:water, proteins, lipids, minerals, carbohydrates, miscellaneous

Body Fluidswater accounts for 50-60% of body weighttotal body water depends on:

skeletal muscle vs. adipose tissue

Fluid Balance: amount of water gained = amount of water lost to environmentDigestive systemUrinary system

Electrolyte Balance:balancing absorption in digestive tract with loss at sweat glands & urine

Acid-Base Balance: pHKidney important for eliminating excess H+ and HCO3- ions

Importance of Water: 99% of Extracellular FluidEssential 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 water1. Intracellular Fluid Compartment (ICF): cytosol

2. Extracellular Fluid Compartment (ECF):*1.) plasma2.) 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


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Each fluid compartment has specific electrolytesECF:

chief cation: chief anions:

ICF:chief cation: chief anion:

Basic Concepts in the Regulation of Fluids and Electrolytes1.) receptors monitoring fluids monitor ECF not ICF

mainly plasma and CSF

2.) no receptors for water or specific electrolytesosmoreceptors monitor osmolarity of fluidplasma volume indicates water balance

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 urineStimulates thirst

2. Aldosterone:Released from

Released in response to plasma Na+ levels and/orK+

Renin also triggers its release

Increases reabsorption of Na+

3. ANPReleased fromReleased in response to plasma Na+ levels and/orK+

Increases secretion of Na+, H2O follows so it?

Fluid Balancewater 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)

normally ECF and ICF are in osmotic equilibrium


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Water Gains:cellular metabolism

Water Losses: urine, fecesinsensible 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:

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 thirstcenter.


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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 excessHypotonic 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

-must be infused with IV of salt (hypertonic) solution

Electrolyte Balance: (salts, acids, bases)salts important for

most important are:

salts: thru foods & watersalts lost thru

GI disorders:

renal mechanisms extremely important: (table 27-2 page 1010 causes & consequences of electrolyteimbalances.)

Sodium Balance (135 145 mEq/L)Na+ salts 90% of all solutes inNa+ important for:

cause mosm of total 300 mosm

Sodium Balance:Gains via digestive tract vs. losses from urine/perspiration

hypernatremia (>145 mEq/L): dehydrationtriggers thirst center in hypothalamus & release of ADH

hyponatremia (


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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 fromregulates water reabsorptionin DCT &

decreased ADH:

increased ADH:

osmoreceptors in hypothalamussense osmolarity of plasma

3. ANF (Atrial Natriuretic Factor)hormone released from

released in response

diuretic and natriureticpromotes 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


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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 and K secretionif pH increases: H+ secretion and K secretion

3. Aldosterone levels:stimulates reabsorp. of Na and excretion of K1:1 exchangeadrenal cortex cells sense

hyperkalemia (>5 mEq/L): from renal failure, chronic acidosiscauses cardiac arrhythmias, muscle spasms

hypokalemia (11 mEq/L): confusion, muscle pain, cardiac arrhythmias, kidney stones

hypocalcemia (


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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, hypotensionhypomagnesemia: hypocalcemia, muscle weakness, cramps, cardiac arrhythmia

Phosphate Balanceimportant 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 BalancepH 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-

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 abnormallyblood pH > 7.45: alkalosis or alkalemia

Both effect all body systems especially nervous & cardioavascular

Acidosis more common because


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acidosis can be fatal CNS degenerates comacardiac contractions become weak, irregularcirculatory collapse from vasodilation

Acids classified as:1. volatile: can leave solution and enter atmosphere

Carbonic acid

2. fixed & organic: from metabolismphosphoric and sulfuric acid from protein metabolism are fixed acids

Lactic acid, pyruvic acid, ketone bodies: organic acids

Volatile Acids: Most CO2 in solution converts to carbonic acid which dissociates

PCO2 most important factor affecting pH in body tissuesPCO2 and pH are inversely related: CO2 levels = H+ = pH

H+ ion concentration regulated by:1. Chemical Buffers2. Respiratory Buffers3. 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:-weak bases: HCO3 and NH3


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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 . ProteinHelp regulate pH in ECF and ICF

B. Sodium Bicarbonate: Carbonic Acid - Bicarbonate

C. PhosphateBuffers pH of ICF and urine

A. Protein Buffer System: depends on amino acidsAmino 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 baseR-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 Systemin 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 acidGoal: 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 baseGoal: 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


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C. Phosphate Buffer System:uses phosphate:

weak acid: NaH2PO4weak base: Na2HPO4

When buffering a strong acid: the weak base is used to convert the strong acid into a weak acidHCl + 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

Limitations of these buffers:Provide temporary solutions do not eliminate H+ ions

Maintenance of AcidBase 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 tochanges in plasma pH

The body normally generates enough organic and fixed acids each day to add 100 mEq of H + toECF

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 acidsmetabolic acidosis: results from their accumulation

H+ enter filtrate and must be buffered

Regulation of H+ ion Secretion in Urinetubule cells respond to pH of ECF and changesH+ 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

2. phosphate buffer system

3. ammonium buffer system

2. Phosphate Buffer SystemH + Na2HPO4 --> NaH2PO4 which leaves in urine

3. Ammonium Buffer Systemuses NH3 produced from the metabolism of

NH3 diffuses into filtrate and: NH3 + H+ --> NH4 (ammonium)


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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 Alkalosiscaused by failure of

PCO2 most important indicator of respiratory functionnormal range: 35 45 mmHg

respiratory acidosis:

Acute: cardiac arrest or drowningChronic: COPD, pneumonia

respiratory alkalosis:Acute: pain, anxiety

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)

secretion of HCO3- (less Cl- is secreted)


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2. Metabolic Acidosis or Alkalosisall other abnormalities of acid-base imbalance except those caused by PCO 2 levels

HCO3 most important indicator of metabolic acidosis/alkalosisnormal values 22 - 28 mEq/L

metabolic acidosis:

1.production of large numbers of fixed or organic acids: ketoacidosis, alcohol poisoning, lactic acidosis2. Impaired H+ excretion at kidneys3. 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 othercompensates

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/LEvery 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 non-

measured anions (lactate, PO4-, acetoacetate)

Interstitial fluid (IF) is too negative

anion-gap metabolic acidosis: (ketoacidosis, respiratory failure (cells use anaerobic metabolism),kidney damage, some poisons, excessive aspirin, antifreeze


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Fluid Electrolyte AcidBase