L5 - Acid Base Balance

Question 1

Why is acid-base balance important?

Answer 1

A small change in pH can have a large effect on body function
Log scale – 1 unit change = x10 change in [H+]

Question 2

What can fluctuations in plasma H+ concentrations have an effect on?

Answer 2

Excitability of muscle and nerve
Enzyme activities
K+ levels

Question 3

pH at venous end of tubule?

Answer 3

7.35

Question 4

pH at arterial end of tubule?

Answer 4

7.45

Question 5

What are the different pH value around the body?

Answer 5

Gastric secretion – 0.7
Cerebrospinal fluid – 7.3
Pancreatic secretions – 8.1
Urine – 5.4

Question 6

What are the different sources of acid and alkali?

Answer 6

Metabolism – large amounts of acid produced
- 15 moles/ day CO¬¬2
- 40 mmol/day H+ (western diet has an excess of H+)
Diet – alkali and acid in food
- 20 mmol/day H+
- Lose 10 mmol/day OH-
Overall excess 70 mmol/day H+

Question 7

What are the 3 different systems involved in acid-base balance?

Answer 7

Blood and tissue buffers (seconds)
Respiration (minutes)
Renal (hours) – only mechanism for the extrusion of acid/alkali

Question 8

Where are buffers present throughout the body?

Answer 8

Blood – plasma and red blood cells
Extracellular and intracellular fluid
Urine
E.g. haemoglobin, inorganic phosphate, weak acids/bases, HCO¬¬-3

Question 9

Carbonic acid/bicarbonate ECF buffer

Answer 9

CO2 + H2O H2CO3 H+ + HCO3

Question 10

What is the Henderson-Hasselbach equarion

Answer 10

pH = pK + log [HCO3]/[H2CO3]
pK is a constant of 6.1 at 37oC
Ratio of [HCO3]/[CO2] = 20:1
6.1+log 20 = 6.1+1.3 = 7.4

Question 11

What does chemical control of ventilation control?

Answer 11

Controls blood gas composition – PO2, PCO2, pH

Question 12

How is ventilation chemically controlled?

Answer 12

Peripheral and central chemoreceptors - negative feedback system
Stimulated by hypoxia, hypercapnia, acidosis – all increase ventilation
- Increase in PO2
- Decrease in PCO2
- Increase in pH

Question 13

What are peripheral chemoreceptors triggered by?

Answer 13

Mainly stimulated by decreased PO2 – hypoxia

Question 14

What do peripheral chemoreceptors do once triggered?

Answer 14

When activated send signals through the sinus, gloss-pharyngeal and the vagus nerve to the medulla

Question 15

What is the structure of carotid receptors?

Answer 15

Small – 2mg
High blood flow (40X higher than the brain per unit mass)
High metabolic rate
Glomus cells
Parasympathetic/sympathetic cells – regulate blood flow Aortic receptors smaller and less well studied

Question 16

What do glomus cells do?

Answer 16

Fire action potentials when O2 drops
Neural phenotype
Type II – supporting

Question 17

What is the mechanism of glomus cells?

Answer 17

1. Inhibition of BK K channels – decreased PO2, increased PCO2, decreased pH
2. Depolarisation and action potential firing
3. Ca channels open increasing intracellular Ca
4. Ach, dopamine, 5-HT, substance P, noradrenaline released
   - Some SIDS babies have higher concentration of carotid body noradrenaline and dopamine
5. Afferent nerve fibre stimulation

Question 18

Sensitivity to PO2 changes with?

Answer 18

Acid/base status

Higher pH = less action potentials

Question 19

Sensitivity to PCO2 changes with?

Answer 19

pH

Higher pH = less action potentials

Question 20

What are central chemoreceptors stimulated by?

Answer 20

Increased CO2 - hypercapnia
Change in PCO2 from 40 - 45 mmHg - doubles ventilation
Same increase in ventilation only seen with 50% fall in PO2

Question 21

What factor is actually measured that triggers central chemoreceptors?

Answer 21

pH

Perfused cerebral ventricles with acidic solution – observed hyperventilation

Question 22

Where are central chemoreceptors located?

Answer 22

Within the brain parenchyma and bathed in brain extracellular fluid
Separated from arterial blood by the blood brain barrier
- BBB has poor ion permeability
- H+ and bicarbonate can’t cross
- CO2 can cross
Increase in arterial PCO2 = increase in brain extracellular fluid PCO2

Question 23

Where are central chemoreceptor neurones located?

Answer 23

Ventrolateral medulla and other brainstem nuclei

Question 24

What are the two types of central chemoreceptor neurone?

Answer 24

Acid activated – serotonin

Acid inhibited – GABA

Question 25

What are the characteristics of central chemoreceptor neurones?

Answer 25

Less non-bicarbonate buffering power (fewer proteins) in the BECF, so larger fall in pH
- Some long term compensation via transport of HCO-3 from blood
Poor ion permeability
- Metabolic disorders change BECF pH by 35% of that observed with respiratory disorders for the same change in blood pH
- Respiratory disorders are better at stimulating the chemoreceptors

Question 26

Do peripheral and central chemoreceptors respond at the same time?

Answer 26

Yes - integrated responses

Question 27

How do peripheral and central chemoreceptors respond to respiratory acidosis?

Answer 27

Both central and peripheral chemoreceptors
Normoxic central 65-80%
Peripheral chemoreceptors respond faster
As PO2 falls response to PCO2 enhanced

Question 28

How do peripheral and central chemoreceptors respond to metabolic acidosis?

Answer 28

Severe cases lead to hyperventilation (Kussmaul breathing)
Decreased PCO2
Peripheral chemoreceptors - acute response role
Central chemoreceptors - longer term role

Question 29

What are the changes in acid-base balance when you breathe faster?

Answer 29

Decrease in CO2 = decrease in [H+] = increase in pH

Increased pH of body fluids = chemoreceptors = decreased breathing rate

Question 30

What are the changes in acid-base balance when you breathe slower?

Answer 30

Increase in CO2 = increase in [H+] = decrease in pH

Decreased pH of body fluids = chemoreceptors = increased breathing rate