Oxygen Concentration and Partial
Pressure in the Alveoli
 The oxygen concentration in
the alveoli, and its partial
pressure is controlled by:
1.
The rate of absorption
of oxygen into the
blood
2. The rate of entry of
new oxygen into the
lungs by the
ventilatory process.
Rate of alveolar
ventilation.
CO2 Concentration and Partial
Pressure in the Alveoli
 Determined by two
factors:
 First, the alveolar PCO2
increases directly in
proportion to the rate of
carbon dioxide excretion
 Second, the alveolar
PCO2 decreases in
inverse proportion to
alveolar ventilation.
By
Dr. Mudassar Ali Roomi (MBBS, M.Phil.)
Assist. Prof. Physiology
Control of respiration
Two types:
1. Nervous control of respiration
2. Chemical control of respiration
Control of repiration
Components:
 Sensors

gather information
 Central controller

integrate signals
 Effectors

muscles
Respiratory centre
 Located bilaterally in
medulla oblongata and pons.
 Composed of
1. Dorsal Respiratory Group
(DRG)
2. Ventral Respiratory
Group (VRG)
3. Pneumotaxic center
4. Apneustic center
Respiratory centre
Pre-Botzinger complex (pre-BOTC)
 A collection of pace-
maker cells at the upper
end of Dorsal Respiratory
Group (DRG)
 Synaptic connection with
DRG
 Function: Discharges
rhythmic respiratory
signals
Dorsal Respiratory Group (DRG)
 Extends most of the length of M.
oblongata
 LOCATION: Neurons located in
nucleus of tractus solitarius and
additional neurons in reticular
substance of medulla
 vagus and glossopharyngeal nerve
terminates at Nucleus of tractus
solitarius
 Both nerves – afferent nerves for resp.
signals to center
 Pace maker neurons send ramp signals
to inspiratory muscles in a Rhythmic
fashion
 Ramp signals controlled by
(a) Pneumotaxic center
(b) Stretch receptors in the
lungs
Significance of ramp signals
 No gasping
 Smooth inflation of lungs
Full cycle of respiration
5 seconds
 2sec inspiration
 3 sec expiration
 Fibers from respiratory
center (DRG) reach the
motor neurons in spinal
cord between C3 & C5 to
form phrenic nerve
 Complete lesion of spinal
cord above C3 will stop
the breathing
 Lesion after C5 will not
affect the respiration
The Hering-Breuer Inflation Reflex
 Muscular portions of the walls of the bronchi and bronchioles throughout
the lungs have stretch receptors
 Transmit signals through the vagi into the dorsal respiratory group of
neurons when the lungs become overstretched.
 Switches Off the inspiratory ramp and thus stops further inspiration
 These signals affect inspiration in much the same way as signals from the
pneumotaxic center
 It also increases rate of respiration
The Hering-Breuer Inflation Reflex
 This reflex is activated when tidal volume increases to
more than three times normal
 Therefore, this reflex appears to be mainly a protective
mechanism for preventing excess lung inflation
Lung “J Receptors.”
 Location: In the alveolar walls in
juxtaposition to the pulmonary
capillaries
 Stimalation: Stimulated especially
when the pulmonary capillaries
become engorged with blood or
 Example: When pulmonary
edema occurs in such conditions as
congestive heart failure.
 Their excitation may give the
person a feeling of dyspnea.
Ventral Respiratory Group (VRG)
 LOCATION: Ventral part of medulla
 Two nuclei
 (1) Nucleus Ambiguus rostrally
 (2) Nucleus Retroambiguus caudally
 Both types of neurons –
INSPIRATORY & EXPIRATORY
 Center remain inactive during quite
breathing
 Active only in increased pulmonary
ventilation, during which signal from
DRG spill over to VRG
 Stimulation of accessory inspiratory
muscles & expiratory muscles
Pneumotaxic
Center
 Location: Upper part of Pons
 Function: Switches off Ramp
Signal
 Controls rate and duration of
Inspiratory ramp signals
 Strong stimulation may reduce
Inspiratory phase to 0.5 sec
respiratory rate ↑ to 30 – 40/min
 Weak stimulation may ↑
Inspiratory phase to 5sec or
more respiratory rate ↓ to 3-5/
min
Apneustic Center
 Located in lower part of pons
 Function: Prevent inspiratory
neurons from being switched
off → prolonged inspiration
 Shortens expiration
 Such Respiration called –
apneusis
CHEMICAL CONTROL OF
RESPIRATION
Following chemical stimuli stimulate the respiration:
1. Excess CO2
2. Excess Hydrogen ion
3. Decreased Oxygen
Central chemosensitive area

Stimulated by CO2 & H+ .Oxygen have no effect
Peripheral chemoreceptors

Stimulated by O2. CO2 & H+ has little effect
Location of Chemosenstive area
 Located bilaterally
beneath the ventral surface
of medulla
 Hydrogen ions are only
the main direct stimulus
for these group of neurons
Peripheral Chemoreceptor
 Carotid bodies through
Hering N to
Glossopharyngeal N
 Aortic Bodies through
Vagus N to DRG
 Both bodies are supplied
by special minute
arteries direct from the
arterial trunk
Stimulation of the Chemoreceptors by Decreased
Arterial Oxygen
Effect of Carbon Dioxide and Hydrogen Ion Concentration on
Chemoreceptor Activity
They have a weak effect but stimulation by way of the
peripheral chemoreceptors occurs as much as five times as
rapidly as central stimulation