Breathing control
The
control of ventilation
is the
physiological
mechanisms involved in the control of
breathing
, which is the movement of air into and out of the lungs. Ventilation facilitates respiration. Respiration refers to the utilization of
oxygen
and balancing of
carbon dioxide
by the body as a whole, or by individual cells in
cellular respiration
.
[1]
The most important function of breathing is the supplying of oxygen to the body and balancing of the carbon dioxide levels. Under most conditions, the
partial pressure of carbon dioxide
(PCO
2
), or concentration of carbon dioxide, controls the
respiratory rate
.
The
peripheral chemoreceptors
that detect changes in the
levels of oxygen and carbon dioxide
are located in the
arterial
aortic bodies
and the
carotid bodies
.
[2]
Central chemoreceptors
are primarily sensitive to changes in the
pH
of the
blood
, (resulting from changes in the levels of carbon dioxide) and they are located on the
medulla oblongata
near to the
medullar respiratory groups
of the
respiratory center
.
[3]
Information from the peripheral chemoreceptors is conveyed along nerves to the respiratory groups of the respiratory center. There are four respiratory groups, two in the medulla and two in the
pons
.
[2]
The two groups in the pons are known as the
pontine respiratory group
.
- Dorsal respiratory group
? in the medulla
- Ventral respiratory group
? in the medulla
- Pneumotaxic center
? various nuclei of the pons
- Apneustic center
? nucleus of the pons
From the respiratory center, the
muscles of respiration
, in particular the
diaphragm
,
[4]
are activated to cause air to move in and out of the lungs.
Control of respiratory rhythm
[
edit
]
Ventilatory pattern
[
edit
]
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
respiratory muscles
(and the
pharyngeal constrictor muscles
).
[5]
Before the end of inhalation, there is a decline in, and end of motor discharge.
Exhalation
is usually silent, except at high
respiratory rates
.
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 receptors
, and other mechanoreceptors in the
lungs
.
[3]
[6]
as well as signals from the
cerebral cortex
and
hypothalamus
.
- Medulla
- 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
[
edit
]
Ventilation is normally unconscious and automatic, but can be overridden by
conscious alternative patterns
.
[3]
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
practices and
meditation
.
[7]
The ventilatory pattern is also temporarily modified by complex reflexes such as sneezing, straining, burping, coughing and vomiting.
Determinants of ventilatory rate
[
edit
]
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
. 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
2
) rises in the blood when the metabolic use of oxygen (O
2
), and the production of CO
2
is increased during, for example, exercise. The CO
2
in the blood is transported largely as bicarbonate (HCO
3
?
) ions, by conversion first to
carbonic acid
(H
2
CO
3
), by the enzyme
carbonic anhydrase
, and then by disassociation of this acid to H
+
and HCO
3
?
. Build-up of CO
2
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
2
(P
CO
2
) 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
CO
2
, and this will stimulate pulmonary ventilation sufficiently to keep the
blood pH constant
at the expense of a lowered P
CO
2
.
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 rate of respiration.
Opioids
and
anesthetics
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
[
edit
]
Receptors
play important roles in the regulation of respiration and include the
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.
[8]
[9]
- Peripheral chemoreceptors
act most importantly to detect variation of the PO2 in the
arterial blood
, in addition to detecting arterial P
CO
2
and pH.
- Mechanoreceptors
are located in the
airways
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
reflexes
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
bronchi
, the
intercostal muscles
and the
diaphragm
.
- Head's paradoxical reflex where sudden inflation of the lung causes a transient respiratory effort or gasp.
References
[
edit
]
- ^
Barrett, Kim E.; Barman, Susan M.; Boitano, Scott; Brooks, Heddwen L. (2012).
Ganong's review of medical physiology
(24th ed.). New York: McGraw-Hill Medical.
ISBN
978-0071780032
.
- ^
a
b
Tortora, Gerard (2008).
Principles of anatomy and physiology
(12. ed.). Hoboken, N.J.: Wiley. pp. 905?909.
ISBN
978-0470-23347-4
.
- ^
a
b
c
Pocock, Gillian; Richards, Christopher D. (2006).
Human physiology : the basis of medicine
(3rd ed.). Oxford: Oxford University Press. pp. 332?336.
ISBN
978-0-19-856878-0
.
- ^
Tortora, G. J. and Derrickson, B. H., (2009). Principles of Anatomy and Physiology ? Maintenance and continuity of the human body. 12th Edition. Danvers: Wiley
- ^
Kuna, Samuel T (2000). "Respiratory-related activation and mechanical effects of the pharyngeal constrictor muscles".
Respiration Physiology
.
119
(2?3): 155?161.
doi
:
10.1016/S0034-5687(99)00110-3
.
ISSN
0034-5687
.
PMID
10722858
.
- ^
Hall, John (2011).
Guyton and Hall textbook of medical physiology
(12th ed.). Philadelphia, Pa.: Saunders/Elsevier. pp. 505?510.
ISBN
978-1-4160-4574-8
.
- ^
Prasad, K.N. (1985). Udupa, R.C. (ed.).
Stress and its management by yoga
(2nd rev. and enl. ed.). Delhi: Motilal Banarsidass. pp. 26 ff.
ISBN
978-8120800007
. Retrieved
17 July
2014
.
- ^
Coates EL, Li A, Nattie EE. Widespread sites of brain stem ventilatory chemoreceptors. J Appl Physiol. 75(1):5?14, 1984.
- ^
Cordovez JM, Clausen C, Moore LC, Solomon, IC. A mathematical model of pH(i) regulation in central CO
2
chemoreception. Adv Exp Med Biol. 605:306?311, 2008.
Further reading
[
edit
]
External links
[
edit
]
- Paul, Anthony D.; et al. (1995).
"Neuronal Connections of a Ventral Brainstem Respiratory Chemosensitive Area"
. In C. Ovid Trouth (ed.).
Ventral brainstem mechanisms and control of respiration and blood pressure
. New York: M. Dekker. pp.
517?523
.
ISBN
0-8247-9514-8
.
OCLC
32169247
.
- Rabbany, Sina Y., "Breathing Coordination", Hofstra University
[1]
- Webber, Charles L., Jr., Ph.D., Pulmonary Curriculum Function:"Neural Control of Breathing", Stritch School of Medicine, Loyola University-Chicago
[2]