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Acid Base balance
1
Content
? Buffer systems and its efficiency
? Requirement of maintenance of acid base
balance
? Causes of acid base imbalances
? Regulation of acid base balance
? Renal
? Respiratory
2
Case report 1
? A patient with a history of chronic lung
disease has suffered from emphysema which
has grown progressively worse over a period
of years. The patient experiences chronic
shortness of breath. Analysis of patient 's
blood reveals the following PCO2=60 mmHg:
[HCO3] =34mM pH=7.38
? What could be the diagnosis?
3
Case report 2
? A man suffering from untreated diabetes
mel itus is admitted to the hospital. Glucose
and acetoacetate are present in his urine and
he exhibits shal ow breathing. Analysis of his
blood indicates [HCO3-] =16 mM and
PCO2=30
? The most likely pH of blood is
4
Relationship of pH to hydrogen ion concentration
5
Scheme demonstrating the relation between
pH and the ratio of bicarbonate concentration
to the concentration of dissolved CO2.
6
? What is pH?
? Negative logarithm of H+ concentration in a solution
? pH Scale:
? Ranges from 0 to 14
? What is pKa?
? Negative logarithm of dissociation constant
? The pH at which an acid is half dissociated, existing as
equal proportions of acid and conjugate base.
7
What is buffer
? A buffer is a mixture of a weak acid and a salt of
its conjugate base that resists changes in pH
when a strong acid or base is added
to the solution
.
? Functions of a buffer depends on:
? pH
? pK
? Salt to acid ratio
8
? Does the dilution change the pH of a buffer?
? pH of a buffer solution is directly proportional
to the salt acid ratio. Dilution does not change
the ratio
? Buffer efficiency:
? Maximum when the ratio of acid/base is
within the range of 10:1 to 1:10
? Over a pH range is equal to pKa?1
9
Requirement of maintenance of acid
base balance
? For optimum function of biomolecules :
Enzymes, transport molecules, nucleic acid
? To avoid disruption of structure and function
of cells
? Several serious health consequences: Acidosis,
alkalosis
10
Buffer systems
? 1. Bicarbonate/Carbonic acid buffer systems
? Extra cel ular buffer
? pK=6.1
? cHCO3-/cdCO2= ?
? 2. Phosphate buffer
? pK= 6.8
? cHPO4-/H2PO4-=
? Intracel ular buffer
? 3. Plasma protein and Hemoglobin: Imidazole
group of histidine: pK=7.3
11
Regulation of Acid-Base balance
1. Respiratory mechanism
2. Renal mechanism
12
Partial pressure of
oxygen and carbon di oxide
13
Respiratory Response to Acid-Base
Perturbations
? responds immediately to a change in acid -base
status
? several hours may be required for the response to
become maximal
? in the early stage plasma pH decreases
? H+ ions equilibrate slowly across the blood -brain
barrier, the pH in CSF remains nearly normal
? Stimulated peripheral chemoreceptor induces
hyperventilation: Plasma pCO2 decreased
?
14
? the PCO2 of the CSF decreases immediately
because CO2 equilibrates rapidly across the blood
?brain barrier, leading to a rise in pH of the CSF
that inhibits the central chemoreceptor
? plasma bicarbonate gradual y fal s because of
acidosis, bicarbonate concentration and pH in the
CSF wil also eventual y fall
? stimulation of respiration becomes maximal from
both central and peripheral chemoreceptors.
15
Role of
RBC and
Hb
16
Role of RBC and Hb
17
Renal mechanisms in the regulation of
Acid-Base Balance
? pH of plasma=
? 7.4
? PH of urine=
? 6.0
18
1. Na+-H+ exchange :
19
2. Reclamation of HCO3-
20
3. Renal production of Ammonia and
excretion of Ammonium ions
21
4. Excretion of H+ as H2PO4-
22
Conditions associated with abnormal acid base
status and abnormal electrolyte composition of
the blood
? Metabolic acidosis
? Metabolic alkalosis
? Respiratory acidosis
? Respiratory alkalosis
23
Simple depiction of the body as a two-vat system
of acid and base
24
Metabolic acidosis (Primary
bicarbonate deficit)
? Decreased plasma HCO3-
? Ratio of cHCO 3- /cdCO 2 is decreased
Causes
? 1. Production of organic acids that exceeds the
elimination: Diabetic ketoacidosis
? 2. Reduced excretion of acids: Renal failure,
RTA
? 3. Excessive loss of bicarbonate:
? Diarrhea (loss of duodenal fluid)
25
? An alcoholic has come to you with a complaint
of severe vomiting. His blood test reveals pH
7.42 and HCO3- 25 mmol/L,
26
Simple depiction of normal gap, anion gap acidosis,
and nonanion gap acidosis.
27
Increase in anion gap
Methanol
Uremia
Diabetic ketoacidosis
Paraldehyde
Iron, Isoniazid, Ibuprofen
Lactic acidosis
Ethylene glycol, Ethanol
Salicylates, starvation ketoacidosis
28
? Methanol:
? Metabolized by the liver to formaldehyde and
formic acid
29
Alcohol
Serum Anion Serum Urine
osmol Gap
acetone oxalate
gap
Ethanol
+
----
-----
-----
Methanol +
+
-----
-----
Isopropanol +
---
+
----
Ethylene
+
+
-----
+
glycol
30
Osmol gap
? OSMg = OSMm - OSMc
The diff erence between the actual osmolality (OSMm),
measure by freezing-point depression, and the calculated
osmolality (OSMc).
? OSMc (mOsm/kg) = 2 Na (mmol/L)
+ glucose (mg/dL) /18
+ urea (mg/dL) /2.8
? presence of unmeasured osmotically active substances:
Volatile alcohols: methanol, isopropanol , ethylene glycol
31
Uremia or renal failure
? (1) decreased ammonia formation,
? (2) decreased Na+-H+ exchange, and
? (3) decreased GFR.
? All result in decreased acid excretion.
? Acidosis usually develops if GFR falls below
20 mL/min.
32
Diabetic ketoacidosis
? -hydroxybutyrate and 2-oxoglutarate
accumulate
? decrease in HCO3- and a high anion gap
? Ketoacids also accumulate in states of
starvation and alcoholic malnutrition
33
Paraldehyde toxicity
? after chronic paraldehyde ingestion
? -hydroxybutyric acid
? Patients with paraldehyde toxicity have a
pungent, apple-like odor to their breath
.
34
Isoniazid, Iron, or Ischemia (" Three I's")
? accumulating organic acids with a predominance
of lactic acid
? production of toxic peroxides that act as
mitochondrial poisons and interfere with normal
cel ular respiration
? Ischemia results in anaerobic metabolism with
accumulation of organic (mainly lactic) acids.
35
Lactic acidosis
? Derived mainly from muscle cells and erythrocytes
? end product of anaerobic metabolism and is normally
metabolized by the liver
? An increase in the concentration of lactate to >3 mmol/L with
the associated increase in H+ is considered lactic acidosis
? caused by severe tissue hypoxia is seen in
(1) severe anemia, (2) shock, (3) cardiac arrest, and (4)
pulmonary insufficiency
? Treatment: origin of lactate (e.g., seizure, hypoxic
tissue) is rectified : rapidly metabolized to CO2, which then
is eliminated
36
Ethylene glycol
? metabolized primarily to glycolic and oxalic acid s
? lead s to an acidosis with high anion and osmolal
gaps
? Precipitation of calcium oxalate and hippurate
crystals in the urinary tract may lead to acute renal
failure
? Patients develop a variety of neurologic symptoms
that may lead to coma
37
Salicylate Intoxication
? blood salicylate concentrations above 30 mg/L
=acidosis develops
? Salicylate, itself an unmeasured anion
? Alters peripheral metabolism, leading to the
production of various organic acids
? stimulates the respiratory center to increase
the rate and depth of respiration
? mixed respiratory alkalosis and metabolic acidosis.
38
Normal anion gap acidosis
? Difference between high and normal anion gap acidosis
? high anion gap acidosis= bicarbonate
is consumed from buffering excess H+
? normal anion gap acidosis= loss of bicarbonate-rich
fluid from the kidney or the gastrointestinal tract :
more Cl- ions are reabsorbed with Na+ or
K+ to maintain electrical neutrality so that
hyperchloremia ensues
39
? Normal anion gap acidosis is divided
into (1) hypokalemic, (2) normokalemic, and
(3) hyperkalemic acidosis
40
? Normal anion gap acidosis with Hypokalemia:
? Gastrointestinal loss
? RTA
41
Gastrointestinal loss
? Diarrhea may cause acidosis as a result of loss
of (1) Na+, (2) K+, and (3)HCO3-
? water, K+, an HCO- 3 in the intestine are not
reabsorbed ,
? The resulting hyperchloremia is due to
replacement of lost bicarbonate with Cl-
? a hypokalemic, normal anion gap metabolic
acidosis develops
42
Renal tubular acidosis, Type I and I
? Loss of bicarbonate due to decreased tubular
secretion of H+ = distal or type I R TA
? Decreased reabsorption of HCO3-= Proximal or
type I RTA
? proximal and distal RTAs may be differentiated by
measurement of urine pH after administration of
acid :
? proximal R TA, urine pH becomes <5.5, whereas
in distal RTA, the distal tubules are compromised
and urine pH is >5.5
43
? Hyperkalemic normal Anion gap Acidosis: RTA type IV
? (1) Failure of the kidneys to synthesize renin,
(2) Failure of the renal cortex to secrete aldosterone,
and
(3) renal tubular resistance to aldosterone.
? inhibits Na+ reabsorption, and both K+ an H+ are thus
abnormal y retained .
? decreased renal ammonia formation and therefore
decreased elimination of H+.
44
Normal Anion Gap
GI fluid loss
Severe diarrhoea
Hypokalemia
Pancreatitis
K+ variable
Renal tubular acidosis
Proximal (type I ) R TA
Urine pH <5.5 , K+ normal or low
Distal (typeI) R TA
Urine pH >5.5 with hypokalemia
Type IV R TA
Urine pH < 5.5 with hyperkalemia
45
Compensation
? Primary compensation: respiratory system
? Stimulation of RS (Kussmaul respiration )
(1) the elimination of carbonic acid as CO2,
(2) a decrease in PCO2 (hypocapnia), and
(3) ultimately a decrease in cdCO2.
? Secondary compensation : by Kidney: takes 2-3 days
? increased excretion of acid and preservation of base by
an increase (1) rate of Na+-H+ exchange, (2) ammonia
formation, and (3) reabsorption of bicarbonate
46
Decrease in Anion Gap
Laboratory error
1.Increase in unmeasured cations
2.Lithium intoxication
3.Increased immunoglobulin
4.Monoclonal gammopathies
5.Nephrotic syndrome
6.Hyperlipidemia
47
Metabolic alkalosis
? (1) excess base is added to the system,
? (2) base elimination is decreased ,
? or (3) acid -rich fluids are lost
? Al lead to a primary bicarbonate excess
? alter the cHCO-3 / cdCO2
patient wil hypoventilate to raise PCO2
? achieving a PCO2 =55 mm Hg
? Above pH 7.55, tetany may develop:
? cause of the tetany is a decrease concentration of
ionized calcium due to increase binding of calcium ions
by albumin as H+ ions decrease
48
? metabolic alkalosis all into
? (1) Cl- responsive,
? (2) Cl- resistant, and
? (3) exogenous base categories
49
1. Cl- Responsive Metabolic Alkalosis
? Occur as a result of hypovolemia
? contraction alkalosis
? Hypovolemia wil result in
? (1) increase reabsorption of Na+,
? (2) increase HCO- 3 absorption and
? (3) excretion of K+ and H+.
? Urine Cl- wil be less than 10 mmol/L, as both the
available Cl- an HCO3 - are reabsorbed with Na+
50
? Common causes of contraction alkalosis
include
? prolonged vomiting or
? nasogastric suction and
? the use of certain diuretics
? Treatment consists of replacing BW with
? (1) water, (2) NaCl tablets, or
? (3) saline in fusion.
51
2. Cl- Resistant Metabolic Alkalosis
? far less common than Cl- responsive MA
? Associated with:
? (1) primary hyper aldosteronism,
? (2) Cushing syndrome, or
? (3) Bartter syndrome, or with excess addition
of exogenous base.
? urine Cl- will be greater than 20 mmol/L.
52
adrenocortical excess
? K+ an H+ are "wasted " by the kidneys
? increased Na+ reabsorption stimulated by
elevated aldosterone or cortisol
? hypokalemia often further contributes to the
alkalosis
? stimulates NH3 production and thus renal H+
excretion as NH 4+
53
3. Exogenous Base
? include (1) citrate toxicity following massive
blood transfusion,
(2) aggressive intravenous therapy with
bicarbonate solutions, and
? (3) ingestion of large quantities of antacids
(Milk alkali syndrome)
54
Conditions leading to Metabolic Alkalosis
Chloride responsive (Urine Cl- < 10 mmol/L)
Contraction alkalosis (Hypovolemia)
Prolonged vomiting
Upper duodenal obstruction
Dehydration
Chloride resistant (Urine Cl- > 10 mmol/L)
Mineralocorticoid Excess
Primary hyperaldosteronism
Bilateral adrenal hyperplasia
Secondary hyperaldosteronism
Glucocorticoid excess
Primary adrenal adenoma
Pituitary adenoma secreting ACTH
Exogenous cortisol therapy
Bartter syndrome (defective renal Cl- absorption )
Exogenous base
Bicarbonate containing iv fluid therapy
Massive blood transfusion ( Sodium citrate overload)
Milk Alkali syndrome
55
Compensatory Mechanisms in
Metabolic Alkalosis
? both respiratory compensation and , if
physiological y possible, renal compensation
? Respiratory compensation:
? Th e increase in pH depresses the respiratory
center,
? causing retention of carbon dioxide
? increase in cH2CO3 and cdCO2
? ratio of cHCO- 3 / cdCO2, which was
original y increased , approaches its normal value
56
Renal compensation
? The kidneys respond to the state of alkalosis
by
? decrease
(1) Na+-H+ exchange,
(2) formation of ammonia,
and (3) reclamation of bicarbonate
? This response is blunted in conditions of
hypokalemia and hypovolemia.
57
Respiratory acidosis
? occurs only through decreased elimination of CO2
? increase in PCO2 (hypercapnia) an dCO2
? decrease in the cHCO3 - / cdCO2 ratio (e.g., the ratio may be
28:1.7 [16:1] or a pH of 7.30 )
? conditions may be divided into those caused by factors that
? directly depress the respiratory center
? mechanical obstruction of the airways
? Chronic obstructive pulmonary disease (COPD) is the most
common cause
58
Conditions leading to Respiratory Acidosis
Factors that directly depress the respiratory centre
Drugs such as narcotics
CNS trauma, tumor
Infections of the CNS
Comatose states
Conditions that affect the Respiratory apparatus
COPD (most common)
Severe pulmonary fibrosis
Disease of the upper airway e,g laryngospasm, tumor
Impair lung motion due to pleural effusion
ARDS
Others Abdominal distension as in peritontitis and ascites
Extreme obesity
Sleep disorder, sleep apnea
59
Compensatory mechanism
? Immediately via buffers
? Over time via kidneys
? Excess carbonic acid present in blood is bufferd
by the hemoglobin and protein
? Buffering of CO2 causes a slight rise in cHCO3-
? immediate post hypercapnic state appear as a
metabolic alkalosis
60
Response of Kidney for respiratory
acidosis
? similarly to the way that they responds to metabolic acidosis
? Increase in
(1) Na+-H+ exchange,
(2) ammonia formation,
and(3) reclamation of bicarbonate
? Partially compensated= the plasma pH is returned about half
way toward normal
Not effective before 6 to 12 hours and is not optimal until 2
to 3 days.
COPD= full renal compensation
? COPD with superimposed metabolic alkalosis due to
prolonged diuretics
61
Respiratory response for respiratory
acidosis
? stimulates the respiratory center
? Increase pulmonary rate and depth of respiration,
provided that the primary defect is not in the
respiratory center
? Elimination of carbon dioxide through the lungs
results in a decrease in c CO2;
62
Respiratory Alkalosis
? decrease in PCO2 (hypocapnia) and the resulting
primary defcit in cdCO2
? increased rate and /or depth of respiration
? excess elimination of acid via the respiratory route
? increase in the cHCO3-/ cdCO2 ratio.
? shifts the normal equilibrium of the
bicarbonate/carbonic acid buffer system
? reducing the hydrogen ion concentration and
increasing the pH
? Also results in a decrease in cHCO3-
63
? causes of respiratory alkalosis have been
classified as
? those with a direct stimulatory effect on the
respiratory center
? and those due to effects on the pulmonary
system.
64
Factors causing respiratory Alkalosis
Nonpulmonary stimulation of respiratory center
Anxiety, hysteria
Febrile state
Metabolic encephalopathy
CNS infection
Cerebrovascular accident
Hypoxia
Drugs and agents such salicylates, cathecholamines
Pulmonary disorder
Pnemonia
pulmonary emboli
Interstitial lung disease
CHF
Respiratory compensation after correction of metabolic acidosis
Others Ventilation induced hyperventilation
65
Compensatory mechanisms for
respiratory alkalosis
? respond in two stages
? 1st stage: erythrocyte and tissue buffers
provide H+ ions that consume a small amount
of HCO3-
? 2nd stage: in prolonged respiratory alkalosis:
renal compensation as metabolic alkalosis
66
ABG parameters in various conditions
of acid-base imbalance
Imbalance
Stage
pH
HCO3
pCO2
Metabolic
Uncompensated
<7.3
Low
Normal
acidosis
Compensated
Approx 7.35
Low
Low
Respiratory Uncompensated
<7.3
Normal
High
acidosis
Compensated
Approx 7.35
High
High
Metabolic
Uncompensated
>7.5
High
Normal
alkalosis
Compensated
Approx 7.45
High
High
Respiratory Uncompensated
>7.5
Normal
Low
alkalosis
Compensated
Approx 7.45
Low
Low
67
Summary
? According to Broensted and Lowry: An acid is defined as a substance , ion
or molecule that yields H+ in sollution and base is an ion , molecule or
substance that can combine with H+ ions.
? Human body produces volatile acids( Carbonic acid) and nonvolatile acid (
Sulfuric acid and lactic acid).
? Buffers are the solution that resist the change in pH on addition of acid or
base.
? The pH of blood is maintained in a narrow range around 7.35-7.45 by
extracellular and intracellular buffering.
? The carbonic-biocarbonate system is the major buffering system.
? The partial pressure of CO2 in blood is 40 mmHg
? The pCO2 is regulated by respiratory system.
? The phosphate buffer system (Na2HPO4/NaH2PO4) operates in the cell
and contributes to only about 1% of the plasma buffering capacity.
68
Summary contd
? Histidine is the most effective amino acid that helps proteins to
work as buffer. Albumin has 16 and Hemoglobin has 38 histidine
residues.
? Kidneys play a very important role in the regulation of extracel ular
pH through reabsorption of bicarbonate and secretion of H+,
synthesis and excretion of ammonia.
? The clinical disorder associated with accumulation of acids in the
tissue and plasma is known as acidosis whereas the build up of
alkali in the body is known as alkalosis.
? The respiratory acidosis is seen in cases of pulmonary diseases such
as COPD.
? Metabolic alkalosis occurs as a result of net gain of HCO3- or loss of
nonvolatile acid
? Assessment of the acid-base imbalance is done by estimation of the
arterial blood pH , cHCO3- and pCO2 along with electrolytes
69
MCQ 1
? A 64 years old man who develops acute renal failure
while recovering from myocardial infarction. Blood
chemistry reveals Na+140 meq/L , K=4 meq/L , Cl- 115
meq/L , Co2 =5 meq/L , pH 7.12 , paCO2=13 mm Hg
, HCo3-=4 meq/L
? A. His anion gap of 14 indicates metabolic acidosis
? B. His anion gap of 20 conclusive of respiratory acidosis
? C. His anion gap of 22 strongly suggestive of metabolic
alkalosis
? D. His anion gap of 21 indicative of high anion gap
metabolic acidosis
70
MCQ2
? A 48 year old man with bronchiectasis presents to the
hospital emergency room with 3 days of increasing
cough, sputum and dyspnea.About 1 month ago his
blood analysis report showed pH 7,38, paO2 55 mmHg,
HCO3- 32 meq/L. His current vital signs are BP117/65,
pulse 123/min, temp 100oF,. His current ABG in the
emergency room pH 7.28, paCO2 70 mmHg, paO2 50 ,
HCO3- 23 meq/L. Which of the following best
characterizes the acid base status of this patient?
? A. Compensated metabolic acidosis
? B. Compensated metabolic alkalosis
? C. Uncompensated metabolic acidosis
? D. Uncompensated respiratory acidosis
71
MCQ3
? A 50 year old chronic alcoholics brought to the
emergency room in semiconscious state. BP was
100/50 and Heart rate 120/min, Resp rate 35/min,
temp 104oFBlood chemistry: Na+= 150 meq/L, K+2.5
meq/L, Cl- 107 meq/L, HCO3- 10 meq/L, pH 7.2,pCO2
=25 mmHg, Alcohol 40 mmol/L (0), Osmolality 370
mOsm/L (280-295), glucose 50 mg/dl, BUN 50 mg/dl
(5-22)What is the acid base status?
? A. Metabolic acidosis
? B. Metabolic acidosis with resp compensation
? C. Metabolic alkalosis
? D. Metabolic alkalosis with resp comp
72
MCQ4
? Which of the following is most appropriate for a
17 year old female suffering from IDDM with the
following blood chemistry report:
? pH 7.2, pO2 108 mmHg, pCo2 12 mmHg, HCO3- 5
meq/L
? A. Metabolic acidosis with resp compensation
? B . Metabolic alkalosis with respiratory
compensation
? C. Metabolic acidosis
? D. Metabolic alakalosis
73
This post was last modified on 05 April 2022