Canonical Question

Applied Resp – Gases

V5 C8.ii Historical V4 F10.ii 1 appearance

Master answer

	Central Chemoreceptor (CCR)	Peripheral Chemoreceptor (PCR)
Location	Below the ventral surface of the medulla Within the BBB	In Glomus cells of: • Carotid bodies: At the bifurcation of the common carotid arteries • Aortic bodies: Above and below the aortic arch.
Mechanism	CO2 rapidly diffuses into CSF • Converted to H+ by carbonic anhydrased • Δ ECF [H+] closes pH sensitive K-channels • ↑ ECF [H+] stimulates ↑ ventilation • CSF has: – Low CSF protein and HCO3 (c.f. plasma) – Poor buffering capacity – Thus ΔCSF pH per ΔPaCO2 is GREATER than that of blood	Type 2 cells: • ↓ PaO2, ↑ PaCO2 and/or ↓pH • ↓ IC [ATP] which leads to ↑ NT production and release • ↑ AP firing rate of AB or CB afferent fibres
Contribution to ↑MV	80%	20%
Response Speed	Fast	Relatively fast compared with CCR

Resp
- CO₂ strongest determinant of ↑MV (2-3 L/min per mmHg)
- Hypoxaemia has a synergistic effect on hypercapnoeic-ventilatory drive
- CO₂ is a potent pulmonary vasoconstrictor → ↑ pulmonary vascular resistance
CVS
- Direct depression of cardiac muscle and vascular SM
  - Peripheral vasodilation → ↓ SVR and ↑ regional BF
  - ↓ heart rate/contractility
- ↑ RV Afterload due to effect on PVR
- ↑ circulating catecholamines
Neuro
- ↑ CBF (4% per mmHg ↑PaCO2) → ↑ ICP
Haeme
- Right shift in OHDC

SNS stimulation
- hyperkinetic circulation →
- ↑ C.O., ↑ BP and arrhythmias
- ↑ cardiac MRO₂ demand
- ↓ cardiac blood supply
“CO₂ nacrosis”
- PaCO₂ > 80 mmHg → ↓ CSF pH
- Disorientation, confusion, headache, unconsciousness

Hypoxia due to displacement of O₂ from alveoli (as per Alveolar gas equation) → further ↑MV as above
Respiratory acidosis →
- ↑ PaCO2 overwhelms buffering capacity of blood
- ↑ serum K+ and Ca2+ (stewart)
↑PaCO₂ can cause anuria

Gladwin 2016

Answer by Gladwin

Exam	Exact wording	Relationship	Success
2012B Q21	Describe the physiological consequences of a progressive rise in blood carbon dioxide levels.	historical_member	—