24 fluid, electrolyte, and acid-base balance

© 2011 Pearson Education, Inc.

PowerPoint® Lecture Presentations prepared byAlexander G. CheroskeMesa Community College at Red Mountain

24Fluid, Electrolyte, and Acid-Base Balance


Section 1: Fluid and Electrolyte Balance

• Learning Outcomes

• 24.1 Explain what is meant by fluid balance, and discuss its importance for homeostasis.

• 24.2 Explain what is meant by mineral balance, and discuss its importance for homeostasis.

• 24.3 Summarize the relationship between sodium and water in maintaining fluid and

electrolyte balance.

• 24.4 CLINICAL MODULE Explain factors that control potassium balance, and discuss

hypokalemia and hyperkalemia.


Section 1: Fluid and Electrolyte Balance

• Fluids constitute ~50%–60% of total body composition

• Minerals (inorganic substances) are dissolved within and form ions called electrolytes

• Fluid compartments

• Intracellular fluid (ICF)• Water content varies most here due to variation in:

• Tissue types (muscle vs. fat)

• Distinct from ECF due to plasma membrane transport

• Extracellular fluid (ECF)• Interstitial fluid volume varies

• Volume of blood (women < men)

© 2011 Pearson Education, Inc.Figure 24 Section 1 1

Total body composition of adult males

Total body composition of adult males and females

Total body composition of adult females

Intracellularfluid 33%

Interstitialfluid 21.5%

Plasma 4.5%

Solids 40%(organic and inorganic materials)

Other body fluids (≤1%)

Adult males

ICF ECF

Other body fluids (≤1%)

Interstitialfluid 18%

Intracellularfluid 27%

Plasma 4.5%

Solids 50%(organic and inorganic materials)

ICF ECF

Adult females


SOLID COMPONENTS

The solid components of a 70-kg (154-pound)individual with a minimum of body fat

(31.5 kg; 69.3 lbs)

Kg

Proteins Lipids Minerals Carbohydrates Miscellaneous


Module 24.1: Fluid balance

• Fluid balance

• Water content stable over time

• Gains

• Primarily absorption along digestive tract

• As nutrients and ions are absorbed, osmotic gradient created causing passive absorption of water

• Losses

• Mainly through urination (over 50%) but other routes as well

• Digestive secretions are reabsorbed similarly to ingested fluids


Dietary Input Digestive Secretions

Water Reabsorption

Food and drink 2200 mL

The digestive tract sites of water gainthrough ingestion or secretion, or waterreabsorption, and of water loss

Small intestinereabsorbs 8000 mL

Colon reabsorbs 1250 mL

150 mL lostin feces

1400mL

1200 mL

9200 mL

5200 mL

Colonic mucous secretions200 mL

Intestinal secretions 2000 mL

Liver (bile) 1000 mLPancreas (pancreaticjuice) 1000 mL

Gastric secretions 1500 mL

Saliva 1500 mL


Module 24.1: Fluid balance

• ICF and ECF compartments balance

• Very different composition

• Are at osmotic equilibrium

• Loss of water from ECF is replaced by water in ICF

• = Fluid shift• Occurs in minutes to hours and restores osmotic equilibrium

• Dehydration• Results in long-term transfer that cannot replace ECF water

loss

• Homeostatic mechanisms to increase ECF fluid volume will be employed


The major factors that affect ECF volume

ICF ECF

Water absorbed acrossdigestive epithelium

(2000 mL)

Metabolicwater

(300 mL)

Water vapor lost in respiration andevaporation frommoist surfaces(1150 mL)

Water lost in feces (150 mL)

Water secretedby sweat glands(variable)

Water lost in urine(1000 mL)

Plasma membranes


Changes to the ICF and ECF when water losses outpace water gains

Intracellularfluid (ICF)

Extracellularfluid (ECF)

The ECF and ICF are inbalance, with the twosolutions isotonic.

ECF water loss Water loss from ECFreduces volume andmakes this solutionhypertonic with respectto the ICF.

IncreasedECF volume

Decreased ICF volumeAn osmotic water shiftfrom the ICF into theECF restores osmoticequilibrium butreduces the ICF volume.


Module 24.1 Review

a. Identify routes of fluid loss from the body.

b. Describe a fluid shift.

c. Explain dehydration and its effect on the osmotic concentration of plasma.


Module 24.2: Mineral balance

• Mineral balance

• Equilibrium between ion absorption and excretion

• Major ion absorption through intestine and colon

• Major ion excretion by kidneys

• Sweat glands excrete ions and water variably

• Ion reserves mainly in skeleton


Mineral balance, the balance between ion absorption (in the digestive tract) and ion excretion (primarily at the kidneys)

Ion Absorption Ion Excretion

ICF ECF

Ion absorption occurs across theepithelial lining of the small intestineand colon.

Ion reserves (primarilyin the skeleton)

Ion pool in body fluids

Sweat glandsecretions(secondarysite of ion loss)

Kidneys(primary siteof ion loss)


Module 24.2 Review

a. Define mineral balance.

b. Identify the significance of two important body minerals: sodium and calcium.

c. Identify the ions absorbed by active transport.


Module 24.3: Water and sodium balance

• Sodium balance (when sodium gains equal losses)

• Relatively small changes in Na+ are accommodated by changes in ECF volume

• Homeostatic responses involve two parts

1. ADH control of water loss/retention by kidneys and thirst

2. Fluid exchange between ECF and ICF


The mechanisms that regulate sodium balancewhen sodium concentration in the ECF changes

Rising plasmasodium levels

The secretion of ADHrestricts water loss andstimulates thirst, promotingadditional waterconsumption.

Osmoreceptorsin hypothalamus

stimulated

HOMEOSTASISDISTURBED

Increased Na

levels in ECF

If you consume largeamounts of salt withoutadequate fluid, as whenyou eat salty potatochips without taking a drink, the plasma Na

concentration rises temporarily.

ADH Secretion IncreasesRecall of Fluids

Because the ECFosmolarity increases,water shifts out of the ICF, increasing ECFvolume and lowering ECF Na concentrations.

HOMEOSTASISRESTORED

Decreased Na

levels in ECF

HOMEOSTASIS

Normal Na

concentrationin ECF

Start


The mechanisms that regulate sodium balancewhen sodium concentration in the ECF changes

Falling plasmasodium levels

HOMEOSTASIS

Normal Na

concentrationin ECF

Start

HOMEOSTASISRESTORED

Increased Na

levels in ECF


Decreased Na

levels in ECF

ADH SecretionDecreases

Osmoreceptorsin hypothalamus

inhibited

Water loss reducesECF volume,

concentrates ions

As soon as the osmoticconcentration of the ECFdrops by 2 percent ormore, ADH secretiondecreases, so thirst issuppressed and waterlosses at the kidneysincrease.



• Sodium balance (continued)

• Exchange changes in Na+ are accommodated by changes in blood pressure and volume

• Hyponatremia (natrium, sodium)

• Low ECF Na+ concentration (<136 mEq/L)

• Can occur from overhydration or inadequate salt intake

• Hypernatremia

• High ECF Na+ concentration (>145 mEq/L)

• Commonly from dehydration



• Sodium balance (continued)• Exchange changes in Na+ are accommodated by

changes in blood pressure and volume (continued)• Increased blood volume and pressure

• Natriuretic peptides released• Increased Na+ and water loss in urine• Reduced thirst• Inhibition of ADH, aldosterone, epinephrine, and

norepinephrine release

• Decreased blood volume and pressure• Endocrine response

• Increased ADH, aldosterone, RAAS mechanism• Opposite bodily responses to above


Rising bloodpressure and

volume

HOMEOSTASIS

Normal ECFvolume

HOMEOSTASISRESTORED

Falling ECF volume


Rising ECF volume by fluidgain or fluid and Na gain

Combined Effects

Responses to Natriuretic Peptides

Increased bloodvolume andatrial distension

Natriuretic peptidesreleased by cardiacmuscle cells

The mechanisms that regulate water balancewhen ECF volume changes

Increased Na loss in urine

Increased water loss in urine

Reduced thirst

Inhibition of ADH, aldosterone,epinephrine, and norepinephrinerelease

Reduced bloodvolume

Reduced bloodpressure

Start


Falling bloodpressure and

volume

HOMEOSTASIS

Normal ECFvolume

Endocrine Responses

Increased renin secretionand angiotensin IIactivation

Combined Effects

Increased aldosteronereleaseIncreased ADH release

Increased urinary Na retention

Decreased urinary water loss

Increased thirst

Increased water intake

Decreased bloodvolume andblood pressure


Falling ECF volume by fluidloss or fluid and Na loss

HOMEOSTASISRESTORED

Rising ECF volume

Start

The mechanisms that regulate water balancewhen ECF volume changes


Module 24.3 Review

a. What effect does inhibition of osmoreceptors have on ADH secretion and thirst?

b. What effect does aldosterone have on sodium ion concentration in the ECF?

c. Briefly summarize the relationship between sodium ion concentration and the ECF.


CLINICAL MODULE 24.4: Potassium imbalance

• Potassium balance (K+ gain = loss)

• Major gain is through digestive tract absorption

• ~100 mEq (1.9–5.8 g)/day

• Major loss is excretion by kidneys

• Primary ECF potassium regulation by kidneys since intake fairly constant

• Controlled by aldosterone regulating Na+/K+ exchange pumps in DCT and collecting duct of nephron

• Low ECF pH can cause H+ to be substituted for K+

• Potassium is highest in ICF due to Na+/K+ exchange pump

• ~135 mEq/L in ICF vs. ~5 mEq/L in ECF


The major factors involved in potassium balance

Factors Controlling Potassium Balance

Approximately 100mEq (1.9–5.8 g) ofpotassium ions are absorbed by thedigestive tract eachday.

Roughly 98 percent of thepotassiumcontent of thehuman body is inthe ICF, ratherthan the ECF.

The K concentration in theECF is relatively low. The rateof K entry from the ICFthrough leak channels isbalanced by the rate of K

recovery by the Na/K

exchange pump.

When potassiumbalance exists,the rate of urinaryK excretionmatches the rateof digestive tractabsorption.

The potassium ionconcentration in the

ECF is approximately5 mEq/L.

KEY

Absorption

Secretion

Diffusion through leak channels

The potassium ionconcentration of theICF is approximately

135 mEq/L.

Renal K lossesare approximately100 mEq per day


The role of aldosterone-sensitive exchange pumpsin the kidneys in determining the potassiumconcentration in the ECF

The primary mechanism ofpotassium secretion involvesan exchange pump thatejects potassium ions whilereabsorbing sodium ions.

Tubularfluid

Sodium-potassium exchange pump

Aldosterone- sensitive exchange pump

The sodium ions are then pumped outof the cell in exchange for potassiumions in the ECF. This is the same pumpthat ejects sodium ions entering thecytosol through leak channels.

KEY

ECF


Events in the kidneys that affect potassium balance

Under normal conditions, thealdosterone-sensitive pumpsexchange K in the ECF forNa in the tubular fluid. Thenet result is a rise in plasmasodium levels and increasedK loss in the urine.

When the pH falls in the ECFand the concentration of H isrelatively high, the exchangepumps bind H instead of K.This helps to stabilize the pHof the ECF, but at the cost ofrising K levels in the ECF.

Distalconvoluted

tubule

Collectingduct


• Disturbances of potassium balance

• Hypokalemia (kalium, potassium)

• Below 2 mEq/L in plasma

• Can be caused by:

• Diuretics

• Aldosteronism (excessive aldosterone secretion)

• Symptoms

• Muscular weakness, followed by paralysis

• Potentially lethal when affecting heart



• Disturbances of potassium balance (continued)

• Hyperkalemia

• Above 8 mEq/L in plasma

• Can be caused by:

• Chronically low pH

• Kidney failure

• Drugs promoting diuresis by blocking Na+/K+ pumps

• Symptoms

• Muscular spasm including heart arrhythmias



CLINICAL MODULE 24.4 Review

a. Define hypokalemia and hyperkalemia.

b. What organs are primarily responsible for regulating the potassium ion concentration of the ECF?

c. Identify factors that cause potassium excretion.


Section 2: Acid-Base Balance

• Learning Outcomes

• 24.5 Explain the role of buffer systems in maintaining acid-base balance and pH.

• 24.6 Explain the role of buffer systems in regulating the pH of the intracellular fluid and the

extracellular fluid.

• 24.7 Describe the compensatory mechanisms involved in the maintenance of acid-base

balance.

• 24.8 CLINICAL MODULE Describe respiratory acidosis and respiratory alkalosis.



• Acid-base balance (H+ production = loss)

• Normal plasma pH: 7.35–7.45

• H+ gains: many metabolic activities produce acids

• CO2 (to carbonic acid) from aerobic respiration

• Lactic acid from glycolysis

• H+ losses and storage

• Respiratory system eliminates CO2

• H+ excretion from kidneys

• Buffers temporarily store H+


The major factors involved in the maintenanceof acid-base balance

Active tissuescontinuously generatecarbon dioxide, which insolution forms carbonicacid. Additional acids,such as lactic acid, areproduced in the course ofnormal metabolicoperations.

Tissue cells

Buffer Systems

Normalplasma pH(7.35–7.45)

Buffer systems cantemporarily store H

and thereby provideshort-term pHstability.

The respiratory systemplays a key role byeliminatingcarbon dioxide.

The kidneys play a majorrole by secretinghydrogen ions into the urine and generatingbuffers that enter thebloodstream. The rate ofexcretion rises and fallsas needed to maintainnormal plasma pH. As a result, the normal pH ofurine varies widely butaverages 6.0—slightlyacidic.



• Classes of acids

• Fixed acids

• Do not leave solution• Remain in body fluids until kidney excretion

• Examples: sulfuric and phosphoric acid• Generated during catabolism of amino acids, phospholipids,

and nucleic acids

• Organic acids

• Part of cellular metabolism• Examples: lactic acid and ketones

• Most metabolized rapidly so no accumulation



• Classes of acids (continued)

• Volatile acids

• Can leave body by external respiration

• Example: carbonic acid (H2CO3)


Module 24.5: Buffer systems

• pH imbalance

• ECH pH normally between 7.35 and 7.45

• Acidemia (plasma pH <7.35): acidosis (physiological state)

• More common due to acid-producing metabolic activities

• Effects

• CNS function deteriorates, may cause coma

• Cardiac contractions grow weak and irregular

• Peripheral vasodilation causes BP drop

• Alkalemia (plasma pH >7.45): alkalosis (physiological state)

• Can be dangerous but relatively rare


The narrow range of normal pH of the ECF, and the conditions that result from pH shifts outside the normal range

The pH of the ECF(extracellular fluid)normally ranges from7.35 to 7.45.

pH

When the pH of plasma falls below7.5, acidemia exists. Thephysiological state that results iscalled acidosis.

When the pH of plasma risesabove 7.45, alkalemia exists.The physiological state thatresults is called alkalosis.

Severe acidosis (pH below 7.0) can be deadlybecause (1) central nervous system functiondeteriorates, and the individual may becomecomatose; (2) cardiac contractions grow weak andirregular, and signs and symptoms of heart failuremay develop; and (3) peripheral vasodilationproduces a dramatic drop in blood pressure,potentially producing circulatory collapse.

Severe alkalosis is alsodangerous, but serious casesare relatively rare.

Extremelyacidic

Extremelybasic



• CO2 partial pressure effects on pH

• Most important factor affecting body pH

• H2O + CO2 H2CO3 H+ + HCO3–

• Reversible reaction that can buffer body pH

• Adjustments in respiratory rate can affect body pH


When carbon dioxide levels rise, more carbonic acidforms, additional hydrogen ions and bicarbonate ionsare released, and the pH goes down.

When the PCO2 falls, the reaction runs in reverse, and

carbonic acid dissociates into carbon dioxide andwater. This removes H ions from solution andincreases the pH.

If PCO2 rises If PCO2

falls

PCO2

40–45mm Hg

pH7.35–7.45

The inverse relationship between the PCO2 and pH

HOMEOSTASIS

H2O CO2 H2CO3 H HCO3 H HCO3

H2CO3 H2O CO2

PCO2

PCO2pH

pH



• Buffer

• Substance that opposes changes to pH by removing or adding H+

• Generally consists of:

• Weak acid (HY)

• Anion released by its dissociation (Y–)

• HY H+ + Y– and H+ + Y– HY


The reactions that occur when pH buffer systems function

HY H YH Y

H

H HYH HY

H

H Y

A buffer system in body fluids generallyconsists of a combination of a weak acid (HY)and the anion (Y) released by its dissociation.The anion functions as a weak base. In solution,molecules of the weak acid exist in equilibriumwith its dissociation products.

Adding H to thesolution upsets the equilibrium and resultsin the formation ofadditional molecules ofthe weak acid.

Removing H from thesolution also upsets theequilibrium and results in the dissociation ofadditional molecules ofHY. This releases H.


Module 24.5 Review

a. Define acidemia and alkalemia.

b. What is the most important factor affecting the pH of the ECF?

c. Summarize the relationship between CO2 levels and pH.


Module 24.6: Major body buffer systems

• Three major body buffer systems

• All can only temporarily affect pH (H+ not eliminated)

1. Phosphate buffer system

• Buffers pH of ICF and urine

2. Carbonic acid–bicarbonate buffer system

• Most important in ECF

• Fully reversible

• Bicarbonate reserves (from NaHCO3 in ECF) contribute



• Three major body buffer systems (continued)

3. Protein buffer systems (in ICF and ECF)

• Usually operate under acid conditions (bind H+)• Binding to carboxyl group (COOH–) and amino group

(—NH2)

• Examples:• Hemoglobin buffer system

• CO2 + H2O H2CO3 HCO3– + Hb-H+

• Only intracellular system with immediate effects

• Amino acid buffers (all proteins)

• Plasma proteins


The body’s three major buffer systems

Buffer Systems

Intracellular fluid (ICF) Extracellular fluid (ECF)

occur in

Phosphate BufferSystem

Protein Buffer Systems Carbonic Acid–Bicarbonate Buffer System

Has an importantrole in buffering thepH of the ICF andof urine

Contribute to the regulation of pH in the ECF and ICF;interact extensively with the other two buffer systems

Is most important in theECF

Hemoglobinbuffer system(RBCs only)

Amino acidbuffers

(All proteins)

Plasmaproteinbuffers


The reactions of the carbonic acid–bicarbonate buffer system

CARBONIC ACID–BICARBONATEBUFFER SYSTEM

BICARBONATE RESERVE

Start

CO2 CO2 H2O H2CO3

(carbonic acid)H HCO3

(bicarbonate ion)NaHCO3

(sodium bicarbonate)HCO3

Na

Body fluids contain a large reserve ofHCO3

, primarily in the form of dissolvedmolecules of the weak base sodiumbicarbonate (NaHCO3). This readilyavailable supply of HCO3

is known asthe bicarbonate reserve.

Addition of H

from metabolicactivity

The primary function of the carbonicacid–bicarbonate buffer system is toprotect against the effects of the organicand fixed acids generated throughmetabolic activity. In effect, it takes the H released by these acids and generatescarbonic acid that dissociates into waterand carbon dioxide, which can easily be eliminated at the lungs.

Lungs


The events involved in the functioning of the hemoglobin buffer system

Tissuecells

Plasma Plasma Lungs

Red blood cells Red blood cells Releasedwith

exhalation

CO2

H2O

H2CO3 HCO3 Hb H H HCO3

Hb H2CO3

H2O

CO2


The mechanism by free amino acids function inprotein buffer systemsStart

Normal pH(7.35–7.45)

Increasing acidity (decreasing pH)

At the normal pH ofbody fluids (7.35–7.45), the carboxylgroups of most aminoacids have releasedtheir hydrogen ions.

If pH drops, the carboxylate ion (COO)and the amino group (—NH2) of a freeamino acid can act as weak bases andaccept additional hydrogen ions, forming acarboxyl group (—COOH) and an aminoion (—NH3

), respectively. Many of theR-groups can also accept hydrogen ions,forming RH.



• Disorders

• Metabolic acid-base disorders

• Production or loss of excessive amounts of fixed or organic acids

• Carbonic acid–bicarbonate system works to counter

• Respiratory acid-base disorders

• Imbalance of CO2 generation and elimination

• Must be corrected by depth and rate of respiration changes


Module 24.6 Review

a. Identify the body’s three major buffer systems.

b. Describe the carbonic acid–bicarbonate buffer system.

c. Describe the roles of the phosphate buffer system.


Module 24.7: Metabolic acid-base disorders

• Metabolic acid-base disorders• Metabolic acidosis

• Develops when large numbers of H+ are released by organic or fixed acids

• Accommodated by respiratory and renal responses• Respiratory response

• Increased respiratory rate lowers PCO2

• H+ + HCO3– H2CO3 H2O + CO2

• Renal response• Occurs in PCT, DCT, and collecting system

• H2O + CO2 H2CO3 H+ + HCO3–

H+ secreted into urine

HCO3– reabsorbed into ECF


The responses to metabolic acidosis Additionof H

Start

CO2 CO2 H2O H2CO3


Lungs(bicarbonate ion)

HCO3 Na NaHCO3

(sodium bicarbonate)

Generationof HCO3

CARBONIC ACID–BICARBONATE BUFFER SYSTEM BICARBONATE RESERVE

Respiratory Responseto Acidosis

Renal Response to Acidosis

Otherbuffer

systemsabsorb H

KIDNEYS

Secretionof H

Increased respiratoryrate lowers PCO2

,

effectively convertingcarbonic acid moleculesto water.

Kidney tubules respond by (1) secreting H

ions, (2) removing CO2, and (3) reabsorbingHCO3

to help replenish the bicarbonatereserve.


The activity of renaltubule cells in CO2

removal and HCO3

production

Tubularfluid

Renal tubule cells ECF

H

H

H

H

Na

Na

CO2 CO2

HCO3

HCO3

H2CO3

HCO3

CO2

H2O

Cl

Cl

Carbonicanhydrase

CO2 generated by the tubulecell is added to the CO2

diffusing into the cell fromthe urine and from the ECF.

Steps in CO2 removal andHCO3

production

Carbonic anhydraseconverts CO2 and water tocarbonic acid, which then dissociates.

The chloride ions exchangedfor bicarbonate ions areexcreted in the tubular fluid.

Bicarbonate ions andsodium ions are transportedinto the ECF, adding to thebicarbonate reserve.



• Metabolic alkalosis

• Develops when large numbers of H+ are removed from body fluids

• Rate of kidney H+ secretion declines

• Tubular cells do not reclaim bicarbonate

• Collecting system transports bicarbonate into urine and retains acid (HCl) in ECF



• Metabolic alkalosis (continued)

• Accommodated by respiratory and renal responses

• Respiratory response

• Decreased respiratory rate raises PCO2

• H2O + CO2 H2CO3 H+ + HCO3–

• Renal response• Occurs in PCT, DCT, and collecting system

• H2O + CO2 H2CO3 H+ + HCO3–

• HCO3– secreted into urine (in exchange for Cl–)

• H+ actively reabsorbed into ECF


The responses to metabolic alkalosisStart

Lungs

Removalof H

CO2 H2O H HCO3H2CO3

(carbonic acid)

HCO3 Na NaHCO3

(sodium bicarbonate)(bicarbonate ion)

CARBONIC ACID–BICARBONATE BUFFER SYSTEM BICARBONATE RESERVE

Generationof H KIDNEYS

Secretionof HCO3

Otherbuffer

systemsrelease H

Respiratory Responseto Alkalosis

Renal Response to AlkalosisDecreased respiratoryrate elevates PCO2

,

effectively convertingCO2 molecules tocarbonic acid.

Kidney tubules respond byconserving H ions and secreting HCO3

.


The events in thesecretion of bicarbonateions into the tubularfluid along the PCT, DCT,and collecting system

Tubularfluid

Renal tubule cells ECF

H2CO3

CO2

H2O

Carbonicanhydrase

H

CO2

HCO3 HHCO3

CO2

Cl Cl

CO2 generated by the tubulecell is added to the CO2

diffusing into the cell from thetubular fluid and from the ECF.

Carbonic anyhydrase convertsCO2 and water to carbonic acid, which then dissociates.

The hydrogen ions are activelytransported into the ECF,accompanied by the diffusionof chloride ions.

HCO3 is pumped into the

tubular fluid in exchange forchloride ions that will diffuseinto the ECF.


Module 24.7 Review

a. Describe metabolic acidosis.

b. Describe metabolic alkalosis.

c. lf the kidneys are conserving HCO3– and

eliminating H+ in acidic urine, which is occurring: metabolic alkalosis or metabolic acidosis?


CLINICAL MODULE 24.8: Respiratory acid-base disorders

• Respiratory acid-base disorders

• Respiratory acidosis

• CO2 generation outpaces rate of CO2 elimination at lungs

• Shifts bicarbonate buffer system toward generating more carbonic acid

• H2O + CO2 H2CO3 H+ + HCO3–

• HCO3– goes into bicarbonate reserve

• H+ must be neutralized by any of the buffer systems

• Respiratory (increased respiratory rate)

• Renal (H+ secreted and HCO3– reabsorbed)

• Proteins (bind free H+)


The events in respiratory acidosis

CARBONIC ACID–BICARBONATEBUFFER SYSTEM BICARBONATE RESERVE

Lungs

CO2 CO2 H2O H2CO2


(bicarbonate ion)HCO3

Na NaHCO3

(sodium bicarbonate)

When respiratory activity does not keeppace with the rate of CO2 generation,alveolar and plasma PCO2

increases.

This upsets the equilibrium and drivesthe reaction to the right, generatingadditional H2CO3, which releases H

and lowers plasma pH.

As bicarbonate ions and hydrogen ionsare released through the dissociation ofcarbonic acid, the excess bicarbonateions become part of the bicarbonatereserve.

To limit the pH effects ofrespiratory acidosis, the excess H must either be tied up byother buffer systems or excreted at the kidneys. The underlyingproblem, however, cannot beeliminated without an increase inthe respiratory rate.


The integrated homeostatic responsesto respiratory acidosis

IncreasedPCO2

Elevated PCO2 results

in a fall in plasma pH

Respiratory Acidosis

Responses to Acidosis

Combined Effects

Respiratory compensation

Renal compensation

Decreased PCO2

Decreased H andincreased HCO3

Stimulation of arterial and CSFchemoreceptors results inincreased respiratory rate.

H ions are secreted andHCO3

ions are generated.

Buffer systems other than thecarbonic acid–bicarbonatesystem accept H ions.


HOMEOSTASISRESTORED

Hypoventilationcausing increased PCO2

Plasma pHreturns to normalStart

Normal acid-base balance

HOMEOSTASIS


CLINICAL MODULE 24.8: Respiratory acid-base disorders

• Respiratory alkalosis

• CO2 elimination at lungs outpaces CO2 generation rate

• Shifts bicarbonate buffer system toward generating more carbonic acid

• H+ + HCO3– H2CO3 H2O + CO2

• H+ removed as CO2 exhaled and water formed

• Buffer system responses

• Respiratory (decreased respiratory rate)

• Renal (HCO3– secreted and H+ reabsorbed)

• Proteins (release free H+)


The events in respiratory alkalosis

If respiratory activity exceeds the rate of CO2 generation, alveolar and plasma PCO2

decline,

and this disturbs the equilibrium and drivesthe reactions to the left, removing H and elevating plasma pH.

CO2 CO2 H2O H2CO2


(bicarbonate ion)HCO3

Na NaHCO3

(sodium bicarbonate)Lungs

CARBONIC ACID–BICARBONATEBUFFER SYSTEM BICARBONATE RESERVE

As bicarbonate ions and hydrogenions are removed in the formation ofcarbonic acid, the bicarbonate ions—but not the hydrogen ions—arereplaced by the bicarbonate reserve.


The integrated homeostatic responses torespiratory alkalosis

StartNormal acid-base balance

HOMEOSTASIS

DecreasedPCO2

Lower PCO2 results

in a rise in plasma pH

Respiratory Alkalosis


Hyperventilationcausing decreased PCO2

Plasma pHreturns to normal

HOMEOSTASISRESTORED

Increased PCO2

Combined Effects

Increased H anddecreased HCO3

Responses to Alkalosis

Respiratory compensation

Renal compensation

Inhibition of arterial and CSFchemoreceptors results in adecreased respiratory rate.

H ions are generated and HCO3

ions are secreted.

Buffer systems other than thecarbonic acid–bicarbonate systemrelease H ions.


CLINICAL MODULE 24.8 Review

a. Define respiratory acidosis and respiratory alkalosis.

b. What would happen to the plasma PCO2 of a patient who has an airway obstruction?

c. How would a decrease in the pH of body fluids affect the respiratory rate?

24 fluid, electrolyte, and acid-base balance

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