RESPIRATION
Dr. Zainab H.H
Dept. of Physiology
Lec.11,12
Objectives
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List the types of respiratory controls
List the types of chemoreceptors and
the main stimulants for each
List the effects of pulmonary receptors
on ventilation
Neural
Control
Control of Respiration
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The respiratory rate changes:
i.
When active - respiratory rate goes up
ii.
When less active or sleeping - the rate goes down.
The respiratory muscles are voluntary BUT you can't
consciously control them when you're sleeping.
So, how is respiratory rate altered & how is respiration
controlled when you're not consciously thinking about
respiration? This is by:
A.
Neural Control
B.
Chemical Control
Two Neurogenic Systems
(both CNS)
1) Involuntary (automatic):
 involve medulla and pons
 limbic systems (emotional response)
 hypothalamus (temperature regulation)
 other subcortical structures
2) Voluntary:
 initiated by the cerebral cortex
Two Neurogenic Systems
(both CNS)
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Notes:
Systems are independent
Both systems require intact innervation of
respiratory muscles (descending pathways and
alpha motorneurons)
The muscles of respiratory ventilation are controlled
by somatic motor system and not the autonomic
system
The autonomic system controls airway smooth
muscle contraction and secretion
Neural Control
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Ventilation is matched to the body’s needs
for O2 uptake and CO2 removal
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Medullary respiratory center receives input
Appropriate signals sent to motor neurons
Rate and depth of ventilation adjusted
Reticular Activating System
(RAS)
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Located in the reticular system of the brain
stem
Activity is associated with the “awake” or
conscious state
When active, stimulates respiratory
ventilation
When RAS activity is reduced, as during
sleep, ventilation is reduced and the PCO2
increases by a few mmHg
Sleep Apnea
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Ventilation ceases temporarily (10+ seconds) during
sleep.
types of sleep apnea
 central apnea - reduced CNS respiratory drive
 obstructive apnea - increased upper airway
resistance (lyryngospasm, bronchospasm,
snoring)
In infants - can lead to SIDS (sudden infant Death
Syndrome).
Other Neural Structures
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Hypothalamus - change in inspiration
associated with temperature regulation
Limbic system - respiratory changes in
emotion
Cerebral cortex - voluntary control
Chemical Control
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This is achieved through the following
stimuli:
A. Arterial PO2 level.
B. Arterial PCO2 level.
C. Arterial H+ Concentration.
CO2 & [H+] act centrally while the O2 levels
act on the peripheral chemoreceptors.
Chemoreceptors
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2 groups of chemo-receptors that monitor
changes in blood PCO2, PO2, and pH.
Central:
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Highly sensitive to PCO2 and [H+]
Located in the medulla oblongata.
Functions by stimulating the respiratory centers
Peripheral:
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Monitors PO2 and arterial H+
Located in the Carotid and aortic bodies.
Control breathing indirectly via sensory nerve fibers to
the medulla (X, IX).
Central Chemoreceptors
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Their input modifies the rate and depth of
breathing to maintain arterial PCO of 40 mm
Hg.
The primary stimulus is [H+]
But [H+] can not cross the Blood Brain
Barrier
the blood PCO2 level has more effect as CO2
readily crosses the BBB.
2
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Effects of Blood PCO2 on
Ventilation
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Is not direct.
Even the indirect effect of CO2 is most
potent. Why?
Because CO2 easily crosses the BBB.
Once it is across the BBB,
CO2 + H2O  H2CO3  H+ + HCO3These increased H+ ions in the brain
stimulate the medullary chemoreceptors.
Effects of Blood PCO2 on
Ventilation
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very large effect
sensitive in the normal range of 40 mmHg
low PCO2 depresses ventilation
very high CO2 is a respiratory depressant
Effects of Blood PCO2 on
Ventilation
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The afferent sensory receptors located in the
CNS and affected by CSF.
Carotid bodies also have CO2 receptors, but
these are less important than the CNS CO2
receptors
The main stimulus is H+ of CSF, which in turn
controlled by PCO2 of blood and to a smaller
extent, blood pH
Effects of Blood PCO2 on
Ventilation
Effects of Blood PCO2 on
Ventilation
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Physiological Significance
homeostatic: maintains PCO2 within the
normal range (38-42 mmHg), thus helps to
maintain brain pH
synergistic with O2: hypercapnia increases
sensitivity to hypoxemia
Effects of Blood H+ on
Ventilation
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Response as a function of pH
especially marked at acidic pH
The afferent endings are carotid body and aortic
body H+ sensitive receptors (rapid response)
CNS medullary H+ receptors (slow H+ leakage
across the BBB, so slow response
Stimulus is pHa and pH CSF
Quantitative Effect of H+
Ions
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The stimulatory effect of H+ ions increases
in the first few hours.
It then decreases in the next 1 to 2 days.
It comes down to about 1/5th the initial
effect.
This is due to Renal readjustment of [H+] in
the circulating blood.
Quantitative Effect of H+
Ions
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The kidneys increase blood HCO3-.
This HCO3- binds with the free H+ ions in the
blood & decreases their concentration.
HCO3- also diffuses slowly past the BBB and
decreases the H+ ions in the brain.
Therefore the effect of H+ ions is:
 Potent: Acutely
 Weak: Chronically.
Effect of O2
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The partial pressure of O2 has no effect on
the central (medullary) chemoreceptors.
It only has an effect on the peripheral
chemoreceptors.
Peripheral Chemoreceptors
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There are two pairs of O2 chemoreceptors:
 Aortic Bodies: located at the arch of aorta.
 Carotid bodies (mainly): located at the
branching of the common carotid arteries.
Their functions are:
 To detect changes in the PO2 & H+
 To transmit nervous signals to the
Respiratory Centers.
Peripheral Chemoreceptors
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These bodies have two types of special cells
called glomus cells.
The type 1 glomus cells have special ion
channels sensitive to PO2.
They fire the nerve endings and send signals
via:
 Aortic bodies: Vagi.
 Carotid bodies: Glossopharyngeal nerve.
Peripheral Chemoreceptors
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Both these bodies receive their own special
blood supply through minute arteries, directly
from the trunk.
Their blood flow is roughly 20 times their own
weight.
They are all the time exposed only to arterial
blood.
 PO2 stimulates these chemoreceptors
strongly.
Effects of Blood PO2 on
Ventilation
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Relatively small effect in the normal range (PO2 >
70 mmHg)
Only important in pronounced hypoxemia (PO2 <
60 mmHg)
High PO2 does not depress ventilation (except for
chronic hypercapnia)
Peripheral chemoreceptors respond to the PO2
and not the total O2 content.
Physiological Significance
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helps to maintain PO2 in conditions of severe
hypoxia (homeostatic)
remain when other chemostimulation is lost
synergistic with CO2 response (hypoxia increases
sensitivity to hypercapnia)
NOTE: If PO2 is very low, then all CNS neurons
including respiratory neurons, become
depressed, so respiratory ventilation is reduced
or ceases activity.
Effects of Blood PO2 on
Ventilation
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Hypoxic drive:
 Emphysema blunts the chemoreceptor
response to PCO2.
 Choroid plexus secrete more HCO3 into CSF,
buffering the fall in CSF pH.
 Abnormally high PCO2 enhances sensitivity of
carotid bodies to fall in PO2.
Effect of CO2 & H+
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They also stimulate the peripheral
chemoreceptors.
But their effects on the central or medullary
chemoreceptors are more powerful.
PCO2 stimulates the peripheral chemoreceptors 5
times as rapidly as it stimulates the central ones.
So this is responsible for the rapid response to
CO2 at the onset of exercise.
Effects of Pulmonary
Receptors on Ventilation
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Lungs contain receptors that influence the
brain stem respiratory control centers via
sensory fibers in vagus.
i.
Pulmonary stretch (mechano-) receptors
ii.
Lung irritant receptors - airway receptors
responding to inhaled irritating
substances; cause hyperpnea and
bronchoconstriction
Effects of Pulmonary
Receptors on Ventilation
Proprioceptors (Respiratory muscle
spindle receptors and joint receptors)
may contribute to respiratory drive in
exercise.
Along with the chemoreceptors, they send
information to the respiratory centers.
iii.
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Factors Influencing
Respiratory Rate and Depth
A.
B.
C.
Physical factors
i.
Increased body temperature
ii.
Exercise
iii.
Talking
iv.
Coughing
Volition (conscious control) - emotional factors
Chemical factors
i.
CO2 levels
ii.
O2 levels
 Chemoreceptors in aorta and carotid arteries
Respiratory Rate Changes
Throughout Life
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Newborns: 40 to 80 respirations per
minute
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Infants: 30 respirations per minute
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Age 5: 25 respirations per minute
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Adults: 12 to 18 respirations per minute
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Rate often increases somewhat with old
age
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A 36-year-old man visits his doctor because his wife has long
complained of his snoring, but recently observed that his
breathing stops for a couple of minutes at a time while he is
sleeping. He undergoes polysomnography and ventilatory
response testing to ascertain the extent and cause of his sleep
apnea.
The activity of the central chemoreceptors is stimulated by
which of the following?
a. A decrease in the metabolic rate of the surrounding brain
tissue
b. A decrease in the PO2 of blood flowing through the brain
c. An increase in the PCO2 of blood flowing through the brain
d. An increase in the pH of the CSF
e. Hypoxemia, hypercapnia, and metabolic acidosis
Every day it’s the same old
thing:
Breathe
Breathe
Breathe