Editing Control of ventilation (section)

=== Determinants of ventilatory rate ===
Ventilatory rate ([[respiratory 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 (medical)|hypoxia]]. These levels are sensed by [[central chemoreceptors]] on the surface of the [[medulla oblongata]] for decreased pH (indirectly from the increase of carbon dioxide in [[cerebrospinal fluid]]), and the [[peripheral chemoreceptors]] in the arterial blood for oxygen and carbon dioxide.  Afferent neurons from the peripheral chemoreceptors are via the [[glossopharyngeal nerve]] (CN IX) and the [[vagus nerve]] (CN X).

The concentration of [[carbon dioxide]] (CO<sub>2</sub>) rises in the blood when the metabolic use of oxygen (O<sub>2</sub>), and the production of CO<sub>2</sub> is increased during, for example, exercise. The CO<sub>2</sub> in the blood is transported largely as bicarbonate (HCO<sub>3</sub><sup>−</sup>) ions, by conversion first to [[carbonic acid]] (H<sub>2</sub>CO<sub>3</sub>), by the enzyme [[carbonic anhydrase]], and then by disassociation of this acid to H<sup>+</sup> and HCO<sub>3</sub><sup>−</sup>. Build-up of CO<sub>2</sub> therefore causes an equivalent build-up of the disassociated hydrogen ions, which, by definition, decreases the pH of  the blood. The pH sensors on the brain stem immediately respond to this fall in pH, causing the respiratory center to increase the rate and depth of [[breathing]]. The consequence is that the [[partial pressure]] of CO{{sub|2}} (P<sub>CO<sub>2</sub></sub>) does not change from rest going into exercise. During very short-term bouts of intense exercise the release of lactic acid into the blood by the exercising muscles causes a fall in the blood plasma pH, independently of the rise in the P<sub>CO<sub>2</sub></sub>, and this will stimulate pulmonary ventilation sufficiently to keep the [[Homeostasis#The extracellular fluid pH homeostat|blood pH constant]]{{Broken anchor|date=2025-03-17|bot=User:Cewbot/log/20201008/configuration|target_link=Homeostasis#The extracellular fluid pH homeostat|reason= The anchor (The extracellular fluid pH homeostat) [[Special:Diff/763872805|has been deleted]].|diff_id=763872805}} at the expense of a lowered P<sub>CO<sub>2</sub></sub>.

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 spindle]]s via a reflex arc involving the spinal cord.

Drugs can greatly influence the rate of respiration. [[Opioids]] and  [[anesthetic]]s tend to depress ventilation, by decreasing the normal response to raised [[carbon dioxide]] levels in the arterial blood. 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]].