Editing Control of ventilation (section)

==Control of respiratory rhythm==

=== Ventilatory pattern ===
[[File:2327 Respiratory Centers of the Brain.jpg|thumb|Respiratory centre and its groups of neurons]]
Breathing is normally an unconscious, involuntary, automatic process. The pattern of motor stimuli during breathing can be divided into an [[inhalation]] stage and an [[exhalation]] stage. [[Inhalation]] shows a sudden, ramped increase in motor discharge to the [[muscles of respiration|respiratory muscles]] (and the [[middle pharyngeal constrictor muscle|pharyngeal constrictor muscles]]).<ref name="Kuna2000">{{cite journal|last1=Kuna|first1=Samuel T|title=Respiratory-related activation and mechanical effects of the pharyngeal constrictor muscles|journal=Respiration Physiology|volume=119|issue=2–3|year=2000|pages=155–161|issn=0034-5687|doi=10.1016/S0034-5687(99)00110-3|pmid=10722858}}</ref> Before the end of inhalation, there is a decline in, and end of motor discharge. [[Exhalation]] is usually silent, except at high [[respiratory rate]]s.

The [[respiratory centre]] in the medulla and pons of the brainstem controls the rate and depth of respiration, (the [[respiratory rhythm]]), through various inputs. These include signals from the peripheral chemoreceptors and central chemoreceptors; from the vagus nerve and glossopharyngeal nerve carrying input from the [[pulmonary stretch receptor]]s, and other mechanoreceptors in the [[lung]]s.<ref name="Pocock"/><ref name="G and H">{{cite book|last1=Hall|first1=John|title=Guyton and Hall textbook of medical physiology|date=2011|publisher=Saunders/Elsevier|location=Philadelphia, Pa.|isbn=978-1-4160-4574-8|pages=505–510|edition=12th}}</ref> as well as signals from the [[cerebral cortex]] and [[hypothalamus]].

* Medulla 
** [[ventral respiratory group]] (includes the [[pre-Bötzinger complex]]). The ventral respiratory group controls voluntary forced exhalation and acts to increase the force of inhalation. Regulates rhythm of inhalation and exhalation.
** [[dorsal respiratory group]] ([[solitary nucleus]]). The dorsal respiratory group controls mostly movements of inhalation and their timing.
* Pons
** [[pneumotaxic center]].
*** Coordinates speed of inhalation and exhalation
*** Sends inhibitory impulses to the inspiratory area
*** Involved in fine tuning of respiration rate.
** [[apneustic center]]
*** Coordinates speed of inhalation and exhalation.
*** Sends stimulatory impulses to the inspiratory area – activates and prolongs inhalations
*** Overridden by pneumotaxic control from the apneustic area to end inhalation

=== Control of ventilatory pattern ===
Ventilation is normally unconscious and automatic, but can be overridden by [[conscious breathing|conscious alternative patterns]].<ref name="Pocock"/> Thus the emotions can cause yawning, laughing, sighing (etc.), social communication causes speech, song and whistling, while entirely voluntary overrides are used to blow out candles, and breath holding (for instance, to swim underwater). [[Hyperventilation]] may be entirely voluntary or in response to emotional agitation or anxiety, when it can cause the distressing [[hyperventilation syndrome]]. The voluntary control can also influence other functions such as the [[heart rate]] as in [[Yoga#Ascetic practices|yoga]] practices and [[meditation]].<ref name="yoga-reticularis">{{cite book|last1=Prasad|first1=K.N. |editor1-last=Udupa |editor1-first=R.C.|title=Stress and its management by yoga |date=1985 |publisher=Motilal Banarsidass |location=Delhi |isbn=978-8120800007 |pages=26 ff |edition=2nd rev. and enl. |url=https://books.google.com/books?id=OluYV0Xe9hEC&pg=PA26 |access-date=17 July 2014}}</ref>

The ventilatory pattern is also temporarily modified by complex reflexes such as sneezing, straining, burping, coughing and vomiting.

=== 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]].

=== Feedback control ===
[[sensory receptor|Receptor]]s play important roles in the regulation of respiration and include the [[central chemoreceptors|central]] and [[peripheral chemoreceptors]], and [[pulmonary stretch receptors]], a type of [[mechanoreceptor]].

* [[Central chemoreceptors]] of the central nervous system, located on the ventrolateral medullary surface, are sensitive to the [[pH]] of their environment.<ref name="Coates">Coates EL, Li A, Nattie EE. Widespread sites of brain stem ventilatory chemoreceptors. J Appl Physiol. 75(1):5–14, 1984.</ref><ref name="Cordovez">Cordovez JM, Clausen C, Moore LC, Solomon, IC. A mathematical model of pH(i) regulation in central CO<sub>2</sub> chemoreception. Adv Exp Med Biol. 605:306–311, 2008.</ref>
* [[Peripheral chemoreceptors]] act most importantly to detect variation of the PO2 in the [[arterial blood]], in addition to detecting arterial P<sub>CO<sub>2</sub></sub> and pH.
* [[Mechanoreceptors]] are located in the [[airway]]s and [[parenchyma]], and are responsible for a variety of reflex responses. These include:
** The [[Hering-Breuer reflex]] that terminates inhalation to prevent over inflation of the lungs, and the reflex responses of [[coughing]], [[airway constriction]], and [[hyperventilation]].
** The upper airway receptors are responsible for reflex responses such as, [[sneezing]], coughing, closure of [[glottis]], and [[hiccups]].
** The [[spinal cord]] reflex responses include the activation of additional respiratory muscles as compensation, gasping response, hypoventilation, and an increase in breathing frequency and volume.
** The nasopulmonary and nasothoracic [[reflex]]es regulate the mechanism of breathing through deepening the inhale. Triggered by the flow of the air, the pressure of the air in the [[nose]], and the quality of the air, impulses from the nasal mucosa are transmitted by the trigeminal nerve to the [[respiratory center]] in the [[brainstem]], and the generated response is transmitted to the [[bronchus|bronchi]], the [[intercostal muscles]] and the [[Thoracic diaphragm|diaphragm]].
** Head's paradoxical reflex where sudden inflation of the lung causes a transient respiratory effort or gasp.