Acid Base Balance

Question 1

Describe the pH ranges in the body

Answer 1

• pH 7.35-7.45 is NORMAL
• pH < 7.35 is ACIDOSIS
• pH > 7.45 is ALKALOSIS

Question 2

Why is pH balance important?

Answer 2

• Vital for bodily and enzyme functions
• Affects hormone and electrolyte balance
• Small changes in pH have severe symptoms

Question 3

How does metabolism produce H+ and how is it excreted?

Answer 3

• Digestion and oxidation of food
• Usually excreted in urine - COMPLICATIONS in renal problems

Question 4

How does metabolism produce CO2 and how is it excreted?

Answer 4

• Cellular respiration
• Converted to carbonic acid
• So excreted by lungs - COMPLICATIONS in respiratory problems

Question 5

What are buffers?

Answer 5

• Solution of weak acid/base and its salt
• Resist changes in pH
• Function almost instantaneously

Question 6

Give examples of buffer systems in the body.

Answer 6

• BICARBONATE SYSTEM - CONVERSION TO CARBONIC ACID
• Cellular proteins and phosphate
• Haemoglobin

Question 7

Describe the first and second lines of defence against pH shifts.

Answer 7

• FIRST LINE - Chemical buffers - Bicarbonate/phosphate buffers
• SECOND LINE - Physiological buffers - respiratory/renal mechanisms

Question 8

What is the reaction that forms the basis of the bicarbonate buffer system?

Answer 8

CO2 + H2O ⇄ H2CO3⇄ HCO3- + H+

Question 9

Describe what occurs in the bicarbonate buffer systems at high and low pHs.

Answer 9

• LOW - HCO3- absorb H+ from ECF and converted to carbonic acid then CO2. Regulated by lungs.
• HIGH - Dissociation of carbonic acid to H+ and HCO3-. Regulated by kidneys

Question 10

What is the Henderson Hasselbach equation?

Answer 10

pH = pKa + log ( [A-]/[HA])
- [A-] = concentration of conjugate base i.e HCO3-
- [HA] = concentration of acid i.e H2CO3

Question 11

How can the Henderson Hasselbach equation be rewritten for the bicarbonate buffer system?

Answer 11

pH = pK + log ([HCO3-]/ S x pCO2)
- pK is dissociation constant of H2CO3 (6.1)
- [HCO3-] - concentration of bicarbonate in mmol/litre
- S - solubility coefficient for CO2 (0.23 if measurede in kPa/0.03 for mmHg)

Question 12

What is the rewritten Henderson Hasselbach equation for the bicarbonate buffer system used for?

Answer 12

• Calculate blood pH changes with [HCO3-] and pCO2

Question 13

What does the ABG measure?

Answer 13

• pH
• pCO2
• HCO3-
• pO2

Question 14

What are the normal ranges for pH, pCO2 and HCO3- and what occurs if ABG values are outside of this range?

Answer 14

• pH 7.35-7.45 –> ACIDOSIS/ALKALOSIS
• pCO2 4.7-6.0 kPa (35-39 mmHg) –> RESPIRATORY ACIDOSIS/ALKALOSIS
• HCO3- 21-29 mmol/litre –> METABOLIC ACIDOSIS/ALKALOSIS

Question 15

How does H+ regulation by the lungs work?

Answer 15

• Respiratory centre in brain responds to pH and CO2 changes in minutes/hours
• Occurs by changing rate of ventilation
• ACIDOSIS - hyperventilation to expel CO2
• ALKALOSIS - hypoventilation to increase CO2

Question 16

Describe H+ regulation by kidneys.

Answer 16

• Most effective pH regulator
• Excrete large amounts of acid/base
• Slower compensation than lungs. Takes hours-days

Question 17

What occurs during H+ regulation by the kidneys during acidosis?

Answer 17

• Greater excretion of H+ into urine
• Excreted as NH4+ and H2PO4-
• HCO3- reabsorption into blood

Question 18

What occurs during H+ regulation by the kidneys during alkalosis?

Answer 18

• Greater excretion of HCO3-
• Greater reabsorption of H+