Chemical Control Of Breathing

Question 1

Normal parameters for serum: PACo2, PaO2, bicarbonate, ph

Answer 1

Packs: 4.7-6 KPa
PaO2: 9.3-13.3 KPa
Bicarbonate:22-26mmol/L
PH: 7.35-7.45

Question 2

What happens to alveolar PO2 and PCO2 in hyperventilation? what could this lead to?

Answer 2

PO2 increases
PCO2 decreases

Could lead to respiratory alkalosis

Question 3

What happens to alveolar PO2 and PCO2 in hypoventilation? What could this lead to?

Answer 3

PO2 decreases
Pco2 increases

Could lead to respiratory acidosis

Question 4

How does hypocapnia occur? What can it lead to?

Answer 4

If PO2 changes without Change in PCO2, correction of Po2 will cause PCO2 to drop

Can lead to respiratory alkalosis

Question 5

How are respiratory acidosis and alkalosis compensated for? How long does this take?

Answer 5

Ph depends on ratio of HCO3- to pCO2 so kidneys…

• increase HCO3- in resp acidosis
• decrease HCO3- in resp alkalosis

2-3 days

Question 6

How does metabolic acidosis come about and how can it be compensated for?

Answer 6

Tissues -> acid -> + HCO3- -> fall in HCO3-> fall PH

Compensated for by increasing ventilation to lower PCO2

Question 7

How does metabolic alkalosis come about? How can it be compensated for?

Answer 7

Plasma HCO3- increases e.g. after vomiting -> plasma Ph rises

Compensated for to a degree by decreasing ventilation (but limited as need O2 above 8kpa)

Question 8

What sensors are part of the respiratory control pathway, where do they send Input to and where do these efferents reach?

Answer 8

Central chemoreceptors (H+), peripheral chemoreceptors (O2, CO2, H+), pulmonary receptors (stretch), joint/ muscle receptors (stretch/tension)

-> respiratory control centre

-> diaphragm
Inspiration -> external intercostal, accessory muscles
Exp -> internal ICM, abdo muscles

Question 9

What makes up the peripheral chemoreceptors, what stimulates them and what effects do they have?

Answer 9

Carotid and aortic bodies

Large falls in pO2 stimulate these -> increase breathing, changes HR, changes blood flow distribution (increasing flow to brain & kidneys

Question 10

What makes up the central chemoreceptors, what stimulates them and what effects do they have? Short and long term

Answer 10

In medulla of brain more sensitive to PCO2 changes. Acidification of CSF and ECF (separated from blood by BBB) from more PCO2 diffusing across stimulates neurones.

• > ventilation
• ve feedback control of breathing

Longer term control: choroid plexus cells control CSf [HCO3-]. This determines which PCO2 is associated with normal CSF PH.

Question 11

What controls normal CSF PH?

Answer 11

CSF [HCO3-] sets the control system to a particular PCO2, can be reset by changing CSF [HCO3-]

Question 12

What occurs in persisting hypoxia? Ph= 7.47

Answer 12

Hypoxia detected by peripheral chemoreceptors -> increases ventilation

But PcO2 will fall further -> decreased ventilation

So CSf composition compensates for altered PCo2 - choroid plexus cells selectively add H+ or HCO3- into CSF, central chemoreceptors accept the PCo2 as normal

Question 13

What occurs in persisting hypercapnia?

Answer 13

Hypoxia and hypercapnia - respiratory acidosis - decreases Ph of CsF - peripheral and central chemoreceptors stimulates breathing but acidic Ph undesirable for neurones therefore choroid plexus needs to adjust PH of CsF by addition of HCO3-, central chemoreceptors accept high PcO2 as normal