© 2011 Pearson Education, Inc.
PowerPoint® Lecture Presentations prepared byAlexander G. CheroskeMesa Community College at Red Mountain
24Fluid, Electrolyte, and Acid-Base Balance
© 2011 Pearson Education, Inc.
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.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.Figure 24 Section 1 2
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.Figure 24.1 1
© 2011 Pearson Education, Inc.Figure 24.1 2
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.Figure 24.1 3
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
© 2011 Pearson Education, Inc.Figure 24.1 4
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.
© 2011 Pearson Education, Inc.
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.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.Figure 24.2 1
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)
© 2011 Pearson Education, Inc.Figure 24.2 2
© 2011 Pearson Education, Inc.Figure 24.2 3
© 2011 Pearson Education, Inc.Figure 24.2 3
© 2011 Pearson Education, Inc.
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.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.Figure 24.3 1
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
© 2011 Pearson Education, Inc.Figure 24.3 1
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
HOMEOSTASISDISTURBED
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.
© 2011 Pearson Education, Inc.
Module 24.3: Water and sodium balance
• 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
© 2011 Pearson Education, Inc.
Module 24.3: Water and sodium balance
• 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
© 2011 Pearson Education, Inc.Figure 24.3 2
Rising bloodpressure and
volume
HOMEOSTASIS
Normal ECFvolume
HOMEOSTASISRESTORED
Falling ECF volume
HOMEOSTASISDISTURBED
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
© 2011 Pearson Education, Inc.Figure 24.3 2
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
HOMEOSTASISDISTURBED
Falling ECF volume by fluidloss or fluid and Na loss
HOMEOSTASISRESTORED
Rising ECF volume
Start
The mechanisms that regulate water balancewhen ECF volume changes
© 2011 Pearson Education, Inc.
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.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.Figure 24.4 1
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
© 2011 Pearson Education, Inc.Figure 24.4 2
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
© 2011 Pearson Education, Inc.Figure 24.4 3
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
© 2011 Pearson Education, Inc.
• 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
CLINICAL MODULE 24.4: Potassium imbalance
© 2011 Pearson Education, Inc.
• 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: Potassium imbalance
© 2011 Pearson Education, Inc.
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.
© 2011 Pearson Education, Inc.
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.
© 2011 Pearson Education, Inc.
Section 2: Acid-Base Balance
• 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+
© 2011 Pearson Education, Inc.Figure 24 Section 2 1
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.
© 2011 Pearson Education, Inc.
Section 2: Acid-Base Balance
• 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
© 2011 Pearson Education, Inc.
Section 2: Acid-Base Balance
• Classes of acids (continued)
• Volatile acids
• Can leave body by external respiration
• Example: carbonic acid (H2CO3)
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.Figure 24.5 1
© 2011 Pearson Education, Inc.Figure 24.5 2
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
© 2011 Pearson Education, Inc.
Module 24.5: Buffer systems
• 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
© 2011 Pearson Education, Inc.Figure 24.5 3
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
© 2011 Pearson Education, Inc.
Module 24.5: Buffer systems
• 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
© 2011 Pearson Education, Inc.Figure 24.5 4
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.
© 2011 Pearson Education, Inc.
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.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Module 24.6: Major body buffer systems
• 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
© 2011 Pearson Education, Inc.Figure 24.6 1
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
© 2011 Pearson Education, Inc.Figure 24.6 4
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
© 2011 Pearson Education, Inc.Figure 24.6 2
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
© 2011 Pearson Education, Inc.Figure 24.6 3
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.
© 2011 Pearson Education, Inc.
Module 24.6: Major body buffer systems
• 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
© 2011 Pearson Education, Inc.
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.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.Figure 24.7 1
The responses to metabolic acidosis Additionof H
Start
CO2 CO2 H2O H2CO3
(carbonic acid)H HCO3
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.
© 2011 Pearson Education, Inc.Figure 24.7 2
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.
© 2011 Pearson Education, Inc.
Module 24.7: Metabolic acid-base disorders
• 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
© 2011 Pearson Education, Inc.
Module 24.7: Metabolic acid-base disorders
• 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
© 2011 Pearson Education, Inc.Figure 24.7 3
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
.
© 2011 Pearson Education, Inc.Figure 24.7 4
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.
© 2011 Pearson Education, Inc.
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?
© 2011 Pearson Education, Inc.
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+)
© 2011 Pearson Education, Inc.Figure 24.8 1
The events in respiratory acidosis
CARBONIC ACID–BICARBONATEBUFFER SYSTEM BICARBONATE RESERVE
Lungs
CO2 CO2 H2O H2CO2
(carbonic acid)H HCO3
(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.
© 2011 Pearson Education, Inc.Figure 24.8 2
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.
HOMEOSTASISDISTURBED
HOMEOSTASISRESTORED
Hypoventilationcausing increased PCO2
Plasma pHreturns to normalStart
Normal acid-base balance
HOMEOSTASIS
© 2011 Pearson Education, Inc.
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+)
© 2011 Pearson Education, Inc.Figure 24.8 3
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
(carbonic acid)H HCO3
(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.
© 2011 Pearson Education, Inc.Figure 24.8 4
The integrated homeostatic responses torespiratory alkalosis
StartNormal acid-base balance
HOMEOSTASIS
DecreasedPCO2
Lower PCO2 results
in a rise in plasma pH
Respiratory Alkalosis
HOMEOSTASISDISTURBED
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.
© 2011 Pearson Education, Inc.
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?
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