Acid Base and Fluid Balance Dr.Nasim Ullah Siddiqui

Homeostasis A delicate balance of fluids, electrolytes, and acids and bases is

required to maintain good health. This balance is called Homeostasis: The bicarbonate buffering system is an important buffer system in the acid-base homeostasis of living things, including humans. As a buffer, it tends to maintain a relatively constant plasma pH and counteract any force that would alter it.

In this system, carbon dioxide (CO2) combines with water to form carbonic acid (H2CO3), which in turn rapidly dissociates to form hydrogen ion and bicarbonate (HCO3

- ).The reaction is catalyzed by the enzyme carbonic anhydrase.

When carbon dioxide dissolves in water, it can do so as a gas dissolved in water or by reacting with water to produce carbonic acid.

Any disturbance of the system will be compensated by a shift in the chemical equilibrium according to Le Chatelier's principle. For example, if the blood gained excess hydrogen ions (a process called acidosis), some of those hydrogen ions would shift to carbon dioxide, minimizing the increased acidity.

This buffering system becomes an even more powerful regulator of acid-base homeostasis when it is coupled with the body's capacity for respiratory compensation, in which breathing is altered to modify the amount of CO2 in circulation. In the above example, the body could increase breathing (respiratory alkalosis) to expel the excess CO2, pulling still more hydrogen ions toward the production of carbon dioxide. The process could continue until the excess acid is all exhaled.

When carbon dioxide dissolves in water, it can do so as a gas dissolved in water or by reacting with water to produce carbonic acid. In the cells of your body, the rate of carbonic acid production is accelerated by the enzyme carbonic anhydrasewhen excess hydrogen ions are added to the system the equilibrium is shifted to the left. This means that some of the added hydrogen ions will react with the bicarbonate ions to produce carbonic acid and the carbonic acid will dissociate into carbon dioxide and water as shown below.

 

Carbonic acid is known as a weak acid because it partially dissociates into the positive Hydrogen ions and negative bicarbonate ions. When hydrogen ions are removed from the reaction, the equilibrium will shift to the right. More carbon dioxide will combine with water and more carbonic acid will be produced and more hydrogen ions and bicarbonate ions will be produced.

Regulation of Acid-Base Balance

Lower concentration of H+, more alkaline, higher pH

The pH is also a reflection of the balance between CO2 (regulated by lungs) and bicarb (regulated by kidneys)

Normal H+ level is necessary to Maintain cell membrane integrityMaintain speed of cellular enzymatic actions

Chemical Regulation

Carbonic acid-bicarbonate buffer system is the first to react to change in the pH of ECF

H+ and CO2 concentrations are directly related ECF becomes more acidic, the pH decreases, producing

acidosis ECF receives more base substances, the pH rises, producing

alkalosis Lungs primarily control excretion of CO2 resulting from

metabolism Kidneys control excretion of hydrogen and bicarb

Acid-Base Balance

Acid-Base balance is:

the regulation of HYDROGEN ions.

pH

The acidity or alkalinity of a solution is measured as pH.

The more acidic a solution, the lower the pH.The more alkaline a solution , the higher the pH.Water has a pH of 7 and is neutral.The pH of arterial blood is normally between 7.35 and

7.45

Hydrogen ions

The more Hydrogen ions, the more acidic the solution and the LOWER the pH.

The lower Hydrogen concentration, the more alkaline the solution and the HIGHER the pH.

Buffer Systems

Regulate pH by binding or releasing Hydrogen

Most important buffer system:Bicarbonate-Carbonic Acid Buffer System

(Blood Buffer systems act instantaneously and thus constitute the body’s first line of defense against acid-base imbalance)

Respiratory Regulation

Lungs help regulated acid-base balance by eliminating or

retaining carbon dioxidepH may be regulated by altering the rate and depth

of respirationschanges in pH are rapid,

occurring within minutes

normal CO2 level 35 to 45 mm Hg

Renal Regulation

Kidneysthe long-term regulator of acid-base balanceslower to respond

may take hours or days to correct pH

kidneys maintain balance by excreting or conserving bicarbonate and hydrogen ions

normal bicarbonate level 22 to 26 mEq/L.

Factors Affecting Balance

Ageespecially infants and the elderly

Gender and Body Sizeamount of fat

Environmental TemperatureLifestyle

stress

Acid-Base Imbalances

Respiratory AcidosisRespiratory AlkalosisMetabolic AcidosisMetabolic Alkalosis

Respiratory acidosis pH ↓

PaCO2 ↑

HCO3 ↓

Respiratory alkalosis pH ↑

PaCO2 ↓

HCO3 ↑

Metabolic acidosis pH ↓

PaCO2

HCO3 ↓

Metabolic alkalosis pH ↑

PaCO2

HCO3 ↑

Respiratory AcidosisMechanism

Hypoventilation or Excess CO2 Production

EtiologyCOPDNeuromuscular DiseaseRespiratory Center Depression Late ARDSInadequate mechanical ventilationSepsis or BurnsExcess carbohydrate intake

Respiratory Acidosis (cont)Symptoms

Dyspnea, Disorientation or comaDysrhythmiaspH < 7.35, PaCO2 > 45mm HgHyperkalemia or Hypoxemia

TreatmentTreat underlying causeSupport ventilationCorrect electrolyte imbalanceIV Sodium Bicarbonate

Respiratory Alkalosis

Risk Factors and etiologyHyperventilation due to

extreme anxiety, stress, or painelevated body temperatureoverventilation with ventilatorhypoxiasalicylate overdosehypoxemia (emphysema or pneumonia)CNS trauma or tumor

Respiratory Alkalosis (cont)

SymptomsTachypnea or HyperpneaChest painLight-headedness, syncope, coma, seizuresNumbness and tingling of extremitiesDifficult concentrating, tremors, blurred visionWeakness, paresthesias, tetanyLab findings

pH above 7.45CO2 less than 35

Respiratory Alkalosis (cont)

TreatmentMonitor VS and ABGsTreat underlying diseaseAssist client to breathe more slowlyHelp client breathe in a paper bag or apply rebreather maskSedation

Metabolic Acidosis

Risk Factors/EtiologyConditions that increase acids in the blood

Renal FailureDKAStarvationLactic acidosis

Prolonged diarrheaToxins (antifreeze or aspirin)Carbonic anhydrase inhibitors - Diamox

Metabolic Acidosis (cont)

SymptomsKussmaul’s respirationLethargy, confusion, headache, weaknessNausea and VomitingLab:

pH below 7.35Bicarb less than 22

Treatmenttreat underlying causemonitor ABG, I&O, VS

Metabolic AlkalosisRisk Factors/Etiology

Acid loss due tovomitinggastric suction

Loss of potassium due tosteroidsdiuresis

Antacids (overuse of)

Metabolic Alkalosis (cont)Symptoms

Hypoventilation (compensatory)Dysrhythmias, dizzinessParesthesia, numbness, tingling of extremitiesHypertonic muscles, tetanyLab: pH above 7.45, Bicarb above 26

CO2 normal or increased w/comp Hypokalmia, Hypocalcemia

TreatmentI&O, VSgive potassiumtreat underlying cause

Interpreting ABGs 1. Look at the pH

• is the primary problem acidosis (low) or alkalosis (high)

2. Check the CO2 (respiratory indicator)• is it less than 35 (alkalosis) or more than 45 (acidosis)

3. Check the HCO3 (metabolic indicator)• is it less than 22 (acidosis) or more than 26 (alkalosis)

4. Which is primary disorder (Resp. or Metabolic)?• If the pH is low (acidosis), then look to see if CO2 or HCO3 is acidosis

(which ever is acidosis will be primary).• If the pH is high (alkalosis), then look to see if CO2 or HCO3 is

alkalosis (which ever is alkalosis is the primary).• The one that matches the pH (acidosis or alkalosis), is the primary disorder.

CompensationThe Respiratory system and Renal systems

compensate each other attempt to return the pH to normal

ABG’s show that compensation is present whenthe pH returns to normal or near normal

If the non primary system is in the normal range (CO2 35 to 45) (HCO3 22-26), then that system is not compensating for the primary.

For example: In respiratory acidosis (pH<7.35, CO2>45), if the HCO3 is

>26, then the kidneys are compensating by retaining bicarbonate.

If HCO3 is normal, then not compensating.

Fluid & Electrolyte Balance

Where’s the water?

Water content varies with age & tissue type

1. Infants – 73%

2. Adult male – 60%

3. Adult female – 50%

4. Elderly – 45%

Fat has the lowest water content (~20%).

Bone is close behind (~22 – 25%).

Skeletal muscle is highest at ~65%.

Functions of Body Fluid

Major component of blood plasmaSolvent for nutrients and waste productsNecessary for hydrolysis of nutrientsEssential for metabolismLubricant in joints and GI tractCools the body through perspirationProvides some mineral elements

Composition of Body Fluids

Body fluids contain ElectrolytesAnions – negative charge

Cl, HCO3, SO4

Cations – positive charge Na, K, Ca

Electrolytes are measured in mEqMinerals are ingested as compounds and are

constituents of all body tissues and fluidsMinerals act as catalysts

Electrolytes in Body Fluids

Normal ValuesSodium (Na+) 35 – 145 mEq/LPotassium (K+) 3.5 – 5.0 mEq/LIonized Calcium (Ca++) 4.5 – 5.5 mg/dLCalcium (Ca++) 8.5 – 10.5 mg/dLBicarbonate (HCO3) 24 – 30 mEq/L

Chloride (Cl--) 95 – 105 mEq/LMagnesium (Mg++) 1.5 – 2.5 mEq/LPhosphate (PO4

---) 2.8 – 4.5 mg/dL

Body FluidsIntracellular fluid (ICF)

found within the cells of the bodyconstitutes 2/3 of total body fluid in adultsmajor cation is potassium

Extracellular fluid (ECF)found outside the cellsaccounts of 1/3 of total body fluidmajor cation is sodium

Terms Osmosis: movement of water across cell membranes from less

concentrated to more concentrated Solutes: substances dissolved in a liquid Osmolality: the concentration within a fluid Diffusion: movement of molecules in liquids from an area of

higher concentration to lower concentration Filtration: fluid and solutes move together across a membrane

from area of higher pressure to one of lower pressure Active Transport: substance moves across cell membranes

from less concentrated solution to more concentrated - requires a carrier

Routes of Fluid Loss

UrineInsensible fluid lossFeces

Electrolytes

SodiumPotassiumChloridePhosphate

MagnesiumCalciumBicarbonate

Electrolytes are important for:

Maintaining fluid balance

Contributing to acid-base regulation

Facilitating enzyme reactions

Transmitting neuromuscular reactions

Electrolyte concentrations are calculated in milliequivalents

mEq/L = ion concentration (mg/L) x number of charges on one ion atomic weight

Na+ concentration in the body is 3300 mg/LNa+ carries a single positive charge.Its atomic weight is approximately 23.

Therefore, in a human the normal value for Na+ is:

3300 mg/L = 143 mEq/L

23

Note: One mEq of a univalent is equal to one mOsm whereas one mEq of a bivalent ion is equal to ½ mOsm. However, the reactivity of 1 mEq is equal to 1 mEq.

Relative electrolyte

concentrations:

Plasma, Interstitial Fluid

& ICF

Sources of intake & output

Regulation of water balance

It is not so much water that is regulated, but solutes.osmolality is maintained at between 285 – 300 mOsm.An increase above 300 mOsm triggers:

ThirstAntidiuretic Hormone release

The Thirst Mechanism

An increase of 2 – 3% in plasma osmolality triggers the thirst center of the hypothalamus.Secondarily, a 10 – 15% drop in blood volume also triggers thirst. This is a significantly weaker stimulus.

DehydrationChronic dehydration leads to oliguria.Severe dehydration can result in hypovolemic shock.

Causes include:

•Hemorrhage•Burns•Vomiting•Diarrhea•Sweating•Diuresis, which can be caused by diabetes insipidus, diabetes mellitus and hypertension (pressure diuresis).

Hypotonic hydrationA severe drop in osmolalityCaused by:

Excessive water intakeRenal dysfunction

Major consequence is hyponatremia.Hyponatremia results in:

Cerebral edema (brain swelling)Sluggish neural activityConvulsions, muscle spasms, deranged behavior.

Treated with I.V. hypertonic mannitol or something similar.

A rather lame

illustrationYou do remember

how osmosis works, don’t you?

Sodium regulation

Blood pressure, sodium, and water

Atrial Naturetic Peptide:The heart’s

own compensatory mechanism.

Reabsorption of

bicarbonate

Generation of new

bicarbonate from

phosphate

Generation of

bicarbonate from

glutamine deamination

Movement of Body Fluids

Osmosis = movement across a semi-permeable membrane from area of lesser concentration to are of higher concentration; high solute concentration has a high osmotic pressure and draws water toward itselfOsmotic pressure = drawing power of water

(Osmolality)Osmolarity = concentration of solution

Movement of Body Fluids

Colloid or Oncotic pressure = keeps fluid in the intravascular compartment by pulling water from the interstitial space back into the capillaries

Solutions

Isotonic SolutionThe same concentration as blood plasma; expand fluid

volume without causing fluid shift

Hypotonic SolutionLower concentration than blood plasma; moves fluid into the

cells causing them to enlarge

Hypertonic solutionHigher concentration than blood plasma; pulls fluid from cells

causing them to shrink

Movement of Body Fluids

Diffusion = Molecules move from higher concentration to lowerConcentration gradient

Filtration = water and diffusible substances move together across a membrane; moving from higher pressure to lower pressure

Edema results from accumulation of excess fluid in the interstitial space

Hydrostatic pressure causes the movement of fluids from an area of higher pressure to area of lower pressure

Active Transport

Requires metabolic activity and uses energy to move substances across cell membranesEnables larger substances to move into cellsMolecules can also move to an area of higher concentration

(Uphill)Sodium-Potassium Pump

Potassium pumped in – higher concentration in ICFSodium pumped out – higher concentration in ECF

Regulation of Body Fluids

Homeostasis is maintained throughFluid intakeHormonal regulationFluid output regulation

Fluid Intake

Thirst control center located in the hypothalamusOsmoreceptors monitor the serum osmotic pressureWhen osmolarity increases (blood becomes more

concentrated), the hypothalamus is stimulated resulting in thirst sensationSalt increases serum osmolarity

Hypovolemia occurs when excess fluid is lost

Fluid Intake

Average adult intake 2200 – 2700 mL per day

Oral intake accounts for 1100 – 1400 mL per daySolid foods about 800 – 1000 mL per dayOxidative metabolism – 300 mL per day

Those unable to respond to the thirst mechanism are at risk for dehydrationInfants, patients with neuro or psych problems, and older

adults

Hormonal Regulation

ADH (Antidiuretic hormone)Stored in the posterior pituitary and released in response to

serum osmolarityPain, stress, circulating blood volume effect the release of

ADHIncrease in ADH = Decrease in urine output = Body

saves waterMakes renal tubules and ducts more permeable to water

Hormonal Regulation

Renin-angiotensin-aldosterone mechanismChanges in renal perfusion initiates this mechanismRenin responds to decrease in renal perfusion secondary to

decrease in extracellular volumeRenin acts to produce angiotensin I which converts to

angiotensin II which causes vasoconstriction, increasing renal perfusion

Angiotensin II stimulates the release of aldosterone when sodium concentration is low

Hormonal Regulation

AldosteroneReleased in response to increased plasma potassium levels

or as part of the renin-angiotensin-aldosterone mechanism to counteract hypovolemia

Acts on the distal portion of the renal tubules to increase the reabsorption of sodium and the secretion and excretion of potassium and hydrogen

Water is retained because sodium is retainedVolume regulator resulting in restoration of blood volume

Hormonal Regulation

Atrial Natriuretic Peptide (ANP)ANP is a hormone secreted from atrial cells of the

heart in response to atrial stretching and an increase in circulating blood volume

ANP acts like a diuretic that causes sodium loss and inhibits the thirst mechanism

Monitored in CHF

Fluid Output Regulation

Organs of water lossKidneysLungsSkinGI tract

Fluid Output Regulation

Kidneys are major regulatory organ of fluid balance Receive about 180 liters of plasma to filter daily 1200 – 1500 mL of urine produced daily Urine volume changes related to variation in the amount and type of fluid

ingested Skin

Insensible Water LossContinuous and occurs through the skin and lungs Can significantly increase with fever or burns

Sensible Water Loss occurs through excess perspirationCan be sensible or insensible via diffusion or perspiration

500 – 600 mL of insensible and sensible fluid lost through skin each day

Fluid Output Regulation

LungsExpire approx 500 mL of water dailyInsensible water loss increases in response to changes in

resp rate and depth and oxygen administration

GI Tract3 – 6 liters of isotonic fluid moves into the GI tract and then

returns to the ECF200 mL of fluid is lost in the feces each day

Diarrhea can increase this loss significantly

Regulation of Electrolytes

Major Cations in body fluidsSodium (Na+)Potassium (K+)Calcium (Ca++)Magnesium (Mg++)

Sodium Regulation

Most abundant cation in the extracellular fluidMajor contributor to maintaining water balance

Nerve transmissionRegulation of acid-base balanceContributes to cellular chemical reactions

Sodium is taken in via food and balance is maintained through aldosterone

Potassium Regulation

Major electrolyte and principle cation in the extracellular fluidRegulates metabolic activitiesRequired for glycogen deposits in the liver and skeletal

muscleRequired for transmission of nerve impulses, normal cardiac

conduction and normal smooth and skeletal muscle contraction

Regulated by dietary intake and renal excretion

Calcium Regulation

Stored in the bone, plasma and body cells99% of calcium is in the bones and teeth1% is in ECF50% of calcium in the ECF is bound to protein (albumin)40% is free ionized calcium Is necessary for

Bone and teeth formationBlood clottingHormone secretionCell membrane integrityCardiac conductionTransmission of nerve impulsesMuscle contraction

Magnesium Regulation

Essential for enzyme activitiesNeurochemical activitiesCardiac and skeletal muscle excitabilityRegulation

DietaryRenal mechanismsParathyroid hormone action

50 – 60% of magnesium contained in bones1% in ECFMinimal amount in cell

Anions

Chloride (Cl-) Major anion in ECFFollows sodium

Bicarbonate (HCO3-)

Is the major chemical base bufferIs found in ECF and ICFRegulated by kidneys

Anions

Phosphate (PO4---)

Buffer ion found in ICFAssists in acid-base regulationHelps to develop and maintain bones and teethCalcium and phosphate are inversely proportionalPromotes normal neuromuscular action and participates in

carbohydrate metabolismAbsorbed through GI tractRegulated by diet, renal excretion, intestinal absorption and PTH

Causes of Electrolyte Imbalances

Excessive sweatingFluid loss leading to dehydrationExcessive vomitingDiuretics like Lasix (K+ depletion)Massive blood lossDehydration may go unnoticed in hot, dry climatesRenal failure

Sodium

Most abundant in extracellular spaceMoves among three fluid compartmentsFound in most body secretions

Hyponatremia – Low Sodium

SeizuresPersonality changesNausea/vomitingTachycardiaConvulsionNormal Na (135-145)

Hypernatremia

Excessive Na in ECFLoss of water

DiarrheaInsensible water lossWater deprivation

Gain of SodiumDiabetes insipidusHeat stroke

Hypokalemia – Low Potassium

Severe leg crampsFlaccid musclesFatigueIrregular pulseChest discomfortEKG changes

T wave flattens

Normal Potassium-3.5-5

Hyperkalemia

CNSNausea and vomiting

Peripheral Nervous SystemTremors, twitching

Heart Bradycardia, peaked T wave

Hypocalcemia – Low Calcium

Tingling of fingersTetanyMuscle crampsPositive Trousseau’s

Carpal spasmPositive Chvostek’s

Contraction of facial muscle when facial nerve tapped

Hypercalcemia

CausesProlonged immobilityOsteoporosisThiazide diureticsAcidosis

Signs/symptomsN/V, weaknessHypoactive reflexesCardiac arrest

Hypomagnesemia

CausesMalnutritionAlcoholismPolyuriaPre-ecclampsia

Signs/symptomsMuscle tremorHyperactive deep reflexesChvostek’s/Trousseau’sDifficulty breathing

Hypermagnesemia

CausesRenal failureExcessive intake

Signs/symptomsLow BPMuscle weaknessAbsent reflexesBradycardia

Cheat Sheet

• Increase pH – alkalosis• Decrease pH – acidosis• Respiratory – CO2• Metabolic (kidneys)– HCO3• CO2 has an inverse relationship with pH• When pH goes down, CO2 goes up• HCO3 follows pH. If pH goes up so does HCO3• CO2 increases, pH decreases – resp. acidosis• CO2 decreases, pH increases – resp. alkalosis• HCO3 increases, pH increases – metabolic alkalosis• HCO3 decreases, pH decreases – metabolic acidosis

Question

• An older client comes to the emergency department experiencing chest pain and shortness of breath. An arterial blood gas is ordered. Which of the following ABG results indicates respiratory acidosis?

1. pH - 7.54, PaCO2 – 28, HCO3 – 22

2. pH – 7.32, PaCO2 – 46, HCO3 – 24

3. pH – 7.31, PaCO2 – 35, HCO3 – 20

4. pH – 7.5, PaCO2 – 37, HCO3 - 28