Acid- Base Balance

Acid- Base Balance

1. why is pH important?

   • although H+ conc is low- it is highly reactive in the body
   • proteins have dissociable groups which readily gain or loose protons
   • NORMAL 7.35-7.45
     • can't go outside of 6.8-7.8
   • in normal animal pH can change due to inconsistant acid base production in metabolism or inconsistant amount in diet

2. definitions

   • acid is a proton donor
   • base- proton acceptor
   • strong acid- fully dissociated in solution
   • weak acid- reaches an equilibrium
   • pH=-log[H+]
   • Ka=[H+][A-]/[HA]
   • buffer= weak acid and conjugate base can accept or donate protons to minimise pH change

3. buffering

   • curve- sigmoidal but almost linear in midrange
   • at midrange the pH change is smallest for a given amount of added acid or base
   • where pH =pKa when buffer is at most efficient capcity
   • bicarbonate as buffer in ECF
     • good buffer because bicarbonate is abundant in body and in an open system so will not run out
     • water and carbon dioxide with carbonic anhydrase- produce carbonic acid which dissociates to proton and carbonate ion
     • carbon dioxide removed by alvolar ventilation in lungs
     • rise or fall in pH are detected in resp centre in brain stem and ventilation rate altered to restore carbon dioxide to 40mmHg
   • proteins as buffers
     • result of their dissociable side groups- Hb v important but plasma proteins do it too
     • Hb Buffering action
       • 1- carbon dioxide into RBC produce carbonic acid and then bicarbonate and protons
       • 2-bicarbonate transported to plasma to act as buffer- RBC takes in chloride ions to balance
     • 3-protons combine with Hb which more readily dissociates with oxygen- so carbon dioxide shifts the curve to the right

4. resp system in pH

   • respiratory acid/base disorders are due to change in arterial carbon dioxide which are regulated by alveolar ventilation
   • respiratoryacidosis- hypoventilation
   • respiratoryalkalosis- hyperventilation
   • control of respiration is in the resp centre of the brain which detects a drop in pH, triggering an increase in resp rate to decrease carbon dioxide and raise pH
   • sensors also alter resp rate
     • in response to carbon dioxide in the blood
   • resp system can respond very quickly

5. Kidney and pH regulation

   • kidney must excrete metabolic acids and bases but maintain alkali reserve
     • pool of bicarbonate ions which must be maintained is the alkali reserve
   • 1- reabsorption of bicarbonate ions
     • proximal tubule reabsorbs bicarbonate ions to maintain alkali reserve
       • does not produce acid urine
     • a-carbon dioxide diffuses into tubular cell from blood or lumen
     • b- produces carbonic acid which dissociates into carbonate ions and protons
     • c-bicarbonate goes into the blood- protons go to the lumen and exchanged for sodium ions
     • d-net one bicarb absorbed and one acid lost
   • 2- excreation of acid urine
     • all bicarbonate has been absorbed so start excreting acid
     • instead protons combine with hydrogen phosphate in the tubule
     • net excretion of acid producing acid urine- only works to pH 4.5 urine
   • 3-excreation of ammonia
     • metabolism produces ammonia which combines with proton to produce NH4+ no loss of sodium, no acid urine, important in prolonged acidosis
   • kidney takes 4-5 days to compensate

6. Acid-base analysis

   • base excess- amount of acid or base added to normal oxygenated blood to restore pH to normal at normal temp and carbon dioxide. normal range -4 to +4
     • decreased BE- metabolic acidosis
     • increased BE- metabolic alkalosis
       • increase base- associate with alkalosis
   • evaluate pH, evaluate resp, evaluate metabolism, define primary process
   • anion gap- difference between measured cations and ions normally 15-25
     • an increase in the gap tends to indicate a metabolic acidosis

7. acid base disturbances

   • respiratory acidosis
     • decrease in pH
     • increase in carbon dioxide
     • BE normal
     • some compensatory metabolic alkalosis
     • causes- airway obstruction, resp depression, neuromuscular disease, disease preventing lung expansion
   • respiratory alkalosis
     • increase in pH
     • decrease in carbon dioxide
     • hyperventilation causes- fear, pain, drugs
   • metabolic acidosis
     • decrease in pH and decrease in BE
     • causes- adding acid to the body or loosing base somewhere
   • metabolic alkalosis
     • increase in pH and increase in BE and carbonate ions
     • causes- diseases including control of acid- vomiting or kidney trouble

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