13_Fluid_and_Electrolyte_Balance محاضرة.pptx

© 2011 Pearson Education, Inc.
PowerPoint® Lecture Presentations prepared by
Alexander G. Cheroske
Mesa Community College at Red Mountain
PowerPoint® Lecture Presentations prepared by
Alexander G. Cheroske
Mesa Community College at Red Mountain
24
Fluid,
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 ‫تنقسم‬
‫السوائل‬
• Intracellular fluid (ICF)
• Water content varies most here due to variation in:
‫تختلف‬
‫معظم‬
‫محتويات‬
‫الماء‬
‫هنا‬
‫نتيجة‬
‫الي‬
‫اختالف‬
‫في‬ :-
• Tissue types (muscle vs. fat)
• Distinct from ECF due to plasma membrane transport
• Extracellular fluid (ECF)
• Interstitial fluid volume varies

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
Intracellular
fluid 33%
Interstitial
fluid 21.5%
Plasma 4.5%
Solids 40%
(organic and inorganic materials)
Other
body
fluids
(≤1%)
Adult males
ICF ECF
Other
body
fluids
(≤1%)
Interstitial
fluid 18%
Intracellular
fluid 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

Figure 24.1 1

Figure 24.1 2
Dietary Input Digestive Secretions
Water Reabsorption
Food and drink 2200 mL
The digestive tract sites of water gain
through ingestion or secretion, or water
reabsorption, and of water loss
Small intestine
reabsorbs 8000 mL
Colon reabsorbs 1250 mL
150 mL lost
in feces
1400
mL
1200 mL
9200 mL
5200 mL
Colonic mucous secretions
200 mL
Intestinal secretions 2000 mL
Liver (bile) 1000 mL
Pancreas (pancreatic
juice) 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 ‫ازاحة‬
‫السوائل‬
• Occurs in minutes to hours and restores osmotic equilibrium
• ‫اعادة‬
‫معادلة‬
‫الضغط‬
‫االسموزي‬
• Dehydration
• Results in long-term transfer that cannot replace ECF water
loss
• Homeostatic mechanisms to increase ECF fluid volume will
be employed

Figure 24.1 3
The major factors that affect ECF volume
ICF ECF
Water absorbed across
digestive epithelium
(2000 mL)
Metabolic
water
(300 mL)
Water vapor lost
in respiration and
evaporation from
moist surfaces
(1150 mL)
Water lost in
feces (150 mL)
Water secreted
by sweat glands
(variable)
Water lost in urine
(1000 mL)
Plasma membranes

Figure 24.1 4
Changes to the ICF and ECF when water losses outpace water gains
Intracellular
fluid (ICF)
Extracellular
fluid (ECF)
The ECF and ICF are in
balance, with the two
solutions isotonic.
ECF water loss Water loss from ECF
reduces volume and
makes this solution
hypertonic with respect
to the ICF.
Increased
ECF volume
Decreased ICF volume
An osmotic water shift
from the ICF into the
ECF restores osmotic
equilibrium but
reduces 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 ‫توازن‬
‫السوائم‬
‫المعدنيه‬
• Equilibrium ‫التوازن‬between ion absorption and
excretion
• Major ion aabsorption through intestine and ccolon
• ‫االمتصاص‬
‫االساسي‬
‫لاليونات‬
‫خالل‬
‫االمعاء‬
‫والقيولون‬
• Major ion excretion by kidneys
• Sweat glands excrete ions and water variably ‫مختلفه‬
• Ion reserves mainly in skeleton

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 the
epithelial lining of the small intestine
and colon.
Ion reserves (primarily
in the skeleton)
Ion pool in body fluids
Sweat gland
secretions
(secondary
site of ion loss)
Kidneys
(primary site
of ion loss)

Figure 24.2 2

Figure 24.2 3

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
• ADH control of water loss/retention by kidneys and thirst
• Fluid exchange between ECF and ICF

Figure 24.3 1
The mechanisms that regulate sodium balance
when sodium concentration in the ECF changes
Rising plasma
sodium levels
The secretion of ADH
restricts water loss and
stimulates thirst, promoting
additional water
consumption.
Osmoreceptors
in hypothalamus
stimulated
HOMEOSTASIS
DISTURBED
Increased Na
levels in ECF
If you consume large
amounts of salt without
adequate fluid, as when
you eat salty potato
chips without taking a
drink, the plasma Na
concentration rises
temporarily.
ADH Secretion Increases
Recall of Fluids
Because the ECF
osmolarity increases,
water shifts out of the
ICF, increasing ECF
volume and lowering
ECF Na concentrations.
HOMEOSTASIS
RESTORED
Decreased Na
levels in ECF
HOMEOSTASIS
Normal Na
concentration
in ECF
Start

Figure 24.3 1
The mechanisms that regulate sodium balance
when sodium concentration in the ECF changes
Falling plasma
sodium levels
HOMEOSTASIS
Normal Na
concentration
in ECF
Start
HOMEOSTASIS
RESTORED
Increased Na
levels in ECF
HOMEOSTASIS
DISTURBED
Decreased Na
levels in ECF
ADH Secretion
Decreases
Osmoreceptors
in hypothalamus
inhibited
Water loss reduces
ECF volume,
concentrates ions
As soon as the osmotic
concentration of the ECF
drops by 2 percent or
more, ADH secretion
decreases, so thirst is
suppressed and water
losses at the kidneys
increase.

• 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

Figure 24.3 2
Rising blood
pressure and
volume
HOMEOSTASIS
Normal ECF
volume
HOMEOSTASIS
RESTORED
Falling ECF
volume
HOMEOSTASIS
DISTURBED
Rising ECF volume by fluid
gain or fluid and Na gain
Combined
Effects
Responses to Natriuretic Peptides
Increased blood
volume and
atrial distension
Natriuretic peptides
released by cardiac
muscle cells
The mechanisms that regulate water balance
when ECF volume changes
Increased Na loss in urine
Increased water loss in urine
Reduced thirst
Inhibition of ADH, aldosterone,
epinephrine, and norepinephrine
release
Reduced
blood
volume
Reduced
blood
pressure
Start

Figure 24.3 2
Falling blood
pressure and
volume
HOMEOSTASIS
Normal ECF
volume
Endocrine Responses
Increased renin secretion
and angiotensin II
activation
Combined Effects
Increased aldosterone
release
Increased ADH release
Increased urinary Na retention
Decreased urinary water loss
Increased thirst
Increased water intake
Decreased blood
volume and
blood pressure
HOMEOSTASIS
DISTURBED
Falling ECF volume by fluid
loss or fluid and Na loss
HOMEOSTASIS
RESTORED
Rising ECF
volume
Start
The mechanisms that regulate water balance
when 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

Figure 24.4 1
The major factors involved in potassium balance
Factors Controlling Potassium Balance
Approximately 100
mEq (1.9–5.8 g) of
potassium ions are
absorbed by the
digestive tract each
day.
Roughly
98 percent of the
potassium
content of the
human body is in
the ICF, rather
than the ECF.
The K concentration in the
ECF is relatively low. The rate
of K entry from the ICF
through leak channels is
balanced by the rate of K
recovery by the Na /K
exchange pump.
When potassium
balance exists,
the rate of urinary
K excretion
matches the rate
of digestive tract
absorption.
The potassium ion
concentration in the
ECF is approximately
5 mEq/L.
KEY
Absorption
Secretion
Diffusion through
leak channels
The potassium ion
concentration of the
ICF is approximately
135 mEq/L.
Renal K losses
are approximately
100 mEq per day

Figure 24.4 2
The role of aldosterone-sensitive exchange pumps
in the kidneys in determining the potassium
concentration in the ECF
The primary mechanism of
potassium secretion involves
an exchange pump that
ejects potassium ions while
reabsorbing sodium ions.
Tubular
fluid
Sodium-potassium
exchange pump
Aldosterone-
sensitive
exchange
pump
The sodium ions are then pumped out
of the cell in exchange for potassium
ions in the ECF. This is the same pump
that ejects sodium ions entering the
cytosol through leak channels.
KEY
ECF

Figure 24.4 3
Events in the kidneys that affect potassium balance
Under normal conditions, the
aldosterone-sensitive pumps
exchange K in the ECF for
Na in the tubular fluid. The
net result is a rise in plasma
sodium levels and increased
K loss in the urine.
When the pH falls in the ECF
and the concentration of H is
relatively high, the exchange
pumps bind H instead of K .
This helps to stabilize the pH
of the ECF, but at the cost of
rising K levels in the ECF.
Distal
convoluted
tubule
Collecting
duct

• 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
imbalance

• 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
imbalance

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 maintenance
of acid-base balance
Active tissues
continuously generate
carbon dioxide, which in
solution forms carbonic
acid. Additional acids,
such as lactic acid, are
produced in the course of
normal metabolic
operations.
Tissue cells
Buffer Systems
Normal
plasma pH
(7.35–7.45)
Buffer systems can
temporarily store H
and thereby provide
short-term pH
stability.
The respiratory system
plays a key role by
eliminating
carbon dioxide.
The kidneys play a major
role by secreting
hydrogen ions into the
urine and generating
buffers that enter the
bloodstream. The rate of
excretion rises and falls
as needed to maintain
normal plasma pH. As a
result, the normal pH of
urine varies widely but
averages 6.0—slightly
acidic.

• 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

Figure 24.5 1

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 from
7.35 to 7.45.
pH
When the pH of plasma falls below
7.5, acidemia exists. The
physiological state that results is
called acidosis.
When the pH of plasma rises
above 7.45, alkalemia exists.
The physiological state that
results is called alkalosis.
Severe acidosis (pH below 7.0) can be deadly
because (1) central nervous system function
deteriorates, and the individual may become
comatose; (2) cardiac contractions grow weak and
irregular, and signs and symptoms of heart failure
may develop; and (3) peripheral vasodilation
produces a dramatic drop in blood pressure,
potentially producing circulatory collapse.
Severe alkalosis is also
dangerous, but serious cases
are relatively rare.
Extremely
acidic
Extremely
basic

• 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

Figure 24.5 3
When carbon dioxide levels rise, more carbonic acid
forms, additional hydrogen ions and bicarbonate ions
are released, and the pH goes down.
When the PCO2 falls, the reaction runs in reverse, and
carbonic acid dissociates into carbon dioxide and
water. This removes H ions from solution and
increases the pH.
If PCO2 rises If PCO2 falls
PCO2
40–45
mm Hg
pH
7.35–7.45
The inverse relationship between the PCO2 and pH
HOMEOSTASIS
H2O CO2 H2CO3 H HCO3 H HCO3 H2CO3 H2O CO2

• 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

Figure 24.5 4
The reactions that occur when pH buffer systems function
HY H Y H Y
H
H HY
H HY
H
H Y
A buffer system in body fluids generally
consists 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 equilibrium
with its dissociation products.
Adding H to the
solution upsets the
equilibrium and results
in the formation of
additional molecules of
the weak acid.
Removing H from the
solution also upsets the
equilibrium and results
in the dissociation of
additional molecules of
HY. 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)
• Phosphate buffer system
• Buffers pH of ICF and urine
• Carbonic acid–bicarbonate buffer system
• Most important in ECF
• Fully reversible
• Bicarbonate reserves (from NaHCO3 in ECF) contribute

• Three major body buffer systems (continued)
• 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

Figure 24.6 1
The body’s three major buffer systems
Buffer Systems
Intracellular fluid (ICF) Extracellular fluid (ECF)
occur in
Phosphate Buffer
System
Protein Buffer Systems Carbonic Acid–
Bicarbonate Buffer
System
Has an important
role in buffering the
pH of the ICF and
of urine
Contribute to the regulation of pH in the ECF and ICF;
interact extensively with the other two buffer systems
Is most important in the
ECF
Hemoglobin
buffer system
(RBCs only)
Amino acid
buffers
(All proteins)
Plasma
protein
buffers

Figure 24.6 4
The reactions of the carbonic acid–bicarbonate buffer system
CARBONIC ACID–BICARBONATE
BUFFER 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 of
HCO3 , primarily in the form of dissolved
molecules of the weak base sodium
bicarbonate (NaHCO3). This readily
available supply of HCO3 is known as
the bicarbonate reserve.
Addition of H
from metabolic
activity
The primary function of the carbonic
acid–bicarbonate buffer system is to
protect against the effects of the organic
and fixed acids generated through
metabolic activity. In effect, it takes the H
released by these acids and generates
carbonic acid that dissociates into water
and carbon dioxide, which can easily be
eliminated at the lungs.
Lungs

Figure 24.6 2
The events involved in the functioning of the hemoglobin buffer system
Tissue
cells
Plasma Plasma Lungs
Red blood cells Red blood cells Released
with
exhalation
CO2
H2O
H2CO3 HCO3 Hb H H HCO3
Hb H2CO3
H2O
CO2

Figure 24.6 3
The mechanism by free amino acids function in
protein buffer systems
Start
Normal pH
(7.35–7.45)
Increasing acidity (decreasing pH)
At the normal pH of
body fluids (7.35–
7.45), the carboxyl
groups of most amino
acids have released
their hydrogen ions.
If pH drops, the carboxylate ion (COO )
and the amino group (—NH2) of a free
amino acid can act as weak bases and
accept additional hydrogen ions, forming a
carboxyl group (—COOH) and an amino
ion (—NH3 ), respectively. Many of the
R-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

Figure 24.7 1
The responses to metabolic acidosis Addition
of H
Start
CO2 CO2 H2O H2CO3
(carbonic acid)
H HCO3
Lungs
(bicarbonate ion)
HCO3 Na NaHCO3
Generation
of HCO3
CARBONIC ACID–BICARBONATE BUFFER SYSTEM BICARBONATE RESERVE
Respiratory Response
to Acidosis
Renal Response to Acidosis
Other
buffer
systems
absorb H
KIDNEYS
Secretion
of H
Increased respiratory
rate lowers PCO2,
effectively converting
carbonic acid molecules
to water.
Kidney tubules respond by (1) secreting H
ions, (2) removing CO2, and (3) reabsorbing
HCO3 to help replenish the bicarbonate
reserve.

Figure 24.7 2
The activity of renal
tubule cells in CO2
removal and HCO3
production
Tubular
fluid
Renal tubule cells ECF
H
H
H
H
Na
Na
CO2 CO2
HCO3
HCO3
H2CO3
HCO3
CO2
H2O
Cl
Cl
Carbonic
anhydrase
CO2 generated by the tubule
cell is added to the CO2
diffusing into the cell from
the urine and from the ECF.
Steps in CO2 removal and
HCO3 production
Carbonic anhydrase
converts CO2 and water to
carbonic acid, which then
dissociates.
The chloride ions exchanged
for bicarbonate ions are
excreted in the tubular fluid.
Bicarbonate ions and
sodium ions are transported
into the ECF, adding to the
bicarbonate 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

Figure 24.7 3
The responses to metabolic alkalosis
Start
Lungs
Removal
of H
CO2 H2O H HCO3
H2CO3
(carbonic acid)
HCO3 Na NaHCO3
(bicarbonate ion)
CARBONIC ACID–BICARBONATE BUFFER SYSTEM BICARBONATE RESERVE
Generation
of H
KIDNEYS
Secretion
of HCO3
Other
buffer
systems
release H
Respiratory Response
to Alkalosis
Renal Response to Alkalosis
Decreased respiratory
rate elevates PCO2,
effectively converting
CO2 molecules to
carbonic acid.
Kidney tubules respond by
conserving H ions and
secreting HCO3 .

Figure 24.7 4
The events in the
secretion of bicarbonate
ions into the tubular
fluid along the PCT, DCT,
and collecting system
Tubular
fluid
Renal tubule cells ECF
H2CO3
CO2
H2O
Carbonic
anhydrase
H
CO2
HCO3 H
HCO3
CO2
Cl Cl
CO2 generated by the tubule
cell is added to the CO2
diffusing into the cell from the
tubular fluid and from the ECF.
Carbonic anyhydrase converts
CO2 and water to carbonic
acid, which then dissociates.
The hydrogen ions are actively
transported into the ECF,
accompanied by the diffusion
of chloride ions.
HCO3 is pumped into the
tubular fluid in exchange for
chloride ions that will diffuse
into 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+)

Figure 24.8 1
The events in respiratory acidosis
BUFFER SYSTEM BICARBONATE RESERVE
Lungs
CO2 CO2 H2O H2CO2
(carbonic acid)
H HCO3
(bicarbonate ion)
HCO3 Na NaHCO3
When respiratory activity does not keep
pace with the rate of CO2 generation,
alveolar and plasma PCO2 increases.
This upsets the equilibrium and drives
the reaction to the right, generating
additional H2CO3, which releases H
and lowers plasma pH.
As bicarbonate ions and hydrogen ions
are released through the dissociation of
carbonic acid, the excess bicarbonate
ions become part of the bicarbonate
reserve.
To limit the pH effects of
respiratory acidosis, the excess
H must either be tied up by
other buffer systems or excreted
at the kidneys. The underlying
problem, however, cannot be
eliminated without an increase in
the respiratory rate.

Figure 24.8 2
The integrated homeostatic responses
to respiratory acidosis
Increased
PCO2
Elevated PCO2 results
in a fall in plasma pH
Respiratory Acidosis
Responses to Acidosis
Combined Effects
Respiratory compensation
Renal compensation
Decreased PCO2
Decreased H and
increased HCO3
Stimulation of arterial and CSF
chemoreceptors results in
increased respiratory rate.
H ions are secreted and
HCO3 ions are generated.
Buffer systems other than the
carbonic acid–bicarbonate
system accept H ions.
HOMEOSTASIS
DISTURBED
HOMEOSTASIS
RESTORED
Hypoventilation
causing increased PCO2
Plasma pH
returns to normal
Start
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+)

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 drives
the reactions to the left, removing H and
elevating plasma pH.
CO2 CO2 H2O H2CO2
(carbonic acid)
H HCO3
(bicarbonate ion)
HCO3 Na NaHCO3
Lungs
BUFFER SYSTEM BICARBONATE RESERVE
As bicarbonate ions and hydrogen
ions are removed in the formation of
carbonic acid, the bicarbonate ions—
but not the hydrogen ions—are
replaced by the bicarbonate reserve.

Figure 24.8 4
The integrated homeostatic responses to
respiratory alkalosis
Start
Normal acid-
base balance
HOMEOSTASIS
Decreased
PCO2
Lower PCO2 results
in a rise in plasma pH
Respiratory Alkalosis
HOMEOSTASIS
DISTURBED
Hyperventilation
causing decreased PCO2
Plasma pH
returns to normal
HOMEOSTASIS
RESTORED
Increased PCO2
Combined Effects
Increased H and
decreased HCO3
Responses to Alkalosis
Respiratory compensation
Renal compensation
Inhibition of arterial and CSF
chemoreceptors results in a
decreased respiratory rate.
H ions are generated and
HCO3 ions are secreted.
Buffer systems other than the
carbonic acid–bicarbonate system
release 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?

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