Physiology 8: Acid Base Balance

Question 1

A person is of normal acid base balance if

Answer 1

• plasma pH is around 7.4 (range from 7.35 to 7.45)
• HCO3- close to 25 mol/litre (range from 23-37mmol/litre)
• arterial PCO2 close to 40mmHg (range from 35-45)

Question 2

if normal acid base balance is disturbed the first and number one priority is to

Answer 2

restore pH to 7.4 as soon as possible which is known as compensation for an acid base disturbance

Question 3

Therefore, compensation is said to be

Answer 3

the restoration of pH irrespective of what happens to bicarbonate and PCO2

Question 4

correction on the other hand

Answer 4

is the restoration of pH, HCO3- and PCO2 to normal

Question 5

classification of disturbances to acid base balance

Answer 5

• respiratory in origin

- non-reparatory in origin

Question 6

respiratory in origin

Answer 6

1. Respiratory acidosis= plasma pH falls

2. Respiratory alkalosis= plasma pH rises

Question 7

non-respiratory origin

Answer 7

1. Metabolic acidosis= plasma pH falls

2. Metabolic alkalosis= plasma pH rises

Question 8

immediate buffering of a chance in pH

Answer 8

• immediate dilution of acid or base present in the exta-cellualr fluid
• Blood buffers are Hb and HCO3- IE in acidosis where there are excess hydrogen ions in the blood bicarbonate buffers them
• this is a very rapid response however, buffers quickly get depleted and the kidney has to replenish stores

Question 9

blood gas analysis can measure

Answer 9

pH and PCO2 and bicarbonate can be calculated

Question 10

kidneys regulate

Answer 10

bicarbonate concentration in the blood

Question 11

lungs regulate

Answer 11

the partial pressure of CO2 in the blood

Question 12

respiratory acidosis caused by

Answer 12

retention of CO2 by the body which is caused by HYPOventilation

Question 13

causes of respiratory acidosis

Answer 13

• COPD
• respiratory restriction
• respiratory depression caused by opitate compounds and general anaesthetics

Question 14

so how do theses respiratory problems generate an acidosis

Answer 14

the excess carbon dioxide in the blood shifts the equilibrium to the right
H20 + CO2 H2CO3- H+ AND HCO3-
- This causes an increase in both the hydrogen and bicarbonate ion concentration, the H+ ions cause the acidosis and you might think the increase in bicarbonate ions should compensate HOWEVER, A BUFFER SYSTEM CANNOT BUFFER ITSELF

Question 15

Uncompensated respiratory acidosis

Answer 15

LOW PH HIGH PCO2

Question 16

Compensation for respiratory acidosis

Answer 16

• There is no extra-cellular buffering because a buffer cannot buffer itself, since the respiratory system is the cause the renal system must compensate: REMEMBER BLOOD CO2 CONCENTRATION DRIVES TUBULAR HYDROGEN ION SECRETION, therefore, the more CO2 retained the more H+ ions secreted into the filtrate

Question 17

renal compensation for respiratory acidosis is as follows

Answer 17

• H+ tubular secretion
• All filtered bicarbonate ions are re-absorbed
• H+ is continually secreted and generates a titratable acid and ammonium
• acid is excreted and new bicarbonate ions are added to the blood, therefore bicarbonate ion concentration increases as a result of the underlying disorder and because bicarbonate is added to the blood

Question 18

CORRECTION OF RESPIRATORY ACIDOSIS

Answer 18

requires lowering PCO2 by restoring normal ventilation

Question 19

respiratory alkalosis problem

Answer 19

excessive loss of CO2 from the body caused by hyperventilation

Question 20

causes of respiratory alkalosis

Answer 20

• low inspired PO2 at high altitudes (which stimulates the peripheral chemoreceptors to cause hyperventilation), panic attacks, brainstem damage

Question 21

how do these repsiraoty problems cause an alkalosis

Answer 21

• they cause the equilibrium to shift to the left
  CO2+ H20 H2C03- HCO3- AND H+
• the shift in equilibrium reduction in bicarbonate and hydrogen ions
• the reduction in hydrogen ions causes the alkalosis

Question 22

decompensated respiratory alkalosis

Answer 22

HIGH PH LOW PCO2

Question 23

Renal compensation for respiratory alkalosis

Answer 23

• since the respiratory system is the problem then the renal system has to compensate
• remember blood PCO2 drives hydrogen ion secretion into the tubules, so the less PCO2 in the blood the less hydrogen ions being secreted into the tubules

Question 24

therefore, compensation for respiratory alkalosis is as follows

Answer 24

• reduction in hydrogen ion secretion into the tubules because there is reduced PCO2 concentration in the blood
• there is reduced bicarbonate re-absorption in the tubules even though the shift in equilibrium has reduced the amount if bicarbonate filtered therefore, bicarbonate gets excreted causing the urine to be ALKALINE
• no titratable acid or ammonium are formed therefore, BICARBONATE IONS ARE NOT ADDED TO THE BLOOD
• so renal compensation for the respiratory alkalosis actually further lowers the bicarbonate

Question 25

Correction of respiratory alkalosis

Answer 25

requires restoration of normal ventilation

Question 26

metabolic acidosis

Answer 26

presence of excess hydrogen ions in the body from a source other than carbon dioxide

Question 27

causes of a metabolic acidosis

Answer 27

ingestion of acids or acid producing foods, H+ ions generated in lactic acidosis and diabetic ketoacidosis, excessive loss of base from the body i.e. ions ever diarrhoea

Question 28

in metabolic acidosis what happens to bicarbonate

Answer 28

it gets depleted as a result of buffering the excess hydrogen ions or because it is lost for the body

Question 29

the problem in metabolic acidosis is not

Answer 29

respiratory in nature therefore, the respiratory system can compensate

• the low plasma pH is sensed by the peripheral chemoreceptors which are stimulated to increase ventilation which blows off more carbon dioxide
• loss of carbon dioxide shifts the equilibrium to the left
• which reduces the plasma concentration of hydrogen ions (normalising the pH) but also causes a reduction in bicarbonate ions

Question 30

uncompensated metabolic acidosis

Answer 30

LOW PH AND LOW BICARBONATE

Question 31

correction for metabolic acidosis

Answer 31

• the reduction in plasma bicarbonate concentration means that very small amounts of bicarbonate get filtered and all of it is re-absorbed
• H+ secretion continues which produces titratable acids and ammonium which means bicarbonate is added to the blood which restores bicarbonate concentration and normal ventilation is restored
• the acid load excreted in the urine makes the urine acidic

Question 32

note for metabolic acidosis

Answer 32

the acid load cannot be immediately excreted by the kidneys which is why respiratory compensation is essential to normalise pH ASAP

Question 33

metabolic alkalosis

Answer 33

excessive loss of hydrogen ions from the body

Question 34

causes of metabolic alkalosis

Answer 34

loss of HCL in severe vomiting, ingestion of alkaline or alkali containing foods, hyperaldosteronism (excessive stimulation of the Na/H+ pump)

Question 35

as a result of the loss of hydrogen ions or the addition of excess base in metabolic alkalosis

Answer 35

the bicarbonate concentration increases

Question 36

uncompensated metabolic alkalosis

Answer 36

high pH

high bicarbonate ion concentration

Question 37

the increase in plasma pH in metabolic alkalosis

Answer 37

stimulates the peripheral chemoreceptors to slow ventilation to retain carbon dioxide causing a shift in the equilibrium to the right causing an increase in hydrogen ions to normalise the pH but also causes an increase in HCO3- IONS

Question 38

correction for metabolic alkalosis

Answer 38

there is a large amount of filtered bicarbonate which cannot all get re-abosrbed therefore, bicarbonate is excreted in the urine which is alkaline and bicarbonate concentration falls back to normal