Physiology of respiration
DR. MOHAMED SEYAM PHD. PT.
ASSISTANT PROFESSOR OF PHYSICAL THERAPY
Physiology of respiration
 Lung
Volumes and Capacities
 Mechanics of Breathing
 Mechanical Properties of the lung
 Pulmonary Circulation
 Lung pressures and ventilation
Neural Control of Breathing
Lung pressures and ventilation
• The thorax and respiratory
muscles
 thoracic cage: ribs (12),
sternum, diaphragm
 pleural space
 respiratory muscles during
inspiration:
- diaphragm
- external intercostal muscles
- accessory muscles
 respiratory muscles during
expiration:
- Diaphragm
- internal intercostal muscles
- abdominal walls
•
Lung pressures

Air flows because of pressure
gradients

pleural pressure (Ppl)

alveolar pressure (PA)

Pressure changes during
respiratory cycle

pneumothorax
1- Total lung capacity (TLC): is the total amount of air contained in
the lungs after a maximum inspiration.
 TLC can be subdivided into four volumes: tidal volume, inspiratory
reserve volume, expiratory reserve volume, and residual volume.
 The vital capacity plus the residual volume equal the TLC, which is
approximately 6000 mL in a healthy, young adult.
2- Tidal Volume (TV) ,The amount of air exchanged during a relaxed
inspiration followed by a relaxed expiration is called the tidal volume
.
 In a healthy, young adult, TV is approximately 500ml per
inspiration. Approximately 350 mL of the tidal volume reaches the
alveoli and participates in gas exchange (respiration).
•
Lung volumes and capacities
Spirometry
 tidal volume (TV)
 inspiratory reserve volume (IRV)
 expiratory reserve volume (ERV)
 residual volume (RV)
 inspiratory capacity (IC)
 functional residual capacity (FRC)
 vital capacity (VC) and forced vital capacity (FVC)
 total lung capacity (TLC)
3-Inspiratory reserve volume (IRV) is the amount of air a person
can breathe in after a resting inspiration (approximately 3000 ml)
4-Expiratory reserve volume (ERV) is the amount of air a person
can exhale after a normal resting expiration (approximately 1000 mL).
5- Residual volume (RV) is the amount of air left in the lungs after a
maximum expiration (approximately 1500 mL).
 RV increases with age and with restrictive and obstructive
pulmonary diseases.
6- Inspiratory capacity (IC):
 is the maximum amount of air a person can breathe in after a
resting expiration (approximately (3500 mL).
7- Functional residual capacity (FRC) :
is the amount of air remaining in the lungs after a resting (tidal)
expiration (approximately 2500 mL).
 It is the sum of the ERV and RV.
 FRC represents the point during ventilation at which the forces
that expand the thoracic wall are in balance with the forces that
tend to collapse the lungs.
8- Vital Capacity (VC): is the sum of the TV, IRV, and ERV.
It is measured by a maximum inspiration followed by a maximum
expiration (approximately 4500 mL).
 Vital capacity decreases with age and is less in the supine position
than in an erect posture (sitting or standing).
 VC decreases in the presence of restrictive and obstructive
diseases.
•
Minute respiratory volume (V, minute ventilation)
V = VT * f (respiratory rate)
•
•
Dead space volume (VD)
Alveolar ventilation (VA): VA = (VT - VD) * f

FEV1: timed forced expiratory
volume in one second

FEV1/FVC = 80%: more
useful for detecting obstructive
vs restrictive lung diseases
Neuroanatomy Of Respiration
Two main descending pathways control the lower motor neurons that
innervate the respiratory muscles which are:
 the corticospinal (pyramidal) and bulbospinal tracts.
 The pyramidal tract is responsible for voluntary control of breathing.
Because the pyramidal motor neurons for respiration are spread over a large
area of the cortex most vascular accidents of the cortex do not cause significant
diaphragmatic impairment
Spontaneous respiration
oSpontaneous respiration is produced by rhythmic discharge of impulses from
respiratory area of the brain via motor nerves to the respiratory muscles.
oThe rhythmic discharge of impulses from the brain can be altered by many factors
such as: changing levels of oxygen, carbon dioxide and hydrogen ions(PH) of the
blood.
oRespiration is controlled by two separate neural mechanisms one
voluntary (from cerebral cortex to respiratory muscles ) and one
involuntary(in the medulla and pons).
oThe final action depends on whether the motor nerves are inhibited or stimulated
BRAIN STEM CONTROL OF BREATHING
The brain stem is the primary site for the central control of respiration.
This control occurs at a subconscious level and results in the rhythmic
contraction and relaxation of the respiratory muscles .
 This automatic state can by temporarily overridden by voluntary
mechanisms or by reflex actions such as coughing or sneezing.
These voluntary mechanisms are essential for speech and phonation .
 Like other central nervous system control systems, the brainstem respiratory
center receives afferent information from several sources.
These include the central chemoreceptors located in the anterolateral surface
of the medulla, the carotid and aortic bodies, and the various stretch receptors
in the lungs.
The respiratory center in the medulla is composed of several nuclear groups
that integrate the afferent information and possess the primary efferent
neurons that control respiration .
These efferent neurons send axons down the ventrolateral spinal
cord that synapse on the anterior horn cells that go to the
respiratory muscles. Other regions in the brain stem such as the
pneumotaxic center modify the output of the respiratory center
Neural Control of Breathing
•
Neural mechanisms

Medullary respiratory centers
inspiratory neurons: set the rhythm
expiratory neurons

receive synaptic inputs from the cortex and
pons

effects of pulmonary stretch receptors
(proprioreceptors)

failure of the respiratory center
by physical damages (concussions, cerebral
edema)
by overdose of chemical substances
(barbiturate, anesthetics)
•
Reflex control of ventilation

Chemoreceptors monitor blood gases and pH

Control centers in the brain stem regulate activity
to respiratory muscles
•
Chemical mechanisms

chemoreceptors
central chemoreceptors (in the medulla): monitor
only H+ in CSF
peripheral chemoreceptors (aortic bodies and
carotid bodies)

control of the alveolar ventilation by the arterial CO2

control of the alveolar ventilation by the arterial H+:
exclusively by peripheral chemoreceptors

control of the alveolar ventilation by the hypoxia:
relatively insensitive to hypoxia
Carotid body oxygen sensor
Central chemoreceptor
Mechanical Properties of the lung
• Lung Distensibility
• Pressure-volume curve
• Compliance (CL= DV/DP)
• Pulmonary surfactant
 surface tension
 Laplace Law: P = 2T/r
 atelectasis
•
Work of breathing
W = force X distance
Factors that affect the amount of work:

lung compliance

surface tension

airway resistance
- R  L  /r4
- diameter of the airways
Bronchoconstriction: histamine
Broncodilation: CO2, EP (2 receptors)
Compliance
Compliance refers to the distensibility elastic recoil of lung tissue or how
easily the lungs inflate during inspiration.
compliance changes with age and the presence of disease
Compliance is the change in volume for a unit change in pressure.
Compliance reflects the ability of the lungs to stretch.
Respiratory conditions that result in paralysis of respiratory muscles or
reduced surfactant, lower compliance increasing the work of breathing.
,
Airway Resistance
Normally the airways widen during inspitation and narrow during expiration,
the amount of resistance to the flow of air depends on:
a- The bifurcation and branching of airways.
b-The size (diameter) of the lumen of each airway.
c-The elasticity of the lung parenchyma
Flow rates
flow rate is determined by the volume of air exhaled divided
by the amount of time it takes for the volume of gas to be exhaled.
Flow rates are Flow rates indicate measurements of the amount of air
moved in or out of the airways over a period of time.
 Expiratory altered as the result of diseases that affect the respiratory
tree and chest wall.
For example, with chronic obstructive pulmonary disease, the expiratory
flow rate is decreased in comparison to normal. That is, it takes a longer
than the normal amount of time to exhale a specific volume of air.
Flow Rates
Flow rates indicate measurements of the amount of air moved in or
out of the airways in a period of time.
Normal sequence of chest wall motions during breathing:
First, the diaphragm contracts and the central tendon moves caudally. Intraabdominal pressure increases and abdominal contents are displaced such that the
anterior epigastric abdominal wall is pushed outward.
Once the central tendon is “fixed” or stabilized on the abdominal organs, the
appositional, vertical fibers pull the lower ribs upward and outward resulting in
lateral movement of the lower chest.
Following abdominal expansion with continued inspiration, the parasternals,
scalenes, and levatores costarum actively rotate the upper ribs and elevate the
manubriosternum, resulting in an outward motion of the upper chest
Pulmonary Circulation
A. Vascular pressure and blood flow
Pulmonary circulation is a low-pressure system
pulmonary arterial systemic pressure: 25 mmHg
pulmonary arterial diastolic pressure: 10 mmHg
mean pulmonary arterial pressure: 15 mmHg
effect of the special gravity of blood on distribution of blood flow in the lung:
poor perfusion in the upper lung (functional dead space volume)
•
•
Hypoxic vasoconstriction
balances blood flow with
ventilation

regional
hypoxia/hypoxemia

hypoxic vasoconstriction a mechanism that
balances the perfusion of
blood with the availability
of regional ventilation

effect of hypoxic
vasoconstriction at the
high altitude
Exercise recruits capillaries
and decreases transit time
Gas exchange:alveoli and cells
•
Factors that affect the rate of gas diffusion through the
respiratory membrane

thickness of respiratory membrane (alveolar-capillary
membrane): normally 0.1 - 0.5 µm
 pulmonary edema
 fibrosis of the lung

surface area of the respiratory membrane: 70 m2 in
the normal adult
 emphysema (dissolution of alveolar walls)

diffusion coefficient
 solubility in water
 molecular weight
 carbon dioxide diffuses 20 times as rapidly as
oxygen

pressure difference across the respiratory membrane
Respiratory membrane
•
Pulmonary pathologies
B. Transport of oxygen
•
Transport of oxygen in the dissolved state

•
only 2% of oxygen transported in the dissolved
state in the water of the plasma and cells
Transport of oxygen by hemoglobin

98% oxygen is carried to the tissues by reversible
combination with hemoglobin

oxygen carrying capacity: 20 ml/100ml blood

oxygen saturation: percent O2 saturation = O2
content/O2 capacity x 100

oxyhemoglobin dissociation curve

factors that affect the oxyhemoglobin curve