Determinants of Ventilatory Rate
Ventilatory rate (minute volume) is tightly controlled and determined primarily by blood levels of carbon dioxide as determined by metabolic rate. Blood levels of oxygen become important in hypoxia. These levels are sensed by chemoreceptors in the medulla oblongata for pH, and the carotid and aortic bodies for oxygen and carbon dioxide. Afferent neurons from the carotid bodies and aortic bodies are via the glossopharyngeal nerve (CN IX) and the vagus nerve (CN X), respectively.
Levels of CO2 rise in the blood when the metabolic use of O2 is increased beyond the capacity of the lungs to expel CO2. CO2 is stored largely in the blood as bicarbonate (HCO3-) ions, by conversion first to carbonic acid (H2CO3), by the enzyme carbonic anhydrase, and then by disassociation of this acid to H+ and HCO3-. Build-up of CO2 therefore causes an equivalent build-up of the disassociated hydrogen ion, which, by definition, decreases the pH of the blood.
During moderate exercise, ventilation increases in proportion to metabolic production of carbon dioxide. During strenuous exercise, ventilation increases more than needed to compensate for carbon dioxide production. Increased glycolysis facilitates release of protons from ATP and metabolites lower pH and thus increase breathing.
Mechanical stimulation of the lungs can trigger certain reflexes as discovered in animal studies. In humans, these seem to be more important in neonates and ventilated patients, but of little relevance in health. The tone of respiratory muscle is believed to be modulated by muscle spindles via a reflex arc involving the spinal cord.
Drugs can greatly influence the control of respiration. Opioids and anaesthetic drugs tend to depress ventilation, especially with regards to carbon dioxide response. Stimulants such as amphetamines can cause hyperventilation.
Pregnancy tends to increase ventilation (lowering plasma carbon dioxide tension below normal values). This is due to increased progesterone levels and results in enhanced gas exchange in the placenta.
Ventilation is temporarily modified by voluntary acts and complex reflexes such as sneezing, straining, burping, coughing and vomiting.
Read more about this topic: Control Of Respiration
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