The Respiratory System
Physiology -2
PHL 226
Dr/ Abdulaziz Bin Saeedan
Pharmacy College
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The Respiratory System
Location:
• Respiratory system is located in the head,
neck and thorax (chest).
• It starts with the nose and ends with the lungs.
• The system is protected by the rib cage.
Functions:
❶ Ventilation of the lungs
❷ Supply all tissues of the body with oxygen.
❸ Production of sound.
❹ Olfactory sensation.
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The Pathway of Air
The respiratory system consists of:
1- The upper respiratory tract.
2- The lower respiratory tract.
❶ The Upper Respiratory Tract:
• It located in the head and neck.
• It consists of:
a- Nose
b- Pharynx
c- Larynx
• No gas exchange occurs in the upper
respiratory tract.
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a- Nose:
 The nasal cavities are lined with a ciliated mucous membrane.
 Dust particles in the inhaled air can damage the lung by irritating the inner
surface of the alveoli.
 Most of these particles fail to reach the lungs because of the ciliated mucous
membrane of the nose.
Functions of the nasal cavities:
1- Filtration of inhaled air (Trapping of dusts and bacteria).
2- Warming of inhaled air.
3- Humidification of inhaled air.
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b- The Pharynx:
• It is a part of the digestive and respiratory systems.
The pharynx is consists of 3 parts:
1- Nasophaynx 2- Oropharynx 3- Laryngopharynx
Functions:
1- It is a passageway of both air and food.
2- It aids in equalization of the air pressure in the middle
ear.
c- The larynx:
• The larynx is a muscular and cartilaginous structure that
located between the pharynx and the trachea.
• It holds the vocal cords.
• This body organ is used by human beings to breathe
and talk.
Function:
 Voice production.
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❷ The Lower Respiratory Tract:
• It is located in the chest and consists of:
a- Trachea b- Bronchial tree c- 2 Lungs
a- The trachea (windpipe) :
 It is a part of the respiratory tract through which air passes
from the nose to the lungs.
 The trachea is flexible.
 The tracheal wall contain a number of C-shaped rings
of cartilage (15 – 20).
 The C shape is open to the posterior surface of the
trachea to be near to the esophagus. This allows the
esophagus to expand easily when food passes through
it.
 At their posterior surface, the tracheal rings contain
smooth muscle (Trachealis muscle) instead of cartilage.
 Trachealis muscle can contract to narrow the trachial
diameter. This response helps to expel irritants in the
airway through coughing.
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 Functions of cartilage rings:
1- Prevent the trachea from collapsing due changes in air pressure
that occurs during respiration.
2- Maintains the trachea in the open position
o The trachea is lined with ciliated mucous membrane which
contain:
1- Mucous-secreting cells (goblet cells).
 They secrete mucous. The respiratory system produces about
125 mL of mucous each day that is removed by the movement
of the cilia.
 The mucous traps dust, bacteria and viruses.
2- Ciliated epithelial cells:
 The surface of each cell has hair-like structures called cilia.
 The cilia beats, so moves the mucous up into the upper
respiratory tract then removed by coughing or by swallowing.
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Functions of the trachea:
1- Conduct air to and from the lungs.
2- Traps dust, bacteria and viruses in its mucous.
NOTE:
 Tracheostomy or tracheotomy is an incision performed
on the anterior aspect of the trachea, usually between the
2nd and 3rd cartilage rings, that allows for the insertion of
a tube for breathing.
b- The bronchial tree:
 The trachea is branched forming the bronchial tree.
 The bronchial tree consists of:
o Bronchi
o Bronchioles
o Alveoli
 The bronchial tree is lined with ciliated mucous
membrane.
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 Bronchi and Bronchioles :
 The trachea divides into 2 tubes called the primary
bronchi (Right & Left).
 The 2 primary bronchi enter the lungs then further
divided into smaller branches called bronchioles.
 Diameter of bronchioles  0.5 - 1 mm.
 The wall of bronchioles contain smooth muscle but
NO cartilage. This allows contraction and relaxation.
 The final branches of the bronchioles are called the
alveolar ducts and ends in the alveoli.
 Alveoli:
 The alveoli are tiny air sacs.
 Walls of alveoli are very thin (about 0.25 mm)
 To facilitate the exchange of gases.
 They are surrounded by a network of capillaries
 To facilitate the exchange of gases.
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 Alveolar membrane is made up of 3 types of
cells:
a. Type I alveolar cells:
 Are very flat cells (less than 0.2 μm thick).
 Their primary function is gas exchange
b. Type II alveolar cells:
 Are secretory cells.
 They secrete lipoprotein called pulmonary
surfactant
 Pulmonary surfactant reduces the surface
tension of water molecules that covers the
alveolar walls (alveolar fluid) so, it is
responsible for maintaining elasticity of the
alveoli.
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NOTES:
• Sometimes newly born infant dies within a few
days because of respiratory difficulty.
• This is due to absence of surfactant in the
alveoli.
• This disease is known as Respiratory Distress
Syndrome or Hyaline Disease.
c. Alveolar Macrophages:
 Are phagocytic cells.
 They phagocytose dust and microorganisms
that may reach to the alveoli.
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 Lungs:
 The lungs are a pair of spongy organs located
on the chest cavity.
o They are highly elastic.
o They respond to the actions of the diaphragm
and the rib cage.
Functions of the lungs
1- Taking in of O2 in the inspired air to supply
O2 to all tissues of the body.
2- Giving away of CO2 in the expired air.
3- Maintaine pH of blood at 7.3 by giving out
CO2 in expired air.
NOTE:
Excess CO2 combines with water to form
carbonic acid which will make the blood highly
acidic.
 The pleura:
 Each lung is covered by a double layers of pleura.
1- Parietal pleura: It is the outer membrane that attached to
the chest wall. It is highly sensitive to pain.
2- Visceral pleura: It is the inner membrane that attached
to the outer surface of the lung. It is not sensitive to pain.
 Space between these 2 layers is known as intra pleural
space or pleural cavity.
 This space is filled with serous fluid called pleural
fluid.
 The pleural fluid acts as a lubricant, So reduces
friction between the lungs and the chest wall during
breathing.
 Without this fluid, breathing would be very difficult
and even painful.
 Pressure within the intra pleural space is called the
intra pleural pressure and the pressure inside the lung
(alveoli) is called the intrapulmonary pressure.
 The intrapleural pressure is always less than the
intrapulmonary pressure because of elastic recoile of
lung alveoli.
 This pressure gradient between the intra pleural space
and the lung is referred to as transpulmonary pressure
that is necessary to keep the lungs expanded at all
times.
 If the transpulmonary pressure is removed, the lungs
will collapse.
Role of the pleura:
1- It gives protection to the lungs
2- Allows the lungs to inflate and deflate without friction.
3. Because of negative pressure of the intra pleural space,
inspiration and expiration becomes easy.
• Pleural diseases include:
1- Pleural effusion:
• It is the accumulation of excessive fluid in the pleural cavity. This can cause
difficulty breathing and chest pain.
2- Pneumothorax:
• It is the accumulation of air in the pleural cavity.
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The Respiratory Membrane
 It is the membrane that separate air (in the
alveoli) from the blood (in the pulmonary
capillaries).
 The respiratory membrane is very thin (less
than 0.5 mm).
 So facilitate the process of gas exchange
 It consists of the alveolar and capillary walls:
1- Alveolar fluid.
2- Alveolar epithelium.
3- Epithelial basement membrane.
4- Interstitial space.
5- Capillary basement membrane.
6- Capillary endothelium.
What are the characteristics of an ideal respiratory surface?
The respiratory surface should have the following characteristics:
1- It should be permeable to the gases.
2- It should be thin (1mm or less) to allow effective diffusion.
3- It should be richly supplied with blood vessels to allow maximum uptake of
oxygen
4- It should have a large surface area to allow maximum uptake of oxygen in
minimum time.
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What is The Partial Pressure (PP)?
 PP is the pressure that generated by a particular gas within a mixture of
gases
• The atmospheric air composed of mixture of gases; mainly:
Nitrogen: 78%,
Oxygen: 21%,
Carbon dioxide: 0.04%,
Water vapour: variable.
• This atmospheric air generates pressure called atmospheric or total
pressure (= 760 mmHg).
• The pressure that generated by O2 is the PP of O2 (PO2) and that
generated by CO2 is the PP of CO2 (PCO2).
• The PP of each gas equals the atmospheric pressure (760 mmHg) times
the % of the gas in the mixture.
Examples:
 PO2 in the air = 0.21
X 760 mmHg = 160 mmHg.
 PCO2 in the air = 0.0004 X 760 mmHg = 0.3 mmHg.
Factors that influence rate of gas diffusion
through the respiratory membrane
1- Thickness of the membrane.
2- Surface area of the
membrane.
3- The PP difference of the gas
between the 2 sides of the
membrane.
Factors
1- Thickness of the membrane:
 Increasing the thickness of the respiratory membrane (as in case of pulmonary
edema) decreases the rate of gas diffusion.
 So, decrease the rate of gas exchange.
NOTE:
 For gas exchange to be efficient, the respiratory membrane must be 0.5 to 1
micrometer thick
2- Surface area of the membrane:
 The total surface area of the respiratory membrane is about 70 m2
 approximately the area of one half of a tennis court.
 Decreasing the surface area of the respiratory membrane (as in case of emphysema,
lobectomy) decreases the rate of gas diffusion.
 So, decrease the rate of gas exchange.
NOTE:
Emphysema is characterized by destruction of the walls of the alveoli producing
abnormally large air spaces that remain filled with air during exhalation
Factors
3- The PP difference of the gas between the 2 sides of the membrane.
 It is the difference between the PP of a gas in the alveolar air and in blood of
the alveolar capillaries.
 Diffusion of a gas occurs from the area of higher pressure to the area of lower
pressure.
 Normally:
• PO2 is greater in the alveolar air than in blood of the alveolar capillaries.
• PCO2 is greater in blood of the alveolar capillaries than in the alveolar air.
 So, O2 pass from the alveoli to the blood and CO2 from the blood to the alveoli.
NOTE:
The partial pressure determines the direction of respiratory gas movement
Respiration
 Respiration is the process that supply all parts of the body with O2 and expel out
the CO2.
 Respiration is very essential for life.
Rate:
 The normal rate of respiration varies with age, exercise and other factors.
 Children breathe more than 20 times / min.
 Adults breathe 16 to 20 times / min.
 Elderly often breathe less than 16 times / min.
 Each day we breathe about 20,000 times.
Eupnea
 Normal respiration.
Dyspnea  Painful or difficult respiration.
Tachypnea  Abnormal respiratory rate: More than 24 times /min.
Bradypnea  Abnormal respiratory rate: Less than 10 times /min.
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Respiration
Respiration includes 4 processes:
I- Pulmonary Ventilation (Breathing).
II- External Respiration.
III- Gas Transport.
IV- Internal Respiration.
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I- Pulmonary Ventilation
(Breathing)
 It is the flow of air in and out of the lungs.
 Lungs are not muscular, so cannot ventilate
themselves.
 There are certain muscles that changes the size of the
chest cavity, so play a role in breathing:
1- The diaphragm:
 It is a sheet of muscle (dome-shaped muscle) located
below the lungs and is essential for breathing.
 The main function of diaphragm:
o It pumps CO2 out of the lungs and pull O2 into the
lungs
2- The intercostal muscles  located between the ribs.
 Pulmonary ventilation include 2 stages:
 Inspiration (Inhalation): i.e, breathing in.
 Expiration (Exhalation): i.e, breathing out.
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I- Pulmonary Ventilation - Breathing
Boyle’s law:
 Movement of air during the process of inhalation and exhalation depends upon
Boyle’s law:
 In a closed container; there is an inverse relationship between pressure of a gas
and the size of the container.
 If the size of the closed container is decreased, the pressure inside the container
is increased and vice versa.
According to Boyle’s law:
o Movement of air in and out of the lungs is due to difference in pressure
o Air moves from the atmosphere into the lungs when the pressure within the
alveoli of the lungs (intrapulmonary pressure) is less than that of the atmospheric
pressure.
o Air moves from the lungs to the atmosphere, when the intrapulmonary pressure
is greater than that of the atmospheric pressure.
o The intrapulmonary pressure depends on movement of the diaphragm and the
intercostal muscles.
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I- Pulmonary Ventilation - Breathing
Inhalation (Inspiration):
 In this process, the diaphragm and the intercostal
muscles contracts.
 The contracting diaphragm flattens and stretches the
elastic lungs downward.
 The contracting intercostal muscles pull the ribcage
out and up, so stretches the elastic lungs out and up.
 These contractions increases the volume of the chest
cavity (including the lungs), so decrease the air
pressure inside the lungs.
 Air flows from high pressure to lower so air flows
from the outside into the lungs.
 Inspiration is an active process
 because it involves muscle contraction.
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I- Pulmonary Ventilation - Breathing
Exhalation (Expiration):





Exhalation is mainly due to:
Relaxation of the diaphragm.
Relaxation of the intercostal muscles.
The elastic recoil of the lungs.
Accordingly:
 The diaphragm returns to its dome-like shape.
 The ribcage returns to its normal position.
 The volume of the chest cavity decreases (including the
lungs), so increase the air pressure inside the lungs.
 Air flows from high pressure to lower so air flows from
the lungs into the outside.
 Exhalation is a passive process
 because it does not involve muscle contraction.
NOTE:
When you hold your breath for half a minute, the
concentration of CO2 in the blood increases
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II- External Respiration
 It is the process of gas exchange between the
alveoli and the blood.
• Gas exchange occurs as a result of diffusion.
• Diffusion of a gas occurs from the area of higher
pressure to the area of lower pressure.
 O2 moves from the alveoli (higher PO2) into the
blood .
 CO2 moves from the blood (higher PCO2) into the
alveoli.
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III- Gas Transport
 It is the process of distributing the O2 into the
body cells and carrying CO2 into the lungs.
 The process of gas transport is carried out by the
Cardiovascular system.
a- Oxygen transport
O2 is carried by blood in 2 forms:
1 - Bound to Hb - 98.5%.
 Oxyhemoglobin
2 - Dissolved in the plasma -1.5%
NOTE:
The solubility of O2 in water is very low,
therefore, 98.5% of the O2 is transported by Hb.
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b- Carbon dioxide transport
• CO2 is carried by blood in 3 forms:
1 - Bicarbonate (HCO3) - 60%
 Formed inside red blood cells when CO2 (released by cells) combines with H2O
forming carbonic acid (H2CO3).
CO2 + H2O  H2CO3
 This process accelerated by the effect of carbonic anhydrase enzyme (in red
blood cells).
 Carbonic acid dissociates into Hydrogen ions (H+) and Bicarbonate ions
(HCO3−).
H2CO3  H+ + HCO3−
 Bicarbonate ions diffuses out of the red blood cell into the plasma
 Hydrogen ions (H+) binds to the protein portion of the Hb (thus having no effect
on pH).
 At the lungs, bicarbonate ions enter the red blood cells and combine with
hydrogen ions to form carbonic acid (H2CO3). HCO3− + H+  H2CO3
 Carbonic acid dissociates into CO2 and water. H2CO3  CO2 + H2O
 CO2 diffuses out of the red blood cells into the alveoli.
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2 - Bound to Hb - 30%

Carbaminohemoglobin
• Formed when CO2 combines with hemoglobin.
3 - Dissolved in the plasma - 10%
Only about 10% of the CO2 generated in the tissues dissolves directly in the plasma.
NOTE:
CO2 is 20 times more soluble in water than O2
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IV. Internal Respiration
• It is the process of gas exchange between the
cells and the blood.
• Gas exchange occurs as a result of diffusion.
• Diffusion of a gas occurs from the area of higher
to the area of lower pressure.
 O2 moves from the blood (higher PO2) into the
cells.
 CO2 moves from the cells (higher PCO2) into the
blood then transported to the lungs.
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Respiratory Volumes & Capacities
 Volumes
• There are 4 respiratory volumes which:
 Do not overlap.
 Can not be further divided.
 When added together equal total lung capacity.
• Respiratory volumes are recorded by Spirometer.
 Capacities
• A capacity is a measure of lung function that consists of 2 or more
volumes.
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Respiratory Volumes
1- Tidal Volume (TV):
Volume of air that inspired or expired in one breath under normal condition (500
mL). OR it is the volume of air exchanged during normal breathing.
2- Inspiratory Reserve Volume (IRV):
Maximum volume of air which can be inspired after inhalation of normal tidal
volume (3000 mL).
3- Expiratory Reserve Volume (ERV):
Maximum volume of air which can be expired after the expiration of normal tidal
volume (1500 mL).
4- Residual Volume (RV):
Volume of air that remains in the lungs after maximum expiration (1200 mL).
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NOTES:
1- Residual Volume is the volume of air that can never be forced out
 so alveoli do not collapse.
2- Residual Volume is the only lung volume which cannot be measured with a spirometer.
3- Dead space is the volume of air that remains in the lungs and respiratory passages (trachea,
bronchi and bronchioles) after maximum expiration, where there is no exchange of gases
(300 mL).
Respiratory Capacities:
1- Total Lung Capacity: The sum of the 4 volumes.
TLC = TV + IRV + ERV + RV
2- Vital Capacity:
 The maximum volume of air that can be exhaled after taking the deepest breath possible
 The maximum amount of air that can be exchanged by a person.
VC = TV + IRV + ERV
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Control of Breathing
• Breathing is normally under unconscious control.
• It is controlled by some areas of the brain called
Respiratory Centers.
The Respiratory centers consists of:
1- The inspiratory center, located in the medulla
oblongata.
2- The apneustic area, located in the pons.
3- The pneumotaxic area, located in the pons.
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Control of Breathing
1- The inspiratory center:
It generates nerve impulses that stimulate contraction of the inspiratory muscles
(diaphragm and intercostal muscles).
2- The apneustic area:
It stimulates the inspiratory center to prolong the contraction of the inspiratory
muscles.
3- The pneumotaxic area:
It inhibits the inspiratory center to limits the contraction of the inspiratory
muscles, and prevents the lungs from overinflating.
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Control of Breathing
• The respiratory centers are influenced by 3 groups of sensory neurons:
a- Central chemoreceptors.
b- Peripheral chemoreceptors.
c- Stretch receptors.
a- Central chemoreceptors:
 Located in the medulla oblongata
 They can detect the pH of the CSF (or concentration of CO2 ).
 When CO2 content of the CSF rises above the normal level, it dissolve in water
forming Carbonic acid (H2CO3). H2O + CO2  H2CO3
 Carbonic acid dissociates into Bicarbonate ions (HCO3−) and Hydrogen ions
(H+), so the pH of the CSF drops (due to H+)  i.e. becomes more acidic.
H2CO3  HCO3− + H+
 The decrease in pH of the CSF, stimulates the central chemoreceptors which
reflexly stimulates the inspiratory center to increase the inspiratory rate.
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Control of Breathing
b- Peripheral chemoreceptors:
 Located in:
 Aortic bodies in the wall of the aortic arch.
 Carotid bodies in the walls of the carotid arteries.
 They can detect the concentration of CO2 in the arterial
blood.
 When CO2 content of the arterial blood rises above the
normal level, it dissolve in water forming Carbonic acid
(H2CO3).
H2O + CO2  H2CO3
 Carbonic acid dissociates into Bicarbonate ions
(HCO3−) and Hydrogen ions (H+), so the pH of the
blood drops (due to H+)  i.e. becomes more acidic.
H2CO3  HCO3− + H+
 The decrease in pH of the arterial blood, stimulates the
peripheral chemoreceptors which reflexly stimulates the
inspiratory center to increase the inspiratory rate.
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Control of Breathing
c- Stretch receptors:
 Located in the walls of bronchi and bronchioles.
 Inflation of the lungs to their physical limit, stimulate stretch receptors, which
reflexly decrease the activity of the inspiratory center.
NOTE:
• Chemoreceptors stimulated more by increased CO2 levels than by decreased O2
levels.
• Thus, the breathing mechanism is controlled by rising levels of carbon dioxide,
not low levels of oxygen.
• The most powerful respiratory stimulus for breathing in a healthy person is
increase of carbon dioxide.
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