Respiration: It refers to interchange of gases between the
atmosphere and the cells of the body.
– External respiration: Gas exchange (O2 & CO2) between the air
– Internal respiration: Gas exchange (O2 & CO2) between the
blood and other tissues and O2 utilization by the tissues of the
Pulmonary Ventilation (breathing): is transport of air to
and from the lungs. Pulmonary ventilation (breathing)
consists of two phases:
– Inspiration (Inhalation) – air in
– Expiration (Exhalation) – air out
Functions of the respiratory system:
1. Gas exchange: Uptake of O2 by cells, and the release of CO2 to the
2. Acid base balance: CO2 +H2O←→ H2CO3 ←→ H+ + HCO3-
3. Pulmonary defense
4. Respiratory heat loss by evaporation of water: It removes heat and
water from body by warming and saturation the inhalated air with
5. Pulmonary metabolism: The lungs converts substances that pass
through the pulmonary blood vessels, for e.g. Angiotensin I →
6. It fascilitates venous return to the heart (Respiratory pump).
7. Production of sound (Vocalization): The larynx contains the paired
vocal folds (vocal cords).
8. Site for olfactory sensation (Smelling)
Physiological anatomy of the respiratory system:
The respiratory system is divided into:
1. Upper respiratory system:
2. Lower respiratory system:
- Larynx (Voice box)
- Trachea (Windpipe)
Nose and Pharynx:
– External nose
– Nasal cavity
– Passageway for air
– Filters and cleans the air
– Humidifies, warms air
– Along with paranasal
sinuses are resonating
chambers for speech
– Common opening for
digestive and respiratory
– Three regions:
– The air passageway between pharynx and trachea.
– The epiglottis prevents food and liquid from entering the larynx
and directed through the pharynx into the esophagus.
– Vocal folds (vocal cords) are primary source of sound production.
- It is a flexible tube, extends from the larynx to the
primary bronchi. It transports air to and from the
- The wall of the trachea contains 16-20 C-shaped
rings of hyaline cartilage, which keep the trachea
- The mucosa of the trachea is ciliated epithelium
with mucus-producing goblet cells. As in the
larynx the cilia sweep upward toward the
Components of the lower respiratory tract:
1. Conducting zone:
- Terminal bronchioles
2. Respiratory zone:
The Site gas exchange (O2 and CO2) between air and blood
which Composed of:
- Respiratory bronchioles
- Alveolar ducts
- Alveolar sacs
Physical properties of the lungs:
- Distensibility (stretchability)
- The expansibility of the lungs and thoracic wall, necessary
for adequate alveolar ventilation.
- Thoracic compliance may be decreased by factors that
produce resistance to distension for e.g., fractured ribs.
- Tendency of the lungs to return to the initial size after
- High content of elastin proteins (Recoil ability).
- The functional units of the lungs are the air sacs
- The alveoli are tiny grape-like sacs which are
branching of the respiratory tree where gas
exchange takes place in the lungs.
- There are about 300 to 400 million alveoli in each
lung with adiameter of about 0.2 milimeter.
- The total surface area is estimated to be about
70m2 in the normal adult human male.
There are three major types of the cell in the alveolar wall:
1. Alveolar type I cells: that form most of the alveolar walls
are simple squamous epithelium.
2. Alveolar type II cells: Also called septal cells secrete
pulmonary surfactant, a lipoprotein which mixes with the
tissue fluid within the alveoli and decreases its surface
tension, permitting inflation of the alveoli.
3. Alveolar macrophages: Within the alveoli are
macrophages that phagocytize pathogens or other
foreign material that may not have been swept out by
the ciliated epithelium of the bronchial tree.
- Alveolar fluid contains a substance that reduces surface tension.
This substance is called surfactant.
- Surfactant is greatly reduces the surface tension of water.
- It is secreted by special surfactant-secreting epithelial cells called
alveolar type II cells into alveoli and respiratory passage.
- Surfactant is a complex mixture of several phospholipids for e.g.,
dipalmitoylphosphatidylcholine (DPPC), proteins, and ions.
- It is important at birth. The lungs remain collapsed until birth.
After birth, the infant makes several strong inspiratory
movements and the lungs expand. Surfactant keeps them from
- Surfactant begins to be produced in late fetal life. For this reason,
premature babies are sometimes born with lungs that lack
sufficient surfactant and their alveoli are collapsed as a result.
This condition is called respiratory distress syndrome (RDS).
A double-layered sac surrounding each lung:
- Parietal pleura( outer layer)
- Visceral pleura (directly on lung)
Pleural cavity: The space between the visceral and parietal pleurae filled with pleural
fluid which is lubricate movement of lung within the cavity.
Mechanics of respiration:
- Diaphragm conrtraction→chest volume↑ (lungs expansion)
→intrapleural pressure ↓→alveolar pressure<atmospheric
pressure→air is sucked into the lungs.
(At the end of an inspiration, alveolar pressure=atmospheric
pressure and airflow stops )
- Diaphragm relaxation→chest volume↓ (lungs conrtraction)
→intrapleural pressure ↑→alveolar pressure>atmospheric
pressure→ air is pushed out of the lungs. Therefore,
expiration is a passive process. (At the end of an exspiration,
also alveolar pressure=atmospheric pressure and airflow
Figure: The mechanics of pulmonary ventilation.
- Muscles of the chest wall help produce changes in
chest volume beyond that produced by the
contraction and relaxation of the diaphragm.
- Contraction of the external intercostal muscles
helps increase the volume of the chest for
stronger inspiration. while contraction of the
internal intercostal muscles helps to decrease
chest volume for stronger expiration.
- Atmospheric pressure: It is the pressure excreted by
the weight of air in atmosphere. It is about 760 mmHg.
- Intrapulmonary pressure (alveolar pressure): It is the
pressure of the air inside the alveoli.
- Intrapleural pressure: It is the pressure of the ﬂuid in
the thin space between the lung pleura (visceral
pleura) and the chest wall pleura (parietal pleura). It is
always less than Intrapulmonary pressure.
- Transpulmonary pressure: It is the difference beween
the alveolar pressure and intrapleural pressure.
Figure: Lung pressure change during inspiration and expiration.
- Tidal volume (TV): The volume of air inspired or expired
during a normal inspiration or expiration; its amount is
about 500 mL in the adult male.
- Inspiratory reserve volume (IRV): The amount of air
inspired forcefully after inspiration of normal tidal
volume (3000 mL).
- Expiratory reserve volume (ERV): The amount of air
forcefully expired after expiration of normal tidal
- Residual volume (RV): The volume of air remaining in
respiratory passages and lungs after the most forceful
expiration (1200 mL).
- Inspiratory capacity (IC): The maximal amount of air which can be
inspired after a normal expiration.
IC = Tidal volume (TV) + Inspiratory reserve volume (IRV)
- Vital Capacity (VC): The maximum amount of air which can be
expired after a maximal inspiration.
VC= Inspiratory reserve volume (IRV) + Tidal volume + Expiratory
reserve volume (ERV)
- Total lung capacity (TLC): The amount of air contained in the lungs at
the end of maximal inspiration.
TLC = Vital capacity + Residual volume (RV)
- Functional residual capacity (FRC): The amount of air that remains in
the lungs at the end of normal expiration.
FRC = Expiratory reserve volume (ERV)+ Residual volume (RV)
- Respiratory rate or frequency: The number of breaths taken per
minute (12 breaths/min).
- Pulmonary ventilation: The amount of air inspired per minute.
Pulmonary ventilation = Respiratory rate X Tidal volume
= 12 breaths/min X 500 ml = 6 L/min.
- Alveolar ventilation: The total volume of fresh air entering the alveoli
= respiratory rate X (Tidal volume – Dead space volume)
= 12 X (500 – 150)= 4.2 L/min
Minute ventilation: Total amount of air moved into and out to of
Respiratory system per min.
- Anatomical dead space: The volume of conducting air
ways at which there is no gaseous exchange (about
150ml) increase with age.
- Alveolar dead space: Volume of inspired air that is not
used for gas exchange .
- Physiological dead space: The sum of Anatomic and
Alveolar dead space.
Gas exchange in the lungs:
Air-Blood barrier (Pulmonary membrane, respiratory
membrane, Alveolar-Capillary barrier):
• The membrane which gas exchange occurs which is about 0.2 µm.
• The alveolar-capillary barrier is composed of:
- A thin alveolar epithelium
- An epithelial basement membrane
- A thin interstitial space between the alveolar basement membrane
and capillary basement membrane.
- A capillary basement membrane that in many places fuses with the
alveolar epithelial basement membrane
- The capillary endothelial membrane
• The total surface area of the respiratory membrane is about 70m2 in
the normal adult human male.
• Dalton’s Law
– Law of Partial Pressures
• “each gas in a mixture of gases will exert a pressure independent
of other gases present”
• The total pressure of a mixture of gases is equal to the sum of the
individual gas pressures.
– Atmospheric components
• Nitrogen = 78% of our atmosphere
• Oxygen = 21% of our atmosphere
• Carbon Dioxide = 0.033% of our atmosphere
• Water vapor, krypton, argon, …. Make up the rest
PATM = PN2 + P02 + PC02 + PH20= 760 mm Hg.
Changes in Partial Pressure:
• In inspired air:
– PO2=160 mmHg
– PCO2=0.3 mmHg
– PH2O=5.7 mmHg
– PN2=596 mmHg
• In alveolar air (mixing with dead space air):
– PO2=104 mmHg
– PCO2=40 mmHg
• In venous blood entering the lungs:
– PO2=40 mmHg
– PCO245 mmHg
• O2 diffuses across the respiratory surfaces into blood, CO2
flows from blood to alveolar air.
• Expired air is saturated with water vapor.
• In expired air:
– PO2=120 mmHg
– PCO2=27 mmHg
– PH2O=47 mmHg
– PN2=565 mmHg
• Arterial blood leaving the lungs:
– PO2=95 mmHg
– PCO2=40 mmHg
• In the tissues, concentrations of gases and pressures
exerted by them vary depending on the amount of
metabolic activity going on in the tissue at any one time.
– PO2=20 mmHg
– PCO2=46 mmHg
• O2 flows from blood to tissues, and CO2 from tissues to
Change in partial pressure in pulmonary capillary and tissue capillary
Transport of gases in the blood:
I. Oxygen Transport:
• O2 is carried in two ways by the blood:
1. Dissolved in plasma: (1.5 %)
The amount of O2 in the plasma is proportional to
the partial pressure. At a pressure of 100 mmHg,
0.3 ml O2 is dissolved in every 100ml blood.
2. Combined with hemoglobin: (98.5 %)
Most of the O2 is carried in combination with
hemoglobin. Each hemoglobin molecule having the
capacity to combine with four molecules of O2.
Factors affecting Hb-O2 dissociation curve:
The ability of Hb to hold O2 is decreased by:
1. A decrease in pH,
2. An increase in carbon dioxide, or
3. An increase in temperature,
4. The presence of 2.3-diphosphoglycerate (2,3-DPG) in
red blood cells stimulate the release of O2 from HbO2.
Transport of Carbon Dioxide:
Carbon dioxide is transported by:
1. Dissolved in plasma (10%):
- The solubility of CO2 in blood is about 20 times that of O2, so that there
is more CO2 than O2 in simple solution.
2. Carbaminohemoglobin (CO2+Hb) (20%):
- The CO2 can combine with amine (-NH2) groups in proteins to form
carbonic compounds. Because more amine groups are available in
hemoglobin than in plasma proteins, more CO2 combines with
hemoglobin than with plasma proteins to form carbamino
3. Bicarbonate ions HC03
- Red blood cells contain the enzyme carbonic anhydrase, which
catalyzes the combination of H2O with CO2.
CO2 + H2O carbonic anhydrase H2CO3 H + + HCO3
Figure: Carbon Dioxide Transport and Chloride Shift
Regulation of Respiration:
The respiratory center is located in the pons and medulla
oblongata of the brain stem:
1. Pons: It consist of two respiratory control center:
a). Apneustic center: It promotes the inspiration.
b). Pneumotaxic center: It mainly controls rate and depth of breathing
(Respiratory rhythm). It inhibits the inspiration.
2. Medullary oblongata: It consist of the following groups of the
a). Dorsal respiratory group (DRG): Mainly cause inspiration (Normal
b). Ventral respiratory group (VRG): cause expiration and inspiration
Dorsal respiratory group (DRG):
- It is located in the dorsal portion of medulla.
- The signal transmitted from this group through the motoneuron
phrenic nerve mainly to diaphragm and cause normal quiet
- The signal are rhythmic and ramp in nature. When signal stop
suddenly will allow the elastic recoil of lung and chest wall to cause
Ventral respiratory group (VRG):
- It is located in the ventral aspect of medulla.
- They send signals to inspiratory and expiratory muscle during active
or labored respiration.
- It is located dorsally in the superior portion of the pons, which mainly
controls the rate and depth of breathing.
- The normal function of the pneumotaxic center is unknown, but it
may play a role in switching off between inspiration and expiration.
Figure: Organization of the respiratory center
Chemical control of respiration:
1. Central chemoreceptors (Chemosensitive area):
- It is located in the ventral surface of medulla and sensitive to change
of PCO2 and H+ concentration.
- The chemoreceptors monitor the H + concentration of CSF, including
the brain interstitial fluid. CO2 readily penetrates membranes,
including the blood-brain barrier, whereas H + and HCO3-
- The CO2 that enters the brain and CSF is promptly hydrated. The
H2CO3 dissociates, so that the local H+ concentration rises. Any
increase in spinal fluid H+ concentration stimulates respiration.
The magnitude of the stimulation is proportionate to the rise in H+
- Thus, the effects of CO2 on respiration are mainly due to its
movement into the CSF and brain interstitial fluid, where it
increases the H+ concentration and stimulates receptors sensitive
to H +.
Figure: Chemoreceptors in the medulla oblongata.
2. Peripheral chemoreceptors:
They are located outside the brain in the:
- Carotid bodies in the bifurcation of the common carotid
- Aortic bodies along the arch of the aorta.
- They are especially important for detecting changes in O2
in the blood, although they also respond to a lesser
extent to changes in CO2 and H+ ion concentrations.
- They are stimulated by decreasing of PO2 in blood, and
also by increasing of PCO2 and H+ ions in blood.
- Eupnea: Normal, comfortable breathing at rest.
- Apnea: Cessation of breathing.
- Dyspnea: Unpleasant, subjective feeling of difficult or
- Polypnea: Increased depth or rate of breathing or both.
- Polypnea: Rapid, shallow breathing.
- Acute respiratory disease syndrome (ARDS): It is a severe
inflammatory disease of the lung. The inflammation leads to
impaired gas exchange.
- Infant respiratory distress syndrome (IRDS): It is a syndrome
caused by lack of surfactant in the lungs of premature infants.
- Pneumothorax: The presence of gas in the intrapleural space (the
space between the visceral and parietal pleurae) causing the
lungs to collapse.
- Pulmonary fibrosis: The normal structure of lungs disrupted by
accumulation of fibrous connective tissue proteins.
- Emphysema: The loss of elasticity in the lungs leads to
prolonged times for exhalation. This leads to a smaller
volume of gas exchanged per breath.
- Chronic bronchitis: It occurs when an abundance of
mucus is produced by the lungs and the air passages
become clogged with mucus.
- Lung cancer: It is a common form of cancer causing the
uncontrolled growth of cells in the lung tissue.
- Pneumonia: It is an infection of the lung parenchyma,
which can be caused by both viruses and bacteria.
Cytokines and fluids are released into the alveolar cavity
in response to infection, causing the effective surface
area of gas exchange in the lungs to be reduced.
Atelectasis: It mean collapse of alveoli due to obstruction of
air way or lack of surfactant.
Asphyxia: In asphyxia produced by occlusion of the airway,
acute hypercapnia and hypoxia develop together.
Drowning: Drowning is asphyxia caused by immersion,
usually in water.
Asthma: It is characterized by airway obstruction, airway
inflammation, and airway hyper-responsiveness to a
variety of stimuli. Proteins released from eosinophils in
the inflammatory reaction may damage the airway
epithelium causing the amount of air flow into the lungs
to be greatly reduced. The usual cause of asthma is
contractile hypersensitivity of the bronchioles in response
to foreign substances in the air such as plant pollen and
irritants in smog.
Hypoxia: It is O2 deficiency at the tissue level.
Traditionally, hypoxia has been divided into four types:
1. Hypoxic hypoxia (anoxic anoxia): in which the PO2 of the
arterial blood is reduced.
2. Anemic hypoxia: in which the arterial PO2 is normal but
the amount of hemoglobin available to carry O2 is
3. Stagnant Hypoxia:
Hypoxia due to slow circulation is a problem in organs
such as the kidneys and heart during shock.
4. Histotoxic Hypoxia:
Hypoxia due to inhibition of tissue oxidative processes is
most commonly the result of cyanide poisoning. Cyanide
inhibits cytochrome oxidase and possibly other enzymes.65
- The term cyanosis means blueness of the skin, and its
cause is excessive amounts of deoxygenated hemoglobin
in the skin blood vessels, especially in the capillaries.
- It is most easily seen in the nail beds and mucous
membranes and in the earlobes, and lips, where the skin
Carbon monoxide (CO) poisoning:
- The CO poisoning is toxic because it reacts with
hemoglobin to form carboxyhemoglobin (COHb)
- The COHb cannot take up O2.
Structure of Avian respiratory system
• The mouth or the nostrils (nares).
• The pharynx
• The trachea (Windpipe).
• Primary bronchi
• Air sacs
- The trachea branches into two primary bronchi at the
syrinx. The syrinx is situated at bifurcation of the
trachea, and it function as sonic membrane (voicebox).
- The primary bronchi enter the lungs and branch off into
dorsobronchi which lead into smaller parabronchi.
- Parabronchi can be several millimeters long and 0.5-2.0
mm in diameter (depending on the size of the bird) and
their walls contain hundreds of tiny, branching 'air
capillaries' surrounded by a profuse network of blood
capillaries. It is within these 'air capillaries' that the
exchange of gases (oxygen and carbon dioxide) between
the lungs and the blood occurs.
- After passing through the parabronchi, air moves into the
Avian air sacs:
• The air sacs is thin walled, about 10 times the
volume of the lungs. It situated between the internal
organs in the thoracic and abdomen.
• The air sac extend into the proximal bones of the
extrimities, and the skull. Replacing bone marrow
with air makes the bird lighter.
• Gas exchange does not occur across the wall of the
• The air sacs function as bellows, to ventilate the
lungs with fresh air with higher O2 content during
both inspiration and expiration. The pulmonary air
flow is continuous and unidirectional through
Most species of the birds have nine air sacs:
• One interclavicular sac
• Two cervical sacs
• Two anterior thoracic sacs
• Two posterior thoracic sacs
• Two abdominal sacs
Figure: The air sacs of birds extend into the humerus (the bone
between the shoulder and elbow), the femur (the thigh bone),
the vertebrae and even the skull.
Figure: Comparison of avian and mammals respiratory system:
a). The avian 'unidirectional' respiratory system where gases are exchanged
between the lungs and the blood in the parabronchi.
b). The mammals 'bidirectional ' respiratory system where gas exchange
occurs in small dead-end sacs called alveoli.
Figure: Avian respiration occurs in two cycle:
First inspiration: The air flows through the trachea and bronchi into the
posterior air sacs.
First expiration: The air flows from the posterior air sacs to the lungs.
Second inspiration: air flows from the lungs to the anterior air sacs.
Second expiration: The air flows from the anterior sacs back through
the trachea and out of the body.
Figure: The cross-current gas-exchange mechanism
operating in the avian lung (between the blood
capillaries and air capillaries).