CHEMICAL REGULATION OF VENTILATION (continued)

B. Carbon Dioxide

1, Response as a function of P_a-CO2

a. very large effect
b. sensitive in the normal range (@ 40 mmHg)
c. low P_a-CO2 depresses ventilation
d. very high CO₂ is a respiratory depressant

Note limitation on breath holding  (see figure =>)

2. Afferent (sensory) receptors

In the CNS near the ventral surface of the medulla, close to the respiratory center; affected by cerebro-spinal fluid (CSF)

Note: Carotid bodies also have CO₂ receptors, but these are less important than the CNS CO₂ receptors

3. Stimulus:

H⁺ of CSF, which is in turn controlled by P_CO2 of blood, and, to a smaller extent, blood pH

 

Note 1: blood-brain (and blood-CSF) barrier permits CO₂ to equilibrate rapidly but is only slowly permeable to H⁺

Note 2: CO₂ response due mainly to changes in CSF pH but changes in arterial pH also contribute due to the effect of P_CO2 on blood pH (see next section)

4. Regulation schematic:

 

5. Physiological significance

a. homeostatic: maintains Pa-CO2 within the normal range (38-42 mmHg), thus
b. helps maintain brain pH constant
c. synergistic with O₂ response: hypercapnia increases sensitivity to hypoxemia