Respiratory role in pH homeostasis and blood gas analysis

Question 1

Why are blood gas samples important?

Answer 1

• Point of Care Testing used for diagnosis, treatment and monitoring of a number of respiratory and cardiac diseases
• Used for intra-operative monitoring and in intensive care

Question 2

What is the normal plasma pH range?

Answer 2

7.35-7.45
- Normal = 7.4
- Lower pH = acidosis - depresses CNS through depression of synaptic transmission (coma)
Alkalosis - above pH 7.45
overexcitability of the PNS then through facilitation of synaptic transmission (spasms, convulsion and death)

Question 3

How is acid produced in the body?

Answer 3

1.) Metabolic production of acid
- Carbonic acid
2. Non-volatile acid produced from nutrient breakdown (50-80 mmol/day)
- protein metabolism
3.) Organic acids from immediate metabolism (lactic acid, ketoacids)

Question 4

What is the line of defence against pH disorders?

Answer 4

1.) Chemical buffers
- Intracellular and extracellular fractions of
2.) Adjusting ventilation to change PaCO2
- restores 50-75% to the way to normal pH
- mins
3. Adjusting renal acid or alkalis excretion :
- Long term regulations
- hrs to days

Question 5

What are Buffer systems?

Answer 5

• Prevent rapid, drastic changes in pH
• Don’t get rid of free H+, just bind it
• Mostly effective around the pKa, the pH at which the acid is 50% ionised
  3 main extracellular buffer systems:
• Protein buffer system (e.g. Hb and plasma proteins)
• Phosphate buffer systems (e.g. in bone)
• Carbonic acid-bicarbonate buffer system

Question 6

Compare the buffer systems in our body

Answer 6

Phosphate - reduction in pH 0.2 units (limited quantity)
Bicarbonate - reduction in pH - 0.04 units (regulated)
Haemoglobin - 0.0015 (Limited quantity)

Question 7

What is the carbonic acid- bicarbonate buffer system?

Answer 7

• Quantitatively the most important ECF buffer, not the strongest
• At pH 7.4, bicarbonate ion concentration (24 mmol/L)- about 20 times that of carbonic acid (1.2 mmol/L)
• pK for the reaction is 6.1
• This system cannot protect against pH changes due to respiratory problems in which there is too much or too little CO2.

Question 8

Describe the bicarbonate buffer system

Answer 8

• Tightly regulated
• CO2 regulated by the lungs
• Chemoreceptors
• HCO3 - and H+ , regulated by the kidneys
• H+ excreted
• HCO3 reabsorbed or excreted
• hours/days

Question 9

What is the Henderson - Hasselbach equation?

Answer 9

• Chemical equation - describes the derivation of pH as a measure of acidity in biological and chemical systems

pH = pK+ log (base)/(acid)

Question 10

What are respiratory and metabolic disturbances?

Answer 10

• Where CO2 is out of the normal range and this is reflected by the measured pH
• i.e. CO2 changes cause the pH disturbance
• Non-respiratory (or metabolic) causes - those where electrolytes (HCO3-/H+) are out of its normal range - this is reflected by the measured pH
• HCO3- changes cause the pH disturbance (similarly, if there is an issue with H+)

Question 11

What is respiratory acidosis?

Answer 11

• PCO2 is high
• Due to decrease in gaseous exchange leading to CO2 retention
• Compensation by the kidneys results in increased H+ secretion over 3-5 days and an increased level of plasma bicarbonate ions.

Causes range from drugs, to Asthma, to Neuromuscular to Miscellaneous (Obesity, Hypoventilation)

Question 12

What is respiratory alkalosis?

Answer 12

• PCO2 low
• Due to alveolar hyperventilation leading to excess removal of CO2
• Compensation by the kidneys results in a decrease in excretion of H+
  and increased excretion of HCO3-

Question 13

What are clinical examples of metabolic pH imbalance?

Answer 13

Metabolic acidosis:
- Diarrhoea
- Renal tubular acidosis
- Renal failure
- Diabetes
- Tissue hypoxia
- Drugs

Metabolic alkalosis :
- Persistent gastric vomiting
- Gastric suction
- Antacids
- Hyperaldosteronism
- Chronic hypokalaemia
- Diuretics

Question 14

What is acid base disturbances?

Answer 14

Respiratory cause - CO2 is the main cause of pH disturbance
- Non-resp (metabolic) cause if HCO3- is the main cause of the pH disturbance
- Compensation refers to the physiological response to an acid-base imbalance
- Your body is trying to get the PH balance again
Correction of the pH disturbance takes longer-kidneys

Question 15

What is blood gas analysis (BGA) and how do we figure out the nature of AB distubances?

Answer 15

1. If pH is normal, high alkalosis or low acidosis
2. Is PaCo2 or HCO3- out of their normal range?
3. Which of these abnormal results corresponds with the acid base state? pH high (alkalosis) this could be due to either a low PCO2 or high HCO3- which one is seen in the data
   4- If the PaCO2 level corresponds with the pH. this is a resp cause.is the culprit, it is non-respiratory cause
4. Look at the other if it is in the normal
   range, it is not yet compensating for the problem