Acid-Base Balance

Question 1

What are the major products of metabolism

Answer 1

Acids - Carbon dioxide and organic acids

Question 2

What is the normal pH range and which two systems maintain it

Answer 2

7. 35 - 7.45
   1) initially by a buffer system in the blood
   2) excess acid excreted by lungs and kidneys

Question 3

What is the definition of acidosis and alkalosis

Answer 3

Acidosis - abnormal decrease in arterial pH (<7.35)

Alkalosis - abnormal increase in arterial pH (>7.45)

Question 4

What are all buffering systems composed of

Answer 4

A weak acid and it’s conjugate base

HB (weak acid) <=======> [H+] + [B-] (conjugate base)

Question 5

How does a buffering system resist pH changes

Answer 5

Addition of acid reacts with the free base ions, moving the equation to the left in order to provide replacement base ions for the ones used to neutralise the acid
Addition of alkali reacts with the free hydrogen ions, moving the reaction to the right to provide replacement hydrogen ions for the ones used to neutralise the base

Question 6

Which buffer systems are most common in the body (intracellular and extracellular)

Answer 6

Intracellular - proteins and phosphate act as buffers

Extracellular - bicarbonate acts as a buffer

Question 7

Describe the buffer system of carbon dioxide and bicarbonate in the blood.

Answer 7

[CO2) + [H20] <===> [H2CO3] <===> [H+] + [HCO3-]

Question 8

What are the main methods of acid/alkali excretion in the body

Answer 8

Excretion via the lungs - rapid response

Excretion via the kidneys - delayed response

Question 9

Describe how the lungs affect pH balance in the blood

Answer 9

The brainstem respiratory centre detects hydrogen ion levels in the blood
- high levels of H+ ions cause an increase in respiratory rate, so that carbon dioxide (acid) is removed from the body
Dysfunction can cause respiratory acidosis (CO2 retention) or respiratory alkalosis (over-excretion)

Question 10

Describe how the kidneys affect pH balance in the blood

Answer 10

Acid is normally excreted in the urine (urinary pH is normally 6.0)
- hydrogen ions are secreted into the urine by the distal nephron
Bicarbonate ions are generated by the renal tubules and secreted into the blood

Question 11

What is respiratory acidosis

Answer 11

Primary disturbance of increased pCO2, leading to decreased pH and compensatory rise in bicarbonate

Question 12

Causes of respiratory acidosis

Answer 12

Depression of respiratory centres
- CVA
- cerebral tumour
- drugs (opiates/sedatives)
- encephalitis
Decreased chest wall movement
- trauma/surgery
- neuromuscular disorder (e.g. myasthenia gravis)
- ankylosing spondylitis
Pulmonary disease (causing type 2 respiratory failure)
- COPD
- pneumonia

Question 13

What is respiratory alkalosis

Answer 13

Primary disturbance of decreased pCO2, leading to increase in pH and compensatory decrease in bicarbonate

Question 14

Causes of respiratory alkalosis

Answer 14

Stimulation of respiratory centre
- CNS disease (e.g. CVA or encephalitis)
- hypermetabolic state (e.g. sepsis, fever and hyperthyroidism)
- exercise  
- hypoxia (e.g. pneumonia)
Excess ventilation
- anxiety
- medications (e.g. aspirin)

Question 15

What is metabolic acidosis

Answer 15

Primary disturbance of decreased bicarbonate or increased hydrogen ions, leading to decreased pH and compensatory decrease in pCO2

Question 16

Causes of metabolic acidosis

Answer 16

Increased anion gap (another source of acid production)
- renal glomerular failure
- overdose (e.g. salicylate)
- causes of respiratory alkalosis
- lactic acidosis (inadequate tissue perfusion e.g. hypovolaemia, ischaemic gut)
- ketoacidosis - diabetes or alcohol
- renal tubular acidosis
Normal anion gap
- excess acid intake e.g. parenteral nutrition

Question 17

What is metabolic alkalosis

Answer 17

Primary disturbance of increased bicarbonate or decreased hydrogen ions, leading to increased pH and compensatory rise in pCO2

Question 18

Causes of metabolic alkalosis

Answer 18

Excess alkali intake - e.g. alkali abuse or overtreatment of acidosis 
Excessive loss of acid - e.g. vomiting
Increased urinary acidification 
- diuretics
- excess aldosterone production
- hypokalaemia

Question 19

What is the anion gap

Answer 19

The difference between the cations (sodium and potassium) and anions (chloride and bicarbonate) in the blood
Normally 10-16mmol/l

Question 20

How to calculate the anion gap

Answer 20

Total sum of cations - total sum of anions

Question 21

Why does the anion gap exist when total number of cations and anions should be equal

Answer 21

The difference equates to the anions that aren’t measured - proteins, phosphate, sulphates and organic acids