Acid-base balance

Question 1

What is the normal pH of arterial blood?

Answer 1

7.4

Question 2

List some sources of H+

Answer 2

Respiratory Acid: carbonic acid production during impaired lung function

Metabolic Acid (Non-respiratory acid) via metabolism:
o Inorganic acids: eg S-containing amino acids → H2SO4 and phosphoric acid is produced from phospholipids
o Organic acids: fatty acids, lactic acid
o On a normal diet, there is a net gain to the body of 50-100 mmoles H+ per day.

Question 3

Whats the major source of alkali production in the body

Answer 3

• Major source of alkali is oxidation of organic anions such as citrate

Question 4

What is the most important extracellular buffer system?

Answer 4

bicarbonate buffer system, H2CO3 ↔ H+ + HCO3-

Question 5

List the range of normal values for pH, pCO2 and HCO3

Answer 5

pH =7.4 Range 7.37-7.43

(Range of pH compatible with life: 6.8-7.8 (US) 7.0-7.6(UK)

pCO2 = 5.3kPa Range 4.8- 5.9
= 40mmHg 36-44

[HCO3-] = 24mmoles/l Range 22-26

Question 6

What are the primary intracellular buffers?

Answer 6

Primary intracellular buffers are proteins, organic and inorganic phosphates and, in the erythrocytes, haemoglobin.

Question 7

List some mechanisms for the reabsorption of HCO3-

Answer 7

o Active H+ secretion from the tubule cells
o Coupled to passive Na+ reabsorption
o Filtered HCO3- reacts with the secreted H+ to form H2CO3. In the presence of carbonic anhydrase on the luminal membrane → CO2 and H2O
o CO2 is freely permeable and enters the cell
o Within the cell, CO2 → H2CO3 in the presence of carbonic anhydrase (present in all tubule cells) which then dissociates to form H+ and HCO3-
o The H+ ions are the source of the secreted H+
o The HCO3- ions pass into the peritubular capillaries with Na+
o Bulk of HCO3- reabsorption occurs in the proximal tubule (>90%)

Question 8

Where does the bulk of HCO3 reabsorption occur?

Answer 8

Proximal tubule

Question 9

What is minimum and maximum pH of urine in humans?

Answer 9

pH = minimum 4.5.-5.0, maximum ≈ 8.0

Question 10

What is the major adaptive response to an acid load?

Answer 10

Ammonium excretion is the major adaptive response to an acid load, generates new HCO3- AND excretes H+.
• Only used for acid loads.

Question 11

How long does it take for ammonium excretion to reach maximal effect?

Answer 11

It takes 4-5 days to reach maximal effect because of the requirements of ↑ protein synthesis.

Question 12

What substances suggest acid/base balances dependent on respiratory causes or metabolic causes?

Answer 12

• Respiratory disorders affect Pco2

* Renal disorders affect [HCO3-]

Question 13

What biochemical factors lead to acidosis and alkalosis?

Answer 13

A decrease in pH (acidosis) is caused by either:
o Decreased HCO3+ (metabolic)
o Increased Pco2 (respiratory)

An increase in pH (alkalosis) is caused by either:
o Increased HCO3+ (metabolic)
o Decreased Pco2 (respiratory)

Question 14

What causes respiratory acidosis?

Answer 14

Respiratory Acidosis: pH has fallen and it is due to a respiratory change, so Pco2 must have increased. Respiratory acidosis results from reduced ventilation and ∴ retention of CO2.

Acute: drugs that depress the medullary respiratory centres, such as barbiturates and opiates. Also caused by bstructions of major airways.

Chronic: lung disease e.g. bronchitis, emphysema, asthma.

Question 15

What is the bodies response to respiratory acidosis?

Answer 15

Response: Need to protect pH so need to ↑ [HCO3-].

Question 16

What causes respiratory alkalosis?

Answer 16

Respiratory Alkalosis: Alkalosis of respiratory origin so must be due to a fall in Pco2 and this can only occur through increased ventilation and CO2 blow-off.

Acute: voluntary hyperventilation, aspirin, first ascent to altitude

Chronic: long term residence at altitude, ↓ Po2 to < 60mmHg (8kPa) stimulates peripheral chemoreceptors to increase ventilation.

Question 17

List some acute causes of respiratory acidosis

Answer 17

Acute: drugs that depress the medullary respiratory centres, such as barbiturates and opiates. Also caused by obstructions of major airways.

Anything that can cause sudden decrease in ventilation

Question 18

List some chronic causes of respiratory acidosis

Answer 18

Chronic: lung disease e.g. bronchitis, emphysema, asthma.

Question 19

List some acute causes of respiratory alkalosis

Answer 19

Acute: voluntary hyperventilation, aspirin, first ascent to altitude

Question 20

List some chronic causes of respiratory alkalosis

Answer 20

Chronic: long term residence at altitude, ↓ Po2 to < 60mmHg (8kPa) stimulates peripheral chemoreceptors to increase ventilation.

Question 21

What acid-base balance disorder can be caused by aspirin use?

Answer 21

Acute respiratory alkalosis

Question 22

What acid-base balance disorder can be caused by barbiturate or opioid use?

Answer 22

Acute respiratory acidosis

Question 23

What acid-base balance disorder can be caused by chronic lung diseases like asthma and bronchitis?

Answer 23

chronic respiratory acidosis

Question 24

What acid-base balance disorder is associated with changes in altitude?

Answer 24

Respiratory alkalosis (can be acute after sudden change or chronic following long term residence at altitude)

Question 25

How does the body respond to respiratory alkalosis?

Answer 25

* To protect pH, [ HCO3-] should ↓ | * Alkaline conditions are dealt with by the HCO3- reabsorptive mechanism.

Question 26

List some causes of metabolic acidosis

Answer 26

Diabetic ketoacidosis Renal failure - fail to excrete H+ adequately Loss of HCO3 in diarrhoea

Question 27

What is Kussmaul breathing and what is it a sign of?

Answer 27

In metabolic acidosis, the body stimulates ventilation so that Pco2 falls. The ↑ in ventilation is in depth rather than rate, may be very striking, reaching a maximum of 30 l/min cf normal 5-6 l/min when the arterial pH falls to 7.0. This degree of hyperventilation = Kussmaul breathing = an established clinical sign of renal failure or diabetic ketoacidosis. Very serious.

Question 28

What is a clinical sign for renal failure or diabetic ketoacidosis?

Answer 28

Kussmaul breathing

Question 29

List some causes of metabolic alkalosis

Answer 29

↑ H+ ion loss- vomiting loss of gastric secretions ↑ renal H+ loss - aldosterone excess, excess liquorice ingestion Excess administration of HCO3- is unlikely to produce a metabolic alkalosis in subjects with normal renal function, but may do so if renal function impaired. Massive blood transfusions can lead to metabolic alkalosis because bank blood contains citrate to prevent coagulation, which is converted to HCO3-, but need at least 8 units to have this effect

Question 30

What acid base balance disorder is associated with liquorice intake?

Answer 30

Metabolic acidosis

Question 31

What acid base balance disorder is associated with diarrhoea?

Answer 31

metabolic acidosis

Question 32

What acid base balance disorder is associated with vomiting?

Answer 32

metabolic alkalosis

Question 33

How can blood transfusions lead to metabolic alkalosis

Answer 33

Massive blood transfusions can lead to metabolic alkalosis because bank blood contains citrate to prevent coagulation, which is converted to HCO3-, but need at least 8 units to have this effect

Question 34

What is the anion gap?

Answer 34

* Anion Gap - The difference between the sum of the principal cations ( Na+ and K+) and the principal anions in the plasma (Cl- and HCO3- ). * Normally 14-18mmoles/L

Question 35

The following blood gas values were seen in a patient. Which simple Acid/Base Disturbance has he got? pH = 7.32, [HCO-3]= 15 mM, PCO2 = 30mmHg (4kPa) 1. Metabolic Acidosis 2. Metabolic Alkalosis 3. Respiratory Acidosis (acute) 4. Respiratory Acidosis (chronic) 5. Respiratory Alkalosis (acute) 6. Respiratory Alkalosis (chronic)

Answer 35

Answer must be 1 - metabolic acidosis. The pH has fallen, which means the condition is an acidosis. As HCO3 and PCO2 have fallen, we can see that the problem must be metabolic (a fall in PCO2 would not give an acidosis so this must be the compensatory mechanism) which is caused by the fall in HCO3.

Question 36

The following blood gas values were seen in a patient. Which simple Acid/Base Disturbance has he got? pH = 7.32, [HCO-3]= 33 mM, PCO2 = 60mmHg (8kPa) 1. Metabolic Acidosis 2. Metabolic Alkalosis 3. Respiratory Acidosis (acute) 4. Respiratory Acidosis (chronic) 5. Respiratory Alkalosis (acute) 6. Respiratory Alkalosis (chronic)

Answer 36

Answer is 4 - chronic respiratory acidosis Here we can see by the pH that this must be acidosis. We can also see by the fact that both PCO2 and HCO3 have increased that the cause must be respiratory (an increase in HCO3 would cause an alkalosis so this must be a compensatory mechanism). We can further see HCO3 is 33, which is well above 27mM, meaning the condition is an example of chronic respiratory acidosis.

Question 37

The following blood gas values were seen in a patient. Which simple Acid/Base Disturbance has he got? pH = 7.45, [HCO-3] = 42 mM, PCO2 = 50mmHg (6.7kPa) 1. Metabolic Acidosis 2. Metabolic Alkalosis 3. Respiratory Acidosis (acute) 4. Respiratory Acidosis (chronic) 5. Respiratory Alkalosis (acute) 6. Respiratory Alkalosis (chronic)

Answer 37

Answer is 2 - metabolic alkalosis Must be alkalosis as pH has increased. As both HCO3 and PCO2 have increased, it must be the HCO3 that has caused this, so it is due to a metabolic cause.

Question 38

The following blood gas values were seen in a patient. Which simple Acid/Base Disturbance has he got? pH = 7.45, [HCO-3]= 21 mM, PCO2 = 30mmHg (4kPa) 1. Metabolic Acidosis 2. Metabolic Alkalosis 3. Respiratory Acidosis (acute) 4. Respiratory Acidosis (chronic) 5. Respiratory Alkalosis (acute) 6. Respiratory Alkalosis (chronic)

Answer 38

Answer is 5 - acute respiratory alkalosis Results suggest alkalosis. As both HCO3 and PCO2 are decreased, we know it must be a respiratory cause. As HCO3 is only down 1mM, it is acute.

Question 39

A 75-year-old man has the following blood gas values: pH = 7.31, PCO2 = 7.7.kPa, (58mmHg), [HCO3-] =36mmoles/l. 1. It is likely that he has renal disease. 2. He may have an acute respiratory infection. 3. It is possible that he may have chronic bronchitis. 4. There will be a decrease in his excretion of ammonium ions. 5. His plasma potassium will be reduced.

Answer 39

Suspected respiratory acidosis, potentially chronic as HCO3 values are so high Only cause of chronic respiratory acidosis suggested here is 3

Question 40

The following acid/base values were obtained: pH = 7.25, [HCO3-] = 12mmoles/l, PCO2 = 3.3kPa (25mmHg) a. They are indicative of a respiratory acidosis b. The reduction in Pco2 is a result of under-breathing c. The subject has probably been taking bicarbonate of soda d. It could be related to impaired renal function e. The subject may have been vomiting very badly

Answer 40

Results suggest metabolic acidosis. Only cause possible here is D - impaired renal function Vomiting causes loss of HCL, resulting in metabolic alkalosis. Taking bicarbonate would also cause alkalosis

Question 41

The following results are obtained from a urine sample from a 25-year-old dancer who has collapsed: pH = 4.5 Osmolality = 285mOsmoles/kg of urine Absence of glucose in urine Which of the following is the most likely to explain the analysis? 1. She has been taking aspirin 2. She is diabetic 3. She has been eating too many bananas 4. She has been taking a diuretic to keep her weight down 5. She has been eating too much liquorice

Answer 41

Diabetes can be immediately discounted, as there is no glucose in the urine. Aspirin use causes acute respiratory alkalosis Liquorice causes metabolic alkalosis. As the urine is isotonic to normal plasma osmolality, there must have been damage to the loop of henle system. The only option here that would result in this is the taking of a diuretic. People can take these as a means of weight loss. Normal urine osmolality is 500-800mOsm/L.

Question 42

A boxer has been taking a loop diuretic to make weight for a world championship bout. He needed to lose 1kg in order to make the featherweight class. He achieves a 1.2kg loss. Where does the weight come from? 1. 400mls from ECF and 800mls from the ICF 2. 600mls from ECF and 600mls from ICF 3. Protein breakdown 4. 1200mls from ECF 5. Breakdown of fat

Answer 42

4. 1200mls from ECF If we lose isotonic fluid, it must all be from ECF. Loop diuretics prevent the reabsorption of water back into plasma.