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Copmpensation mechanism acd & base
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COMPENSATORY MECHANISM OF ACID - BASE BALANCE
By
Dr KHALED SALEH ALGARIRI
2014
Alkalosis refers to excess removal of H+ from the body fluids
Acidosis refers to excess addition of H+
Acidosis- a decline in blood pH ↓Metabolic acidosis: due to a decrease in
bicarbonate. ↓Respiratory acidosis: due to an increase in
carbonic acid. ↑Alkalosis- a rise in blood pH ↑
Metabolic alkalosis: due to an increase in bicarbonate.↑
Respiratory alkalosis : due to a decrease in carbonic acid. ↓
4
pH
Acidosis Alkalosis
respiretory
[HCO3-]
↓[HCO3
-]↑
PaCO2↑ PaCO2
↓
metabolicmetabolic respiretory
HCO3-
5
Affect all body systems
Particularly nervous and cardiovascular
systems
Both are dangerous But acidosis is more common Because normal cellular activities generate
acids
Acidosis and Alkalosis
Types of Acids in the Body
1. Fixed acids
2. Organic acids
3. Volatile acids
The body produces more acids than bases
Acids take in with foods.Cellular metabolism produces CO2.Acids produced by metabolism of lipids
and proteins.
Volatile acid
H2CO3 CO2+ H2O
CO2
CO2 CO2
Fixed acid
H2SO4 H3PO4
Uric acidLactic acid
Ketone body(H+ < 0.05 –0.10 mol /d)
(H+ 15 –20 mol /d)
F 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
The Basic Relationship between PCO2 and Plasma pH
pH
PCO2
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 thepH.
pH
7.35–7.45HOMEOSTASIS
If PCO2 falls
H HCO3 H2CO3 H2O CO2
pH< 7.35: Acidosis
pH > 7.45: Alkalosis
The body response to acid-base imbalance
is called compensationThe body gears up its homeostatic
mechanism and makes every attempt to restore the pH to normal level.
May be complete if brought back within normal limits
Partial compensation if range is still outside norms.
Defenses against changes in hydrogen ion concentration
There are three primary systems that regulate the H+ concentration in the body fluids to prevent acidosis or alkalosis:
(1) the chemical acid-base buffer systems of the body fluids
(2) the respiratory center(3) the kidneys, which can excrete either
acid or alkaline urine, thereby readjusting the extracellular fluid H+ concentration toward normal during acidosis or alkalosis
Buffer system
A chemical substance that minimizes changes in pH by releasing or binding hydrogen ions
Buffer SystemMost buffers composed of weak acid and
weak baseThe purpose of the buffer is to help the
body maintain pHThree important buffer system: 1. H2CO3/HCO3 buffer system (Most important in ECF)
2. H2PO4-/HPO4-2 buffer system (Buffers pH of ICF and urine)
3. Protein buffers (plasma proteins and hemoglobin)
The Carbonic Acid–Bicarbonate Buffer SystemCarbon dioxide
Most body cells constantly generate carbon dioxide Most carbon dioxide is converted to carbonic acid,
which dissociates into H+ and a bicarbonate ion
Is formed by carbonic acid and its dissociation products
Prevents changes in pH caused by organic acids and fixed acids in ECF
The Carbonic Acid–Bicarbonate
Buffer System 1. Cannot protect ECF from changes in pH
that result from elevated or depressed
levels of CO2
2. Functions only when respiratory system
and respiratory control centers are
working normally
3. Ability to buffer acids is limited by
availability of bicarbonate ions
The Phosphate Buffer System Consists of anion H2PO4
– (a weak acid)
Works like the carbonic acid–bicarbonate
buffer system
Is important in buffering pH of ICF
Protein Buffer Systems Depend on amino acids
Respond to pH changes by accepting or
releasing H+
If pH rises:Carboxyl group of amino acid
dissociatesActing as weak acid, releasing a
hydrogen ionCarboxyl group becomes carboxylate ion
Protein Buffer Systems At normal pH (7.35–7.45)
Carboxyl groups of most amino acids have already given up their H+
If pH drops: Carboxylate ion and amino group act as weak
bases Accept H+
Form carboxyl group and amino ion
Protein Buffer Systems Carboxyl and amino groups in peptide bonds
cannot function as buffers
Other proteins contribute to buffering
capabilities Plasma proteins Proteins in interstitial fluid Proteins in ICF
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
The Hemoglobin Buffer System
CO2 diffuses across RBC membrane
No transport mechanism required
As carbonic acid dissociates: Bicarbonate ions diffuse into plasma In exchange for chloride ions (chloride shift)
Hydrogen ions are buffered by hemoglobin
molecules
Figure 23-24 A Summary of the Primary Gas Transport Mechanisms
Systemiccapillary
Cells inperipheral
tissues
Chlorideshift
CO2 pickup
Figure 23-24 A Summary of the Primary Gas Transport Mechanisms
Alveolarair space
Pulmonarycapillary
CO2 delivery
Buffer Systems
Intracellular fluid (ICF)
Phosphate BufferSystem
Protein Buffer Systems
The phosphatebuffer systemhas an importantrole in bufferingthe pH of the ICFand of urine.
Protein buffer systems contribute to the regulationof pH in the ECF and ICF. These buffer systems interactextensively with the other two buffer systems.
Hemoglobin buffersystem (RBCs only)
Amino acid buffers(All proteins)
Plasma proteinbuffers
The carbonic acid–bicarbonate buffersystem is mostimportant in the ECF.
Carbonic Acid–Bicarbonate BufferSystem
Extracellular fluid (ECF)
occur in
Respiratory and Renal Mechanisms Support buffer systems by:
1. Secreting or absorbing H+
2. Controlling excretion of acids and bases
3. Generating additional buffers
Respiratory control of pH
Within body fluids, CO2 and H2O coming together to form H2CO2 which breaks down into HCO3- and H+
HCO3- and H+ are constantly forming H2CO3 which can split apart to form CO2 and H2O
Respiratory control of pHWhen we breathe more quickly, more
CO2 leaves the body
When we breathe more slowly, less CO2 leaves the body
Renal control of pH
The kidneys control acid-base balance by excreting either an acidic or a basic urine
Excreting an acidic urine reduces the amount of acid in the extracellular fluids
Excreting a basic urine removes base from the extracellular fluids
Under normal conditions, almost all HCO3- are reabsorbed from the tubules, thereby conserving the buffer system of the extracellular fluid
This reabsorption of HCO3- are accomplished through the process of H+ secretion by the tubules
Renal control of acid base balanceThe kidneys regulate extracellular fluid H+
concentration through three mechanisms:
1. Secretion of H+2. Reabsorption of filtered HCO3-3. Production of new HCO3-
About 80 to 90 per cent of the bicarbonate reabsorption (and H+ secretion) occurs in the proximal tubule
In the thick ascending loop of Henle, another 10 per cent of the filtered bicarbonate is reabsorbed
The remainder of the reabsorption takes place in the distal tubule and collecting duct
Secretion of Hydrogen Ions 1. CO2 arrive at the kidney
tubules cells 2. Within tubular cells, CO2
combine with H2O to form H2CO3 by carbonic anahydrase
3. Then H2CO3 split to HCO3 and H+
4. The H+ is secreted from the cell into the tubular lumen by Na-H counter transport
5. The HCO3- generated in the cell moves into the peritubular capillary blood
6. The epithelial cells in PCT, thick ascending limb and DCT all secrete H+ into the tubular lumen
Reabsorption of HCO3- from filtrate 1. HCO3- that is filtered by
the glomerulus combines with H+ to form H2CO3 which eventually becomes CO2 and H2O
2. The H2CO3 formed dissociate into CO2 and H2O
3. CO2 diffuses into tubular cell where it recombine with H2O under the influence of CA to generate H2CO3
4. Then H2CO3 split to HCO3 and H+
5. The HCO3- generated in the cell moves into the renal interstitial fluid and the peritubular capillary blood
Generating new HCO3-
H+ combined with HCO3- in the tubular fluid which results in reabsorption of HCO3-
If high H+ (as in acidosis), the kidneys generate new HCO3- by phosphate and ammonia buffers mechanisms
Phosphate buffer system carries excess H+ into the urine and generate new bicarbonate
Phosphate buffer composed of HPO4= and H2PO4-
Excess H+ can combine with HPO4=
After H+ combines with HPO4= to form H2PO4 , it can be excreted as a NaH2PO4 carrying with it the excess H+
Excretion of excess H+ and generation of new HCO3- by ammonia buffer system
The glutamine delivered to the kidneys is transported into the epithelial cells of the proximal tubules, thick ascending loop of Henle, and distal tubulesGlutamine is metabolized to form two NH4+ and two HCO3-
The HCO3 is transported across the basolateral membrane along with reabsorbed Na+ into the peritubular capillariesIn chronic acidosis the dominant mechanism by which acid is eliminated is excretion of NH4+
The Detection of Acidosis and
Alkalosis
Includes blood tests for pH, PCO2’
and HCO3– levels
Recognition of acidosis or
alkalosisClassification as respiratory or
metabolic
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