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Acid-Base Balance
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Acid-Base Balance
Normal pH of body fluids Arterial blood is 7.4 Venous blood and interstitial fluid is 7.35 Intracellular fluid is 7.0
Alkalosis or alkalemia – arterial blood pH rises above 7.45
Acidosis or acidemia – arterial pH drops below 7.35
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin CummingsFigure 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
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Acid–Base Balance - Hydrogen Ions (H+) Are gained
At digestive tract Through cellular metabolic activities
Are eliminated At kidneys and in urine
Must be neutralized to avoid tissue damage Acids produced in normal metabolic activity
Are temporarily neutralized by buffers in body fluids
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pH and enzyme function
Hydrogen ion concentration has a widespread effect on the function of the body's enzyme systems.
The hydrogen ion is highly reactive and will combine with bases or negatively charged ions at very low concentrations.
Proteins contain many negatively charged and basic groups within their structure.
Thus, a change in pH will alter the degree ionization of a protein, which may in turn affect its functioning.
At more extreme hydrogen ion concentrations a protein's structure may be completely disrupted (the protein is then said to be denatured).
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Sources of Hydrogen Ions Most hydrogen ions originate from cellular
metabolism Breakdown of phosphorus-containing proteins
releases phosphoric acid into the ECF Anaerobic respiration of glucose produces lactic
acid Fat metabolism yields organic acids and ketone
bodies Transporting carbon dioxide as bicarbonate
releases hydrogen ions
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Volatile acid comes from carbohydrate and fat metabolism Can leave solution and enter the atmosphere (e.g. carbonic
acid – H2CO3) Breaks in the lungs to carbon dioxide and water In the tissues CO2 reacts with water to form carbonic acid,
which dissociate to give hydrogen ions and bicarbonate ions This reaction occurs spontaneously, but happens faster with
the presence of carbonic anhydrase (CA) PCO2 and pH are inversely related
Types of acids in the body
CO2 + H20 H2CO3 HCO3- + H+
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Types of acids in the body Fixed acids
Acids that do not leave solution (e.g. sulfuric and phosphoric acids – produced during catabolism of amino acids)
Eliminated by the kidneys Organic acids
by-products of anerobic metabolism such as lactic acid, ketone bodies
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Buffers Buffers - compound
that limits the change in hydrogen ion concentration (and so pH) when hydrogen ions are added or removed from the solution.
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Buffer systems Two types of buffer in the body Chemical buffers
Bicarbonate, phosphate and protein systems Substance that binds H+ and remove it from the solution if its
concentration rises or release it if concentration decreases Fast reaction within seconds
Physiological respiratory (fast reaction – few minutes) or urinary (slow
reaction – hours to days) Regulates pH by controlling the body’s output of bases,
acids or CO2
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Acid-Base Balance Hydrogen ion and pH balance in the body
Figure 20-18
Fatty acidsAmino acids
CO2 (+ H2O)Lactic acidKetoacids
CO2 (+ H2O)
H+ input
H+ output
Plasma pH7.38–7.42
Buffers:• HCO3
– in extracellular fluid• Proteins, hemoglobin, phosphates in cells• Phosphates, ammonia in urine
H+
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin CummingsFigure 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.
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Chemical Buffer Systems Three major chemical buffer systems
Bicarbonate buffer system Phosphate buffer system Protein buffer system
Any drifts in pH are resisted by the entire chemical buffering system
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Bicarbonate Buffer System A mixture of carbonic acid (H2CO3) and its salt, sodium
bicarbonate (NaHCO3) (potassium or magnesium bicarbonates work as well)
If strong acid is added: Hydrogen ions released combine with the bicarbonate ions
and form carbonic acid (a weak acid) The pH of the solution decreases only slightly
If strong base is added: It reacts with the carbonic acid to form sodium bicarbonate (a
weak base) The pH of the solution rises only slightly
This system is the only important ECF buffer
CO2 + H2O H2CO3 H+ + HCO3¯
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Figure 27-9 The Basic Relationship between PCO2 and Plasma pH
PCO2
40–45mm Hg HOMEOSTASIS
If PCO2 rises
When carbon dioxide levels rise, more carbonic acidforms, additional hydrogen ions and bicarbonate ionsare released, and the pH goes down.
PCO2
pH
H2O CO2 H2CO3 HCO3H
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Figure 27-9 The Basic Relationship between PCO2 and Plasma pH
pH
PCO2
When the PCO2 falls, the reaction runs in reverse, andcarbonic acid dissociates into carbon dioxide and water.This removes H ions from solution and increases thepH.
pH7.35–7.45
HOMEOSTASIS
If PCO2 falls
H HCO3 H2CO3 H2O CO2
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Phosphate Buffer System Nearly identical to the bicarbonate system Its components are:
Sodium salts of dihydrogen phosphate (H2PO4¯),
a weak acid Monohydrogen phosphate (HPO4
2¯), a weak base
This system is an effective buffer in urine and intracellular fluid
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Protein Buffer System
Plasma and intracellular proteins are the body’s most plentiful and powerful buffers
Some amino acids of proteins have: Free organic acid groups (weak acids) Groups that act as weak bases (e.g., amino groups)
Amphoteric molecules are protein molecules that can function as both a weak acid and a weak base
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Figure 27-11 The Role of Amino Acids in Protein Buffer Systems
Neutral pH
If pH fallsIf pH rises
Amino acidIn alkaline medium, aminoacid acts as an acid
and releases H
In acidic medium, aminoacid acts as a base
and absorbs H
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Buffer Systems in Body Fluids
Figure 27.7
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Physiological Buffer Systems – respiratory system The respiratory system regulation of acid-base balance
is a physiological buffering system The respiratory buffering system takes care of
volatile acids – by-products of glucose and fat metabolism
CO2 + H2O H2CO3 H+ + HCO3¯
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Physiological Buffer Systems – respiratory system During carbon dioxide unloading, hydrogen ions are
incorporated into water When hypercapnia or rising plasma H+ occurs:
Deeper and more rapid breathing expels more carbon dioxide
Hydrogen ion concentration is reduced Alkalosis causes slower, more shallow breathing, causing H+
to increase
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pH Disturbances
The reflex pathway for respiratory compensation of metabolic acidosis
Figure 20-19
Respiratorycontrol centers
in themedulla
Plasma H+
( pH)Plasma
PCO2
Carotid and aorticchemoreceptors
Centralchemoreceptors
PlasmaPCO2
Plasma H+
( pH)
by Law of Mass Action
by Law of Mass Action
Action potentials in somaticmotor neurons
Muscles of ventilation
Rate and depth of breathing
Neg
ativ
e fe
edba
ck
Negative feedback
Sensory neuron Interneuron
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Physiological Buffer Systems – kidneys
Chemical buffers can tie up excess acids or bases, but they cannot eliminate them from the body
The lungs can eliminate carbonic acid by eliminating carbon dioxide
Only the kidneys can excrete the body of metabolic acids (phosphoric, uric, and lactic acids and ketones) and prevent metabolic acidosis
The ultimate acid-base regulatory organs are the kidneys
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Physiological Buffer Systems – kidneys The kidney takes care of the non-volatile acid products
By-products of protein metabolism and anaerobic respiration
The kidneys must prevent the loss of bicarbonate ions (re-absorb) that is being constantly filtered from the blood.
Both tasks are accomplished by secretion of hydrogen ions Only about 10% of the hydrogen ions secreted will be
excreted As a result of the H+ excretion the urine is usually acidic
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Renal compensation when pH is low When H+ or PCO2 in plasma is high – acidosis The kidneys will:
Secrete H+ in nephron and excretion will increase All the filtered HCO3- will be reabsorbed Produce HCO3- to increase its blood levels
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Renal Compensation
Hydrogen Ions
Are secreted into tubular fluid along:
Proximal convoluted tubule (PCT)
Distal convoluted tubule (DCT)
Collecting system
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Renal Compensation The ability to eliminate large numbers of H+ in a normal
volume of urine depends on the presence of buffers in urine
Major Buffers in Urine
Glomerular filtration provides components of:
Carbonic acid–bicarbonate buffer system
Phosphate buffer system
Tubule cells of PCT
Generate ammonia
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Reabsorption of Bicarbonate
In a person with normal acid-base balance all the HCO3- in
the tubular fluid is consumed by neutralizing H+ - no HCO3
- in the urine
HCO3- molecules are filtered by the glomerulus and than
reabsorbed and appear in the peritubular capillary (most in the PCT).
The re-absorption is not direct – the luminar surface of the tubular cells can not absorb HCO3
-
The kidney cells can also generate new HCO3- if needed
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Renal compensation when pH is high When H+ or PCO2 in plasma is low – alkalosis The kidneys will:
Inhibit secretion of H+ in nephron and excretion will decrease
Reduced HCO3- reabsorption; will appear in urine and level in plasma decrease
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Acid–Base Balance Disturbances Respiratory Acid–Base Disorders
Result from imbalance between: CO2 generation in peripheral tissues
CO2 excretion at lungs
Cause abnormal CO2 levels in ECF
Metabolic Acid–Base Disorders Result from:
Generation of organic or fixed acids Conditions affecting HCO3
- concentration in ECF
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Respiratory Acidosis and Alkalosis Result from failure of the respiratory system to balance pH PCO2 is the most important indicator of respiratory
inadequacy PCO2 levels
Normal PCO2 fluctuates between 35 and 45 mm Hg
Values above 45 mm Hg signal respiratory acidosis Values below 35 mm Hg indicate respiratory alkalosis
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Respiratory Acidosis and Alkalosis Respiratory acidosis is the most common cause of
acid-base imbalance Occurs when a person breathes shallowly, or gas
exchange is slowed down by diseases such as pneumonia, cystic fibrosis, or emphysema
Respiratory alkalosis is a common result of hyperventilation
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Metabolic Acidosis Metabolic acidosis is the second most common cause of acid-base
imbalance Can be a result of:
Failure of the kidney to excrete metabolic acids Renal acidosis is either the inability of kidney to excrete
H+ or to re- absorb bicarbonate ion Diarrhea – most common reason of metabolic acidosis
Loss of large amounts of sodium bicarbonate in the feces (which is normal component of the feces)
Diabetes mellitus – results in breakdown of fat that releases acids
Ingestion of acids Acetylsalicylic acid (aspirin) Methyl alcohol (forms acid when metabolized)
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Metabolic Alkalosis Is caused by elevated HCO3
– concentrations Bicarbonate ions interact with H+ in solution
Forming H2CO3
Reduced H+ causes alkalosis Typical causes are:
Vomiting of the acid contents of the stomach Intake of excess base (e.g., from antacids) Constipation, in which excessive bicarbonate is
reabsorbed
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Acid-base imbalances To be able to assess the type of imbalance:
1. look at the pH – to decide acidosis/alkalosis 2. look at PCO2 and HCO3- to decide
respiratory/metabolic If PCO2 causes the acidosis/alkalosis – it is respiratory If HCO3- causes the acidosis/alkalosis – it is metabolic
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To determine compensation: Uncompensated= abnormal pH and change in one
blood parameter Partially compensated= all 3 values of pH,
HCO3-, CO2 are abnormal Fully compensated= pH is normal, both HCO3-
and CO2 are abnormal Corrected= all parameters are normal
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The response to acidosis caused by the addition of H
Additionof H
Start
(carbonic acid) (bicarbonate ion)H
Otherbuffer
systemsabsorb H
KIDNEYS
Increased respiratoryrate lowers PCO2
,
effectively convertingcarbonic acidmolecules to water.
LungsCO2 CO2 H2O
Respiratory Responseto Acidosis
Secretionof H
H2CO3 HCO3 HCO3
Na
BICARBONATE RESERVE
NaHCO3
Generationof HCO3
Renal Response to Acidosis
(sodium bicarbonate)
Kidney tubules respond by (1) secreting H
ions, (2) removing CO2, and (3) reabsorbing
HCO3 to help replenish the bicarbonate
reserve.
CARBONIC ACID-BICARBONATE BUFFER SYSTEM
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BICARBONATE RESERVE
Removalof H
H
(carbonic acid) (bicarbonate ion)H2CO3 HCO3
Otherbuffer
systemsrelease H
Generationof H
Secretionof HCO3
KIDNEYS
H2OCO2 Lungs
Respiratory Responseto Alkalosis
Decreased respiratoryrate elevates PCO2
,
effectively convertingCO2 molecules tocarbonic acid.
Renal Response to Alkalosis
HCO3 NaHCO3Na
(sodium bicarbonate)
Kidney tubules respond byconserving H ions andsecreting HCO3
.
The response to alkalosis caused by the removal of H
Start
CARBONIC ACID-BICARBONATE BUFFER SYSTEM
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