CHEMICAL REGULATION OF VENTILATION

A. Oxygen

1. Response as a function of P_a-O2

a. relatively small effect in the normal range ( P_a-O2 > 70 mmHg )
b. only important in pronounced hypoxemia ( P_a-O2 < 60 mmHg ) (final defense)
c. high P_a-O2 does not depress ventilation (except for chronic hypercapnia)

2. Receptors: carotid (mainly) and aortic oxygen-sensitive chemoreceptors in carotid and aortic bodies

Note:  Oxygen receptors (sensory nerve endings) are chemoreceptors separate from the carotid sinus baroreceptors (pressoreceptors)

3. Stimulus: P _a-O2 ¯

Note: not C_a-O2 because of very high chemoreceptor blood flow means that
P_a-O2 @ P_c-O2

4. O₂ response enhanced by P _a-CO2 increase (right shift of response; hypercapnia increases sensitivity to hypoxemia

Note problem of administering high oxygen to a chronic hypercapnic patient

5. Regulation schematic:

6. Physiological significance

a. homeostatic: helps maintain P_a-O2 in conditions of severe hypoxia
b. remains when other chemostimulation is lost (e.g. deep anesthesia)
c. synergistic with CO₂ response (hypoxemia increases sensitivity to hypercapnia)

Note:  If Pa-O2 is very low, then all CNS neurons, including respiratory neurons, become depressed, so respiratory ventilation is reduced or ceases entirely.