Breathing is mostly an involuntary, automatic process.
Because its major function is to supply the body with oxygen and remove carbon dioxide,
the rate and depth of breathing is generally regulated by carbon dioxide status or the
need for oxygen.
For example, breathing automatically accelerates with physical exercise when the body’s need
for oxygen is increased.
Basically, various receptors in the body feed information about its metabolic state to the
respiratory center in the brainstem, which responds by changing the firing pattern of
inspiratory and expiratory neurons.
Inspiratory neurons fire during inspiration, while expiratory neurons only fire during
deep expiration, since quiet expiration is a passive process.
The fibers of these neurons descend to the cervical and thoracic spine where they synapse
with motor neurons.
Motor neurons then travel in several nerves to respiratory muscles, changing the way these
muscles contract, adjusting thereby the rate and depth of breathing to suit the body’s
needs.
Of most importance are phrenic nerves which control the diaphragm, and intercostal nerves
which innervate intercostal muscles.
While the functional anatomy of human respiratory center is complex and not entirely clear,
the current consensus is that the primary center is composed of several areas in the
medulla: the dorsal respiratory group, DRG, mainly associated with inspiration; the ventral
respiratory group, VRG, mostly concerned with expiration; and the pre-Bötzinger complex,
possibly coupled with two other oscillators, thought to be the intrinsic rhythm generator,
similar to the pacemaker in the heart.
The medullar areas also communicate with two other areas in the pons to fine-tune the respiration
control: the pneumotaxic center which seems to inhibit inspiration, while the apneustic
center stimulates it.
The most important factor regulating breathing rate is the concentration of carbon dioxide.
Changes in carbon dioxide leads to changes in pH, and these are detected by chemoreceptors.
Central chemoreceptors located on the surface of the medulla monitor pH changes in the cerebrospinal
fluid; while peripheral chemoreceptors found in the aortic and carotid bodies respond to
fluctuations in pH, carbon dioxide, as well as oxygen levels in the blood.
Peripheral receptors transmit signal to the respiratory center via the vagus and glossopharyngeal
nerves.
An increase in carbon dioxide, such as during exercise, causes a decrease in pH, which is
sensed by central or arterial chemoreceptors and leads to deeper, faster breathing; more
carbon dioxide is exhaled, and blood pH returns to normal.
The respiratory center also receives input from various mechanoreceptors in the lungs,
which transmit information about the mechanical status of the lungs via the vagus nerve.
For example, pulmonary stretch receptors present in smooth muscle of the airways are activated
when the lungs are excessively inflated, and trigger the inflation reflex, which stops
inspiration and prolongs expiration.
Other receptors respond to inhaled irritants and are responsible for defensive respiratory
reflexes such as bronchoconstriction or coughing.
The limbic system and hypothalamus also send information to the respiratory center and
allow pain and emotional state to affect breathing.
For example, pain or strong emotion may induce gasping, crying; while anxiety may cause uncontrollable
hyperventilation.
While breathing is mostly involuntary, some degree of voluntary control is possible, for
example, during singing, playing wind instruments, or holding breath under water.
In this case the control originates from the primary motor cortex, which sends signals
directly to the spinal cord, bypassing the respiratory center in the brainstem.
There are limits, however, to the extent one can control their breath even though it’s
possible to increase these limits with training.