Download ACID-BASE BALANCE Acid-base balance means regulation of [H + ]

ACID-BASE BALANCE
 Acid-base balance means regulation of [H+] in the body fluid.
 Only slightly changes in [H+] from the normal value can cause marked
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alteration in the rates of chemical reactions in the cell.
For this reason the regulation of [H+] is one of the most important aspects
of homeostasis.
1
pH = log -------------- = - log [H+]
[H+]
For the bicarbonate buffer system the formula may be expressed as Follow:
[HCO3-] in mmol/l
pH = 6.1 + log -------------------------------[CO2] in mmol/L
This is called the Henderson— Hasselbalch equation and by using it
one can calculate the pH of a solution.
 Acidosis :low pH -high [H+]
 Alkalosis :high pH -low [H+]
Defense against change in [H+]
To prevent acidosis or alkalosis, several control systems
are available:
1. Acid-base buffer system
2. Respiratory system
3. Renal system
1. Acid-base buffer system
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present in all body fluids
combine immediately with any acid or alkali
can act within a fraction of a second
2. Respiratory system
 change rate of breathing ….. change rate of CO2
washing(acid gas) …….. readjust [H+] back to normal
within 1 -15 minutes.
3. Renal system
 Renal excretion of acidic or alkaline urine ….. readjust
the [H+] back to normal within hours to several days.
 The most powerful of all the acid-base regulatory
systems.
Acid-base buffer system
[1] The bicarbonate buffer system
 It consists of mixture of (H2CO3) - (NaHCO3) [carbonic
acid- sodium bicarbonate].
 When a strong acid such as HCl is added:
 HCl + NaHCO3 →H2CO3 + NaCl
 strong HCl is converted into a very weak carbonic acid .
 When a strong base such as NaOH is added to this buffer,
the following takes place;
 NaOH + H2CO3→ NaHCO3 + H2O
 The net result is exchange of the strong base NaOH for the
weak base NaHCO3.
[2] Phosphate buffer system
 It is composed of NaH2PO4 and Na2HPO
 The phosphate buffer is especially important in
the tubular fluid of the kidney because of high
concentration in the tubules.
[3] The protein buffer system
 NH2 -Protein-COOH → Undissociated protein(buffer).
 Behave as weak acids and weak bases.
 The most plentiful buffer of the body is the plasma and cells
proteins.
Hb as a buffer System
 There are 38 histidine units in the Hb molecules.
 Histidine is an amino acid which contains additional—
NH2 and -COOH groups, thus Hb is a good buffer.
 Reduced Hb + H+
H-Hb which is a very weak acid.
Respiratory Regulation of Acid-Base Balance
 Recalling the Henderson-Hasselbalch equation
[HCO3-] in mmol/l
 pH = 6.1 + log --------------------------------
[CO2] in mmol
 Inverse relationship between CO2 concentration in the body
fluids and pH.
 The CO2 concentration in the ECF depends on:
 the rate of metabolic productionof CO2.
 the rate of pulmonary ventilation(CO2 washing).
 Since alveolar ventilation can be reduced to zero or
increased to about 15 times normal, thus, activity of the
respiratory system has marked effects on pH of the body
fluids.
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On the other hand, a change in blood CO2 concentration
or blood [H+] affects the rate of alveolar ventilation by a
direct action of H+ on the respiratory center .
 The overall buffering power of the respiratory system is one
to two times as great as that of all the chemical buffers
combined.
Renal regulation of acid-base balance
 The kidneys regulate H+ conc. principally by increasing or
decreasing the HCO3- in the body fluid.
 Normally H+ secretion = HCO3- filtration, and they titrate each
other in the renal tubules the end products being CO2 and water.
The basic mechanism by which the kidney corrects either
acidosis or alkalosis is by incomplete titration of H+ against
HCO3-, leaving one or the other of these to pass into the
urine and therefore to be removed from the extracellular
fluid.
Clinical applications
1. Respiratory acidosis(CO2 retention)
 Alveolar ventilation is not enough to wash CO2 produced
by metabolism.
 increased PCO2 , carbonic acid and fall of pH
 Pathological causes are:
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airway obstruction
Pneumoinia(infection of alveoli)
chest deformities
paralysis of respiratory muscles
 All decrease gas exchange resulting in low PO2 and high
PCO2.
2-Respiratory alkalosis (CO2 washout or deficit)
 Overvbreathing ………. loss of carbonic acid …….occurs in:
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Hysteria
Mechanical respirator
Meningitis or encephalitis(depression of the …..?)
3-Metabolic acidosis
 Accumulation of acids other than carbonic acid in the
body or as a consequence of body depletion of the base
bicarbonate.
 Causes are :
 Increased lactic acid (vigorous exercise).
 Ketoacidosis: accumulation of ketone bodies
(uncontrolled diabetes mellitus).
 Renal diseases(failure of …………?)
4- Metabolic alkalosis
 Abnormal loss of HCl (prolonged or severs vomiting).
 newborn children with pyloric obstruction.
 Excessive ingestion of alkaline drugs such as sodium
bicarbonate for the treatment of peptic ulcer.