acid base balance lecture – 8 dr. zahoor 1. acid base balance 2 acid base balance refers to...
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ACID BASE BALANCE
Lecture – 8
Dr. Zahoor
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ACID BASE BALANCE Acid Base Balance refers to regulation of free
(unbound) H+ concentration in the body fluids Acids liberate free H+ ion, whereas bases
accept them Strong acid gives greater percentage of free
H+ e.g. HCL is strong acid Weak acid give less H+ ion e.g. carbonic acid
H2CO3 is weak acid
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BASE Base is a substance that combines with free
H+ and removes it from the solution Strong base combines with H+ more readily
than weak base
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pH (Potential Hydrogen) pH is used to express H+ ion present H+ in ECF is normally 4×10-8 or 0.00000004
equivalent per liter (3 million times less than Na+ in ECF)
pH = log1/[H+] High H+ corresponds to low pH (acidity) Low H+ corresponds to high pH (towards
alkaline)
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pH Normal pH = 7.4 How it is calculated ? Normal H+ ion concentration is 40nEq/L and
can change normally 3-5nEq/L These small values are difficult to express,
therefore we express H+ concentration on logarithm scale using pH units
pH = log 1/[H+] OR pH= -log H+ Normal H+ is 40nEq/L (0.00000004Eq/L) Therefore Normal pH = - log [ 0.00000004] pH = 7.4
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Acid Base Balance pH 7 is neutral in CHEMISTRY H2O pH is 7 (equal number of H+ and OH- ions
are formed when H2O disassociates) Solution having pH less than 7.0 is acidic
(have more H+ ion) Solution having pH more than 7.0 is alkaline
(less H+ ion)
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pH consideration in chemistry and physiology
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ACIDOSIS & ALKALOSIS IN BODY Arterial blood pH is 7.45 Venous blood pH is 7.35 Average blood pH is 7.4 Acidosis when blood pH falls below 7.35 Alkalosis when blood pH is above 7.45
Remember – Reference point for body’s pH determination is 7.4. Why?
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ACIDOSIS & ALKALOSIS IN BODY Because body’s pH of 7.4 is taken neutral for
the body and is reference point for acidosis or alkalosis for the body.
PH compatible with life 6.8- 8
IMPORTANT – An arterial pH less than 6.8 or greater than 8.0 is not compatible with life
Death can occur in few seconds, therefore, pH of body fluids is carefully regulated
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ACIDOSIS & ALKALOSIS IN BODY pH regulation is important because changes in
H+, alter nerve, enzyme and K+ activity which will affect CVS, CNS and body metabolic processes
Acidosis causes depression of CNS, disorientation, coma and death
Alkalosis causes over excitability of CNS and peripheral nervous system (muscle twitches and muscle spasm)
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REGULATION OF H+ ION IN THE BODY There are 3 lines of defense against changes in
H+ ion to keep the pH of ECF 7.4 in the body 3 lines of defense are: 1. Chemical buffer system – respond in
seconds 2. Respiratory mechanism of pH control –
respond in minutes 3. Renal mechanism of pH control – respond in
hours to days
We will study each one, mainly renal mechanisms
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REGULATION OF H+ ION IN THE BODY1. Chemical Buffer System In Chemical Buffer System either an acid or
a base is added or removed from the solution
In body, 4 buffer systems are i). H2CO3 : HCO3 buffer system
ii). Protein buffer system iii). Hemoglobin buffer system iv). Phosphate buffer system
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2. Respiratory mechanism of pH control
Respiratory system regulates H+ by controlling the rate of CO2 removed
Respiration plays important role in acid base balance by pulmonary ventilation and excretion of H+ by generating CO2
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2. Respiratory mechanism of pH control
When there is metabolic acidosis, respiratory center in the brain stem is stimulated, therefore, increase ventilation occurs and CO2 is eliminated which helps to remove H+ ion
H + HCO3 H20 + CO2
CO2 is removed by ventilation, therefore, decreased H+ occurs in body fluid
Respiratory system is very important for every day removal of H+ ion
Respiratory system can bring pH back to normal only 50 to 75%
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3. Renal mechanism of pH control
Kidneys regulate acid base balance by 1. H+ secretion and excretion 2. HCO3 reabsorption
3. Renal buffers i). Phosphate buffers ii). Ammonia mechanism
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3. Renal mechanism of pH control1. H+ ion secretion The proximal, distal and collecting tubule all
secrete H+ ion Normally urine pH is 6 as H+ is excreted Mechanism of H+ ion secretion H+ secretary process begins in the
tubular cells with CO2 diffused into tubular cells from plasma, tubular fluid or CO2 metabolically produced in tubular cell
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Active Secretion of H+ ion in to tubular cell and reabsorption of HCO3 ion
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1. H+ ion secretion H+ ion is secreted in PCT by both primary H+
ATP pump and secondary active transport via Na+ - H+ anti-porters (anti-porters transport Na+ and H+ in opposite directions where Na+ is reabsorbed and H+ is secreted)
H+ secretion in distal and collecting tubule occurs in intercalated cells type A
Intercalated cells type A secrete H+ ion and reabsorb HCO3
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Primary Active Secretion of H+ ion in the intercalated epithelial Type A cell in DCT and CT
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2. HCO3 Reabsorption
As we have seen in previous diagram ( slide 18 )
Filtered HCO3 disappears, but is coupled with appearance of another HCO3 from the cell into the plasma
Two HCO3 are different, but HCO3 going to the plasma is considered to have been reabsorbed
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3. Renal buffersi) Filtered Phosphate bufferii) Secreted ammonia
Filtered Phosphate buffer Secreted H+ ion is buffered by phosphate
buffer system H+ secreted in the tubule combines with
phosphate
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Buffering of Secreted H+ by Filtered phosphate (NaHPO4
-) and Generation of “New” HCO3
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Secreted ammonia as Urinary Buffer When acidosis exist, the tubular cells secrete
NH3 in the tubular fluid, once normal urinary phosphate buffers are saturated
NH3 is synthesized from amino acid glutamine within the tubular cell
NH3 combines with H+ in the tubular fluid to form ammonium (NH4)
NH4 is excreted from tubular fluid
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Production and Secretion of NH4+ and HCO3
- by Proximal Tubular Cell
“New” HCO3-
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Buffering of Hydrogen Ion Secretion by Ammonia (NH3) in the Collecting Tubules
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pH Regulation Kidneys are powerful third line of defense
against changes in H+ ion Kidneys requires hours to days to compensate
for changes in body fluid pH H+ is secreted by energy depending H+
carrier until tubular fluid (urine) becomes 800 times more acidic than plasma
At this point, kidney can not acidify urine any more and pH of urine is 4.5
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pH Regulation In Alkalosis, H+ ion is secreted less in PCT and
type A intercalated cells There is decreased HCO3 reabsorption
Secretion of HCO3 occurs by intercalated type B-cells in DCT and CT. HCO3 is excreted in urine.
In Alkalosis, kidney makes urine alkaline
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ACID BASE DISORDERS Acid Base imbalance can occur from
respiratory dysfunction or metabolic disturbances
Acid Base disorders - Respiratory acidosis – increased CO2
- Respiratory alkalosis – decreased CO2
- Metabolic acidosis – decreased HCO3
- Metabolic alkalosis – increased HCO3
There can be combined disorders
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ACID BASE DISORDERSRespiratory Acidosis Occurs due to CO2 retention, therefore, there
is increased CO2 due to hypoventilation
Increased CO2 generates more H+ ion Causes - Depression of respiratory center e.g. drugs - Nerve and muscle disorders
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ACID BASE DISORDERSRespiratory Alkalosis Occurs due to decreased CO2
e.g. hyperventilation, therefore, decreased H+ is formed
Causes of Respiratory Alkalosis - Fever - Anxiety - Aspirin poisoning - High altitude
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ACID BASE DISORDERSMetabolic Acidosis It is characterized by decreased HCO3
CO2 remains normal Causes of Metabolic Acidosis - Severe Diarrhea – HCO3 is lost from GIT
- Diabetes mellitus – there is keto acidosis due to abnormal fat metabolism
- Renal failure – kidney can not excrete H+ ion, therefore, there is increase H+ ion
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ACID BASE DISORDERSMetabolic Alkalosis In Metabolic Alkalosis, there is increased HCO3
and decreased H+ ion Causes - Vomiting – Loss of H ion due to loss of
gastric juice - Ingestion of alkaline drugs e.g. NaHCO3 for
acidity
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pH in Uncompensated Acid-Base Abnormalities
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Analysis of Simple Acid-BaseDisorders
Figure 30-10; Guyton and Hall
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