8 Chemical Control of Breathing

Question 1

What is hypoxaemia?

Answer 1

Fall in arterial pO₂

Question 2

What do hypocapnia and hypercapnia mean?

Answer 2

• Hypercapnia: Rise in alveolar (hence arterial) pCO₂
• Hypocapnia: Fall in alveolar (hence arterial) pCO₂

Question 3

Hypoventilation can result in respiratory acidosis. Explain why?

Answer 3

pH has decreased and ventilation rate decrease

Removal of CO₂ from lungs= less rapid

pCO₂ in alveoli has increased

[dissolved CO₂] increases relative to [HCO₃^-] so ratio not 20:1

(Opposite for hyperventilation)

Question 4

How do the kidneys respond to persistent respiratory acidosis?

Answer 4

Reduce excretion of HCO₃^-

(Opposite for hyperventilation)

Question 5

What happens to the HCO₃^- concentration in the blood if there is excess metabolic production of acid (eg diabetic ketoacidosis)? What is the consequence of this?

Answer 5

Excess acid- buffered by HCO₃^- so used up–> concentration decreases

pH falls- metabolic acidosis

Question 6

What are the 2 ways in which disturbances in the pH of the blood can be countered?

Answer 6

• Alteration of alveolar pCO₂ - alter ventilation rate
• Corrected by kidneys (excretion of HCO₃^-)

Question 7

What effect will these changes have on ventilation rate?

Drop in pO₂

Large increase in pCO₂

Drop in pH

Answer 7

Question 8

How are the ventilation changes brought about (as a result of changes in pO₂/pCO₂/pH)?

Answer 8

Chemoreceptors- detect changes in arterial pO₂/pCO₂/pH

Send impulses to respiratory centres in brainstem–> modulate rate and depth of ventilation

Question 9

What are the 2 types of chemoreceptors that respond to changes in blood pO2/pCO2/pH?

Answer 9

Peripheral and central

Question 10

What can peripheral chemoreceptors detecting changes in pO2/pCO2/pH be found and what nerves carry the impulses to the brainstem respiratory centre?

Answer 10

• Carotid bodies
  
  • Glossopharyngeal nerves (IX)
• Aortic bodies
  
  • Vagus nerves (X)

Question 11

What changes in the blood pO₂ do peripheral chemoreceptors detect and what changes do the bring about? (Carotid and aortic bodies)

Answer 11

• Changes detected: Respond to large falls in pO₂
• Changes made: Increase tidal volume, rate of respiration, direct more blood to brain and kidneys, increased pumping of blood by the heart

(Respiratory drive from hypoxia remains in persitent hypoxia)

Question 12

What changes in the blood pCO₂ and pH ​do peripheral chemoreceptors detect and what changes do the bring about? (Carotid and aortic bodies)

Answer 12

• Changes detected:
  
  • Directly activated by changes in pH
  • (respond to LARGE changes in pCO₂)
• Changes made:
  
  • Low pH: increased respiratory rate and tidal volume

Question 13

Where can central chemoreceptors be found?

Answer 13

Ventral surface of medulla

Exposed to cerebro-spinal fluid

Question 14

What do the central chemoreceptors respond to and what changes do they bring about?

Answer 14

Respond to: rise in arterial pCO₂ (pH decrease)

Changes: impulses from chemoreceptors travel to brainstem respiratory centres - change respiration

Question 15

Why is CSF pH corrected more quickly than blood pH?

Answer 15

Small volume of CSF

Lower protein content- overall buffering capacity is less

Question 16

CSF is separted from the blood by a blood-brain barrier. How is its pH therefore determined?

Answer 16

Free passage of CO₂ across blood-brain barrier (NOT HCO₃^-)

pH determined by its own HCO₃^- conc to dissolved CO₂ buffer system

[HCO₃^-] determined by activity of choroid plexus cells in CSF

Question 17

What happens to the central chemoreceptors if pCO₂ remains elevated for any length of time? (eg with lung disease)

Answer 17

Choroid plexus cells - reset central chemoreceptors–> no longer sensitive to existing pCO₂

_{(So in long standing hypocapnia- drive from increased ventilation is due to hypoxia (hypoxic drive) via peripheral chemorecpetors)}

Question 18

Define hyperventilation.

Answer 18

Ventilation increase without change in metabolism

Question 19

Write out the equation that demostrates how pH of the blood is determined by the concentration of dissolved CO₂ in the blood and [HCO₃^-]

Answer 19

Question 20

What happens in the body if pH rises above 7.6?

Answer 20

Free calcium concentration drops (tetany: condition marked by intermittent muscular spasms)

Question 21

What may cause metabolic alkalosis? How can it be compensated for?

Answer 21

Vomiting (loss of protons)

Plasma [HCO₃^- rises]

Decrease degree of ventilation

Question 22

For info:

Question 23

What will happen to total oxygen content of the blood if a normal individual breathes air at twice normal atmospheric pressure?

Answer 23

Question 24

Fill in the missing gaps in the table:

Answer 24