Acid Base Balance

Question 1

What is normal H+ conc in ECF?

Answer 1

35-45 nmol/l

Question 2

What is normal pH and how do you calculate it?

Answer 2

pH 7.35 -7.46
pH = log 1/ [H+]

Question 3

How is H+ made and excreted?

Answer 3

Metabolism of proteins, carbohydrates and fats produce carbon dioxide, water and hydrogen ions

Production of H+ (50-100 mmol/day)

Excretion by the kidney

Question 4

Where do H+ ions travel?

Answer 4

Hydrogen ion produced in the tissues is transported via the circulation to the kidneys where it is excreted

Question 5

What is buffering of H+ in the body?

Answer 5

One can add 14 mmol of hydrogen ion per litre of body water with an increase in hydrogen ion concentration of only 36 nmol/L.

ECF buffering of hydrogen is at the expense of bicarbonate
Bicarbonate buffering of H+ is only effective in the short term.
To maintain normal homeostasis the kidney needs to excrete hydrogen ions and regenerate bicarbonate.

Question 6

How is bicarbonate handled in the nephron?

Answer 6

Bicarbonate reabsorption in the proximal tubule

H+ excretion & bicarbonate regeneration

Question 7

How is CO2 handled in the body?

Answer 7

Production of CO2 (20,000-25,000 mmol/day)
Excreted by lungs

Question 8

How does respiration control CO2?

Answer 8

Respiration is controlled by chemoreceptors in the hypothalamic respiratory centre

In health any increase in CO2 stimulates respiration thus tending to maintain a stable concentration of CO2

20-25,000 mmol/day of CO2 is excreted through the lungs in expired air.

In health CO2 produced in the tissues is transported via the circulation to the lungs where it is excreted

Question 9

How does CO2 make H+?

Answer 9

By reacting with water to make bicarbonate and hydrogen ions (HHb)

Question 10

How do the kidneys and lungs control acid base balance?

Answer 10

Kidneys- control bicarbonate
Lungs- control CO2

Question 11

How do you calculate H+ using CO2 and bicarb?

Answer 11

k x (CO2 conc/ HCO3 conc)

Question 12

What are the causes of metabolic acidosis?

Answer 12

1. Increased H+ production e.g. diabetic ketoacidosis
2. Decreased H+ excretion e.g. Renal tubular acidosis
3. Bicarbonate loss e.g. intestinal fistula

Question 13

What is respiratory compensation in metabolic acidosis?

Answer 13

This stimulates the respiratory centre
Identified by a fall in pCO2
H+ returns towards normal

Question 14

What are the causes of acute respiratory acidosis?

Answer 14

Primary abnormality is increased CO2 producing increased H+ (decreased pH) and a slight increase in bicarbonate (2-4 mmol/L).

May be due to:
Decreased Ventilation
Poor Lung Perfusion
Impaired Gas Exchange

Question 15

What happens in chronic respiratory acidosis?

Answer 15

Over the course of a few days this leads to increased renal excretion of H+ combined with generation of bicarbonate.
H+ may return to near normal but pCO2 and bicarbonate remain elevated

Question 16

What are the causes of metabolic alkalosis?

Answer 16

H+ loss (e.g. pyloric stenosis)
Hypokalaemia
Ingestion of Bicarbonate

Question 17

What is the compensation of metabolic alkalosis?

Answer 17

This tends to inhibit the respiratory centre
Identified by a rise in pCO2
H+ returns towards normal

Question 18

What are the causes of respiratory alkalosis?

Answer 18

Due to Hyperventilation
Voluntary
Artificial ventilation
Stimulation of respiratory centre

Question 19

What happens in chronic respiratory alkalosis?

Answer 19

If prolonged this leads to decreased renal excretion of H+ and less bicarbonate generation
H+ may return to near normal but pCO2 and bicarbonate remain low

Question 20

How do you assess an ABG?

Answer 20

H+/pH tells us whether there is an overt acidosis or alkalosis
pCO2 tells us whether there is a respiratory disturbance (primary or secondary)
pO2 does not directly affect acid-base status but gives an indication of respiratory function and tissue oxygenation
Note: Bicarbonate predominantly reflects metabolic disturbances but is also affected by respiratory disturbances

Question 21

Abnormality?

Answer 21

Metabolic Acidosis with partial compensation

Question 22

64 year old female

3 week history of intermittent vomiting

abdominal pain

weight loss

O/E

Dehydrated

Jaundiced

Hypotensive

Oliguric

Urea 28.1 mmol/l (2.5-8.0)

Creatinine 387 mmol/l (60-125)

Sodium 129 mmol/l (135-145)

Potassium 1.6 mmol/l (3.5-5.5)

Bicarbonate 56 mmol/l (22-30)

Total protein 89 g/l (64-83)

Gas abnormality? Diagnosis?

Answer 22

Metabolic alkalosis with partial respiratory compensation

Hyperchloraemic hypokalaemia

Diagnosis: pyloric stenosis

Question 23

How does vomiting cause a metabolic alkalosis?

Answer 23

Loss of HCl in vomit produces a metabolic alkalosis

(Low H+, high bicarbonate)

Loss of fluid produces dehydration

(Raised urea, creatinine and total protein)

Dehydration stimulates renin/angiotensin/aldosterone mechanism

Low potassium as it is lost in vomit and urine

Question 24

pH 7. 55 (7.35-7.45)

H+ 28 nmol/l (35-46)

pCO2 3.0 kPa (4.7-6.0)

pO2 14.4 kPa (10.0-13.3)

Bicarbonate 20 mmol/l (22-30)

Abnormality?

Answer 24

Respiratory alkalosis (with little compensation)

Question 25

pH 7. 41 (7.35-7.45)

H+ 39 nmol/l (35-46)

pCO2 10.4 kPa (4.7-6.0)

pO2 7.8 kPa (10.0-13.3)

Bicarbonate 47 mmol/l (22-30)

Abnormality?

Answer 25

Compensated Resp acidosis/ met alkalosis

Resp acidosis - e.g. COPD

Met Alkalosis- e.g. hypokalaemia, loss of H+, too many rennies ;)

Question 26

A young woman was admitted to hospital 8 hours after she had taken an overdose of aspirin.

Arterial blood:

pH 7. 46 (7.35-7.45)

H+ 35 nmol/l (35-46)

pCO2 2.0 kPa (4.7-6.0)

pO2 17.8 kPa (10.0-13.3)

Bicarbonate 10 mmol/l (22-30)

Abnormality?

Answer 26

Respiratory alkalosis with partial compensation

Question 27

Arterial blood:

pH 6. 93 (7.35-7.45)

H+ 116 nmol/l (35-46)

pCO2 9.7 kPa (4.7-6.0)

pO2 65.8 kPa (10.0-13.3)

Bicarbonate 15 mmol/l (22-30)

Abnormality?

Answer 27

Mixed respiratory and metabolic acidosis