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Tuesday, January 09, 2018

Acid-Base Balance (Homeostasis)

Homeostasis can be defined as a property of an organism or system that helps it maintain its parameters within a normal range of values. It is key to life, and failures in homeostasis can lead to diseases like hypertension and diabetes according to

PH = Log [H+]
      - Log [40[ = 7.4                     - 1

 HA + NaHCO  ------ NaH+H2CO3             - 2

 H+ + HCO3 ------ H2CO3  -------  H2O + H2O + CO2   - 3

 PH = PK + Log [A-] (HH)                  - 4

PH = 6.1 + Log [HCO3]
                [H2CO3]          - 5 

 [H+] = 24 + PCO2
                   HCO3                                          - 6


HA + NaHCo3 NaA  +  H2Co3
H+    +     HCo3   ----- H2Co3  CO2   +  H2O
PH = PK + Log  base/acid (H. H.)
PH = 6.1  + Log.  Bicarbonate/carbonic acid.  (PC02)
Buffers represent 1st line of action on introduction of acid into a system or body.
However, effects are not sustained
Some are largely incompetent and can readily be overwhelmed.
Two principal classes of buffer exist (These are Extra-cellular and intracellular buffers)


1. Bicarbonate - Carbonic acid (abundant)
2. Haemoglobin - (very powerful)
3. Proteins
4. Bicarbonate - Carbonic acid in renal tubules.( more widespread)
5. Mono-hydrogen Phosphate – Di-hydrogen phosphate buffer.
6. Sodium hydrogen exchange in distal renal tubules.
7. Ammonium - Ammonia buffer in the distal renal tubules.

Pulmonary mechanism.
   PH = 6.1 + Log HCO3/H2CO3 = HCO3/PCO2
  - Carbonic acid is in equilibrium with dissolved CO2 in water, measurement of partial pressure of CO2 (PCO2) can be used in clinical estimate of Carbonic acid concentration.
  - Ph depends on ratio of the two concentrations and not absolute value of either of them.
  - increased CO2 ------ increased respiratory rate --- increased excretion of CO2 ----- reduced PCO2 ----- increased PH.
  - Decreased respiratory rate leads to increased CO2 concentration ----- increased PCO2 ----- reduced PH.
  - Limitations - Lungs are incapable of generating bicarbonates if required.

Factors leading to increased H+ secretion in the PCT

- Increased plasma PCO2. - Hypokalaemia (causes m. alkalosis by shift in H+ into the cells) - Decreased arterial perfusion - Administration of mineralocorticords

Factors leading to reduced secretion of H+

* Reduced PCO2. * Expansion of Extracellular fluid * Mineralocorticoid deficiency.

Causes of Metabolic Acidosis

PTA (ineffective bicarbonate reclamation systems)
DTA (inability of the kidney to maintain normal H+ gradient.
CRI (decreased renal tubular mass resulting in the inability of the kidney to generate sufficient NH3.
Decreased filtering of PO4-.
Syndromes - that limit the ability of the kidneys to secrete NH3.
DKA (incomplete metabolism of lipids, and subsequent production of intermediate products of metabolism)
Salicylism (uncoupling of oxidative phosphorylation)
Diarrhoea (excessive loss of bicarbonate in stool)
Starvation and Aminoaciduria.
Hypoxaemia/shock.( non-oxidative pathway and production of organic acids.


1. Excessive loss of H+
2. Exogenous addition of bicarbonate to ECF
3. Excessive renal reabsorption of HCO3- (marked K+ depletion)
4. Contraction of the ECF

Contraction of the ECF

Metabolic Alkalosis stimulates the buffer systems to minimize changes in PH.
Respiratory rate may be depressed, leading to increase in PCO2.
Renal threshold of bicarbonate may also be exceeded.
    (Presence of hypokalaemia and volume depletion may interfere with the correction making metabolic-alkalosis refractory.)
Hypochloraemia and Hypokalaemia are often present.


Gastric outlet obstruction
Prolonged gastric aspiration, persistent vomiting
Excessive use of bicarbonate
Increased renal reabsorption of bicarbonate, (potassium depletion, Primary hyperaldosteronism, Cushing syndrome, Bartter's syndrome.
 Contraction of the extra-cellular compartment.
Increased filtered load of sodium.
Increased PCo2


1.Inadequate excretion of CO2 in the presence of adequate production.
2. New steady state between production and excretion is realized.
Brain stem injury 
GB syndrome
Sedative overdose
Severe airway obstruction (as would occur massive pulmonary embolism, Pulmonary oedema, Severe pneumonia bronchospasm, Chronic Obstructive Airway Disease Poliomyelitis)


Base Excess - is measured by titrating whole blood with a strong acide of a PH of 7.4 at a PCO2 of 40 mmHg at 37oC.
Base deficit - is measured by titrating whole blood with a strong base to a PH of 7.4 at PCO2 of 40mmHg at 37oC.
 Anion Gap - concentration of most abundant serum cations (sodium) - concentration of the 2 most abundant anions (HCO3 & CI-) = anion gap; = 12mEq/l (range of 8-16 mEq/l) - (due to the unmeasured anions of PO4, SO4, proteins, organic acids that exceed those of unmeasured cations of K+, Ca++ , Mn+.

Causes of Increased Anion Gap

Renal failure (increased production or impaired clearance of sulphate and phosphate)
DKA - (Increased production of aceto-acetic acid, beta hydroxybutyrate.
Lactic acidosis (increased production of lactate)
Hyperglycaemia (increased production of organic acids)
Penicillin ingestions (intermediate products, which are essentially weak acids))
Salicylate poisoning.

Causes of Reduced Anion - Gap

Nephrotic Syndrome (loss of albumin.
Lithum ingestion (Lithium as a cation)
Multiple myeloma (presence of cationic proteins).

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