Blood Gases, pH and Buffer system

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Blood Gases, pH and Buffer system

• Part 1

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Introduction

• An important aspect of clinical chemistry is
  information on a patient's acid-base balance and
  blood gas homeostasis.
• These data often are used to assess patients in
  life-threatening situations.
• This lecture discusses
• the body's mechanisms to maintain acid-base
  balance and
• exchange of gases, carbon dioxide and oxygen,

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Definitions Acid, Base, Buffer

• Acid
• a substance that yields H ions in H2O.
• Base
• a substance that yields a hydroxyl ion (OH).
• Buffer
• the combination of a weak acid and its salt, is a
  system that resists changes in pH.

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Acid, Base, Buffer

• The relative strengths of acids and bases, their
  ability to dissociate in water, are described by
  their dissociation constant (ionization constant
  - K value)
• pK defined as the negative log of the ionization
  constant
• that is pH where the protonated and unprotonated
  forms are present in equal concentration.

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Strong acids vs. Strong Base

• Strong acids
• have pK value of less than 3.0
• Strong base
• have a pK value greater than 9.0

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Acid-Base balance

• Maintenance of hydrogen ions
• Body produces 40-80 mmol of H/day,
• normal concentration of H in ECF ranges from
  36-44 nmol (pH, 7.34-7.44)
• Any deviation from the values the body will try
  to compensate.
• gt44 nmol/L altered consciousness, coma- death
• lt36 nmol/L neuromuscular irritability, tetany,
  loss of consciousness- death.

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Acid-Base balance

• Because pH is the negative log of the cH
• Decrease in H ion Increase pH
• Increase H ions Decrease pH
• Arterial blood pH is controlled by
• Buffers
• Respiratory System
• and Kidneys

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Buffer System Regulation of H

• First line of defense to changes in H consist
  of
• weak acid (H2CO3) its salt (HCO3-)
• Add acid to H2CO3 HCO3- system
• the HCO3- combines with H from the acid to form
  H2CO3.
• Add a base to the system
• H2CO3 combines with OH to form H2O and HCO3
• Keeps the body at the correct pH (7.35-7.45)

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Buffer System Regulation of H

• Bicarbonate carbonic acid system has low
  buffering capacity but still an important buffer
  system for 3 reasons
• H2CO3 dissociates into CO2 H2O allowing H to
  be eliminated as CO2 by lungs
• Changes in CO2 modify the ventilation rate
• HCO3- conc. can be altered by the kidneys

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Other systems

• HPO42 ? H2PO4 system
• Proteins are capable of binding H
• Hemoglobin

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Regulation of Acid-Base Balance Lungs and
Kidneys

• The lungs and kidneys play important roles in
  regulating blood pH.
• The lungs regulate pH through retention or
  elimination of CO2
• by changing the rate and volume of ventilation.
• The kidneys regulate pH by
• excreting acid, primarily in the ammonium ion,
• and by reclaiming HCO3- from the glomerular
  filtrate.

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Regulation of Acid-Base Balance by Lungs

• End product of aerobic metabolic process is CO2
• diffuses out the tissue into plasma and RBC
• In Plasma RBCs
• a small amount of CO2 is dissolved
• or combined with proteins to form carbamino
  compounds.
• Most of the CO2 combines with H2O to form H2CO3,
  which quickly dissociates into H and HCO3-

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Regulation of Acid-Base Balance by Lungs

• The dissociation of H2CO3 causes the HCO3-
  concentration to increase in the RBCs and diffuse
  into the plasma.
• To maintain electroneutrality chloride diffuses
  into the cell (chloride shift)

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Regulation of Acid-Base Balance by Lungs

• In the lungs
• The process is reversed.
• Inspired O2 diffuses from the alveoli into the
  blood and is bound to hemoglobin, forming
  oxyhemoglobin (O2Hb).
• The H that was carried on the (reduced)
  hemoglobin in the venous blood is released to
  recombine with HCO3- to form H2CO3,
• which dissociates into H2O and CO2.

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Regulation of Acid-Base Balance by Lungs

• The CO2 diffuses into the alveoli and is
  eliminated through ventilation.
• The net effect of the interaction of these two
  buffering systems is a minimal change in H
  concentration between the venous and arterial
  circulation.

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Interrelationship of the bicarbonate and
hemoglobin buffering systems
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Regulation of Acid-Base Balance by Lungs

• When the lungs do not remove CO2 at the rate of
  its production (hypovent.) it accumulates in the
  blood,
• causing an increase in H concentration.
• If, however, CO2 removal is faster than
  production (hypervent,)
• the H concentration will be decreased.
• Consequently, ventilation affects the pH of the
  blood.

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Regulation of Acid-Base Balance by Lungs

• A change in the H concentration of blood that
  results from nonrespiratory disturbances causes
  the respiratory center to respond
• altering the rate of ventilation in an effort to
  restore the blood pH to normal.
• The lungs, by responding within seconds, together
  with the buffer systems, provide the first line
  of defense to changes in acid-base status.

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Kidney system

• Main role is reabsorption of bicarbonate
• Kidneys respond to increase or decrease in
  hydrogen ions by selectively excreting or
  reabsorbing
• Hydrogen ions
• Sodium
• Chloride
• Phosphate
• Ammonia
• Bicarbonate

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Re absorption of Bicarbonate

• Reabsorption of bicarbonate (HCO3 ) takes place
  in the renal tubule cells.
• Na is exchanged for H ion.
• H ion combines with the HCO3 and carbonic
  acid dissociates into H2O and CO2.
• CO2 diffuses into the tubule cells combining with
  H2O forming H2CO3.
• Reabsorption of bicarbonate in the blood system.
• Urinary H combines with HPO4 and NH3.

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Acid-Base Disorders

• Acidosis (decrease pH) ? acidemia
• Alkalosis (increased pH) ? alkalemia
• metabolic or respiratory
• A disorder caused by ventilatory dysfunction is
  termed primary respiratory acidosis or alkalosis.
• A disorder resulting from a change in the
  bicarbonate level is termed a nonrespiratory
  disorder.

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Acid-Base Disorders

• Body's cellular and metabolic activities are pH
  dependent
• The body tries to restore acid-base homeostasis
  whenever an imbalance occurs.
• This action is termed compensation
• Done by altering the factor not primarily
  affected by the pathologic process.
• eg., if the imbalance is of nonrespiratory
  origin, the body compensates by altering
  ventilation.
• For disturbances of the respiratory component,
  the kidneys compensate by selectively excreting
  or reabsorbing anions and cations.

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Acid-Base Disorders

• Lungs can compensate immediately but
• the response is short term
• and often incomplete.
• The kidneys compensate are slower to respond (2-4
  days) but
• the response is long term
• and potentially complete

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Metabolic (non-respiratory) Acidosis

• Decrease pH, increase H
• Bicarbonate decreased (lt24 mmol/L)
• Caused by
• acid producing substance or process
• or reduce excretion of acids
• Compensation
• 1o - Respiratory compensation
• Hyperventilation, decrease CO2 in circulation.
• 2o - Renal compensation
• increase H ion loss by increasing PO4 and NH4
  excretion and retain HCO3

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Respiratory Acidosis

• Caused by hypoventilation
• decrease the elimination of CO2 in the lungs, it
  builds up in the blood
• decrease in pH, increase in H and HCO3
• Diseases emphysema, drugs , congestive heart
  failure, bronchopneumonia.
• Compensation
• Renal compensation
• increase H excretion increase reabsorption of
  HCO3-

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Metabolic (non-respiratory ) alkalosis

• HCO3- increased, H decreased, pH increased
• Causes of nonrespiratory alkalosis
• excess administration of NaHCO3
• ingestion of HCO3 producing salts such as
  Na-lactate, citrate or acetate
• excessive loss of acid through vomiting
• Compensation
• Respiratory compensation
• Hypoventilation with CO2 retention
• Renal compensation
• excrete HCO3 and retain H ions.

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Respiratory alkalosis

• Decreased CO2, decreased H, increased pH
• Causes of respiratory alkalosis include
• hypoxemia
• chemical stimulation of the respiratory center by
  drugs, such as salicylates
• pulmonary fibrosis.
• Compensation
• Renal compensation
• decrease renal excretion of H ions, HCO3 excreted.