Exercise 16
CONTROL OF BREATHING
The three basic elements of the respiratory control system are SENSORS that gather informa9on
and feed it to the CENTRAL CONTROLLER in the brain which coordinates the informa9on and, in
turn, sends impulses to the EFFECTORS (respiratory muscles), which allows ven9la9on.
a. Central Controller
a.1. The Respiratory Centers
The basic rhythm of breathing is controlled by respiratory centers located in the brainstem.
• Within the medulla, a paired group of neurons known as the inspiratory center, or the
dorsal respiratory group, sets the basic rhythm by automa9cally ini9a9ng inspira9on.
o A second group of neurons in the medulla, the expiratory center or ventral
respiratory group, appears to func9on mainly during forced expira9on,
s9mula9ng the internal intercostal and abdominal muscles to contract.
• In addi9on, other respiratory centers within the pons (pneumotaxic & apneus9c center)
modify inspira9on and allow for smooth transi9ons between inspira9on and expira9on.
Their precise roles, however, are not fully understood.
a.2. Cerebral Cortex
Because the cerebral cortex has connec9ons with the respiratory center, we can voluntarily
alter our paNern of breathing. We can even refuse to breathe at all for a short 9me.
Voluntary control is protec9ve because it enables us to prevent water or irrita9ng gases from
entering the lungs. The ability to not breathe, however, is limited by the buildup of CO2 and
H+ in the body. When PCO2 and H+ concentra9ons increase to a certain level, the inspiratory
area is strongly s9mulated, nerve impulses are sent along the phrenic
and intercostal nerves to inspiratory muscles, and breathing resumes, whether the person
wants it to or not. It is impossible for us to kill ourselves by voluntarily holding our breaths. If
breath is held long enough to cause fain9ng, breathing resumes when consciousness is lost.
Nerve impulses from the hypothalamus and limbic system also s9mulate the respiratory
center, allowing emo9onal s9muli to alter respira9ons as, for example, in laughing and
crying.
b. Sensors
An important characteris9c of the human respiratory system is its ability to adjust breathing
paNerns to changes in both the internal milieu and the external environment. It responds to
changes in the PCO2, pH, and PO2 of arterial blood, which are important factors that alter
ven9la9on. O2 has liNle central effect, but significant peripheral effects; CO2 and pH have
significant central effects but limited peripheral effect.
• The central chemoreceptors in the medulla monitor the pH associated with CO2
levels within the cerebrospinal fluid in the fourth ventricle. The chemoreceptors
synapse directly with the respiratory centers.
• The peripheral chemoreceptors are found in two loca9ons: (1) the aor9c bodies
within the aor9c arch and (2) the caro9d bodies at the bifurca9on of the common
caro9d arteries. The peripheral chemoreceptors monitor the PCO2, pH and PO2 of
arterial blood.
Chemoreceptors are more sensi9ve to changes in PCO2. Oxygen content of blood decreases
more slowly because of the large “reservoir” of oxygen aNached to hemoglobin. on the
central chemoreceptors. Carbon dioxide readily diffuses from the blood into the
cerebrospinal fluid in the fourth ventricle. Here, carbon dioxide combines with water to form
carbonic acid, which dissociates into hydrogen ions and bicarbonate ions. Most of the
hydrogen ions within the cerebrospinal fluid are derived from this chemical reac9on:
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3 –
MAJOR FACTORS AFFECTING BREATHING
FACTOR RECEPTOR RESPONSE EFFECT
↓ PO2 P e r i p h e r a l ↑ ven9la9on = ↑ PO2
chemoreceptors ↑ RR
↑ PCO2 C e n t r a l ↑ ven9la9on = ↓ PCO2
chemoreceptors ↑ RR
↑ H+ or ↓ pH C e n t r a l ↑ ven9la9on = ↓ PCO2 ! ↑ pH
c h e m o r e c e p t o r s ↑ RR
(CSF)
P e r i p h e r a l
chemoreceptors
(arterial)
Stretch of 9ssues Stretch receptors in Inhibits inspira9on P r e v e n t s
the lungs overinfla9on of
lungs during forceful
breathing
Several other factors influence ven9la9on. These factors include:
1. Voluntary control
• By sending signals from the cerebral cortex to the respiratory muscles, we can
voluntarily change our breathing rate and depth when holding our breath,
speaking, or singing. However, chemoreceptor input to the respiratory centers
will eventually override conscious control and force you to breathe.
2. Pain and emo9ons
• Pain and strong emo9ons, such as fear and anxiety, act by way of the
hypothalamus to s9mulate or inhibit the respiratory centers.
• Laughing and crying also significantly alter ven9la9on.
3. Pulmonary irritants
• Dust, smoke, noxious fumes, excess mucus and other irritants s9mulate
receptors in the airways.
• This ini9ates protec9ve reflexes, such as coughing and sneezing, which forcibly
remove the irritants from the airway.
4. Lung hyperinfla9on
• Stretch receptors in the visceral pleura and large airways send inhibitory signals
to the inspiratory neurons during very deep inspira9ons, protec9ng against
excessive stretching of the lungs. This is known as the infla9on, or Hering-Breuer,
reflex.
c. Nega9ve Feedback
Chemoreceptor regula9on of breathing is a form of nega9ve feedback. The goal of this
system is to keep the pH of the blood stream within normal neutral ranges, around
7.35-7.45. Nega9ve feedback responses have three main components: the chemoreceptors
that regulate the levels of CO2, O2, and H+ in the blood, the medulla and pons form the
control center, and the respiratory muscles are the effectors.
In cases of acidosis (↑ PCO2, ↓ pH or PO2) input
from the central and peripheral chemoreceptors
causes the inspiratory area to become highly
ac9ve, and the rate and depth of breathing ↑↑↑.
Rapid and
deep breathing, called hyperven9la9on, allows the
inhala9on of
more O2 and exhala9on of more CO2 un9l PCO2
and H+ are lowered
to normal.
Consider a case in which a person is
hyperven9la9ng from an anxiety aNack. Their ↑
ven9la9on rate will remove too much carbon
dioxide from their body. Without that carbon
dioxide, there will be ↓ carbonic acid in blood, so
the concentra9on of hydrogen ions ↓ and the pH of
the blood ↑, causing alkalosis. In response, the
chemoreceptors detect this change, and send a
signal to the medulla, which signals the respiratory
muscles to ↓↓↓ the ven9la9on rate so carbon
dioxide levels and pH can return to normal levels.
Chemoreceptor feedback also adjusts for oxygen
levels to prevent hypoxia, though only the
peripheral chemoreceptors sense oxygen levels. In
cases where oxygen intake is too low, feedback ↑↑↑
ven9la9on to increase oxygen intake.
SUMMARY
The point of respira9on is to allow you to obtain oxygen, eliminate carbon dioxide, and regulate
the blood’s pH level. Respira9on rate (breaths per minute) and depth (volume of air inhaled and
exhaled with each breath) are adjusted to maintain arterial PCO2 of 40 mmHg. For example,
when you exercise, demand for oxygen increases because the cells require more ATP. In turn,
more carbon dioxide is produced by cells and diffuses to the blood. The rise in carbon dioxide
leads to a decrease in pH, causing the blood to be more acidic. The brain is especially sensi9ve to
pH levels; as pH levels in the blood fall, the brain s9mulates more rapid breathing and deeper
breathing. The effect is to draw more air into the lungs, to transport more oxygen to the cells,
and lower pH and CO2 levels.
