8 - Chemical Control of Breathing

Question 1

What is the difference between hypoxia and hypoxaemia?

Answer 1

Hypoxaemia is a low O2 in the blood, due to dissociation curve body can tolerate small changes

Question 2

What are normal values for:

• PaCO₂
• PaO₂
• Bicarbonate
• pH

Answer 2

Question 3

What is hypo and hyper capnia?

Answer 3

Hyper = increase pCO₂

Hypo = decreased pCO₂

Small changes in pCO₂ can have big effects on pH

Question 4

During exercise pCO₂ increases and pO₂ decreases, how is this overcome?

Answer 4

Question 5

What happens to partial pressures during hyperventilation and what can this lead to?

Answer 5

• Can lead to rise in pH as alveolar pCO₂ decreases leading ro respiratory alkalosis

- This is corrected by kidneys secreting more HCO₃- returning the ratio of HCO₃ to CO₂ to normal but buffer base concentration is reduced –> compensated respiratory alkalosis (2-3 days)

Question 6

What happens to partial pressures during hypoventilation and what can this lead to?

Answer 6

• Dissolved CO₂ rises so plasma pH falls so respiratory acidosis

- Kidney reduces secretion of HCO₃- so compensated respiratory acidosis. Takes 2-3 days

Question 7

When does hypocapnia occur?

Answer 7

Question 8

When does hypoxia start to occur?

Answer 8

• 100% saturation at 8kPa so pO₂ can fall quite a lot before hypoxia

Question 9

What happens if the pH of blood falls below 7 or rises above 7.6?

Answer 9

• Below 7 enzymes are denatured
• Above 7.6 free calcium concentration drops so tetany

Question 10

How does metabolic acidosis occur and how does the body compensate for this?

Answer 10

• Tissues produce acid so HCO₃- reacts with them lowering pH
• Can increase ventilation to lower pCO₂ to restore pH
• Buffer base is deplete until corrected by the kidney

Question 11

How does metabolic alkalosis occur and how does the body compensate for this?

Answer 11

• If plasma HCO₃- rises, e.g after losing protons in vomiting or blood loss, the pH falls
• Can only be slighlty compensated by lowering breathing as don’t want to risk hypoxia so mainly corrected by kidneys

Question 12

What is the difference between correcting respiratory acidosis/alkalosis to metabolic acidosis/alkalosis?

Answer 12

• Respiratory is mainly restored by the kidney altering HCO₃- levels
• Metabolic is usually by changes in ventilation rate and therefore alveolar pCO₂

Question 13

What factors affect changes in ventilation rate the most?

Answer 13

These changes are detected by chemoreceptors and send impulses to the respiratory centre in the brain stem to alter rate and depth of respiration

Main things, drop in ph and small increase in co2
o2 doesn’t change unless DRASTIC

Question 14

What are the two types of chemoreceptors and what changes do they detect?

Answer 14

H+ means changes in pH

Question 15

Where are peripheral chemoreceptors and what is their response when levels of CO_2, O₂ and H+ change?

Answer 15

- Carotid and aortic bodies

- O₂: large falls in pO₂ cause an increase in tidal volume and resp rate. Also direct more blood to brain and kidney and increase pumping of heart. Do not adapt to chronic hypoxia

- CO₂: Not very sensitive, only when large changes. Not crucial for controlling resp but when they do respond they respond quickly to large changes in pCO₂

• pH: carotid bodies detect low pH and increase tidal volume and resp rate

Question 16

Where are central chemoreceptors and what is their response when levels in the blood change?

Answer 16

- Ventral surface of the medulla, exposed to CSF

• Respond to drop in pH, which occurs when high pCO₂ as the blood brain barrier only allows CO₂ across not HCO₃ so pH is affected directly by pCO₂
• Impulses to brain are negative feedback so if low pH in CSF means high pCO₂ SO tidal volume and ventilation rate increase

Question 17

How is bicarbonate concentration in the CSF controlled if bicarbonate cannot pass the blood brain barrier?

Answer 17

- Choroid Plexus pumps HCO₃- in and out of CSF

• This plexus is long term solution if issues with hypoxia or lung disease

Question 18

What happens to chemoreceptors in persistent hypoxia and hypercapnia?

Answer 18

- Hypoxia detected by peripheral so will increase ventilation but CO₂ will fall so will decrease ventilation so CSF compensates for changed pCO₂ by use of choroid plexus and accepting new pCO₂ as normal in central chemoreceptors

- Hypercapnia with hypoxia will lead to respiratory acidosis and will damage the neurons. Central receptors accept the CO₂ as normal and make changes to CSF using HCO₃- and H+

Question 19

What is the difference in the response to falling pO₂ and falling pCO₂?

Answer 19

O₂: increased ventilation only when fall is large. Respiratory drive lasts as long as the hypoxia so chemoreceptors do not reset

CO₂: small changes in levels lead to rapid changes in ventilation. Central chemoreceptors reset central chemoreceptors so if have hypoxic episode in chronic hypercapnia this will be detected and driven by peripheral receptors

Question 20

What is the most important stimulus in minute to minute control of ventilation in a healthy person?

Answer 20

Effect of a change in arterial pCO₂ and therefore pH on central chemoreceptors