THE RESPIRATORY
SYSTEM
FUNCTIONS


Exchange of gases (O2 & CO2) between
environment and tissues.
It plays a role in the regulation of pH of the
extracellular fluid.
MECHANICS OF BREATHING

Anatomical consideration:
The lungs
 Thoracic cage (muscles, ribs and vertebrae)
 Diaphragm
 Pleural space (visceral and parietal pleurae)


Chest movement:
External intercostal muscle ( anteroposterior
diameter)
 Internal intercostal muscle ( anteroposterior
diameter)
 Diaphragm ( vertical diameter)


Inspiration:





Active process
 volume of thoracic cavity  lower the intrapleural
pressure
The lungs expand  lowers the intra-alveolar
pressure  draw air into the lungs
Quiet breathing: 1/10th of inspiratory muscles are
active
Deep breathing: increased in pulmonary ventilation
10 folds (60L/min)

Expiration:
Passive
 Relaxation of muscles of inspiration
 Recoil of chest wall  rises of intrapleural pressure
 Elastic recoil of the lungs  rises of intra-alveolar
pressure  forced air out of the lungs


Accessory muscles of respiration:


Accessory muscle of inspiration: sternocleidomastoid,
anterior serrati and the scaleni
Accessory muscle of expiration: the internal
intercostal muscles and abdominal recti.
Intrapleural pressure:
Negative pressure vary between -2mmHg at the end of
expiration to –6mmHg during inspiration.
CAUSES OF THE NEGATIVE
INTRAPLEURAL PRESSURE
1. The lungs





Elastic recoil of the lungs  tends to collapse in an
inward direction
The surface tension of the fluid lining the alveoli
Surfactant normally reduced this surface tension
It’s a lipoprotein substance secreted by type II
alveolar epithelium
Decrease surfactant in the newborn causes hyaline
membrane disease of the newborn or (respiratory
distress syndrome)
2. The chest wall
Tends to recoil outward
 At equilibrium, there are two opposing forces lead to
a negative pressure in the pleural cavity

3. Pleural capillaries and lymphatics

The intrapleural space is rich in blood capillaries and
lymphatics, which tend to absorb fluid from the
pleural cavity  adds to the negativity of
intrapleural pressure
Intra-alveolar (intrapulmonary) pressure
Pressure within the alveoli of the lungs varies with
different stages of respiratory cycle
 During inspiration: quiet inspiration, the pressure
falls to –1mmHg but with forcible inspiration it falls
to about –70mmHg
 During expiration: in quiet expiration, it rises to
1mmHg but during forcible expiration it rises to
100mmHg (Valsalva manoeuver)

LUNG VOLUMES AND CAPACITIES

Tidal volume:


Is the volume of air inspired or expired at each breath.
500mL in normal quiet breathing (0.5L).
Respiratory minute volume or (pulmonary
ventilation):
Is the volume of air breathed in or out of the lungs each
minute.
 500 (tidal volume) x 12 (respiratory rate) = 6000mL/min.


Inspiratory reserve volume:


Inspiratory capacity:


The volume of air inspired by a maximal inspiratory effort
after normal inspiration (3.3L).
The volume of air inspired by a maximal inspiratory effort
after normal expiration. Equal to the tidal volume plus the
inspiratory reserve volume (3.8L).
Expiratory reserve volume:

The volume of air expired by a maximal expiratory effort
after normal expiration (1L).

Vital capacity:


Residual volume:


The volume expired by a maximal expiratory effort
after maximal inspiration. Equal to the tidal volume
plus inspiratory reserve volume plus expiratory
reserve volume (4.8L).
The volume of air that remains in the lung after
maximal expiration. Cannot be measured by
spirometer or gas meter. Increased by age and lung
disease (1.2L).
Functional residual capacity (FRC):

The volume of air that remains in the lung after
normal expiration. Cannot be measured by
spirometer (2.2L).

Total lung capacity:



The maximal volume of air that can be
accommodated in the lungs. Equal to tidal volume
plus inspiratory reserve volume plus expiratory
reserve volume plus residual volume (6L).
Both residual volume and functional residual capacity 
by age while vital capacity  by age.
The lung volumes and capacities are lower in females (20
– 25%) than in the male.
DISTRIBUTION OF PULMONARY
VENTILATION
Definitions
Ventilation: is the process dealing with the air
movement between the lung and atmospheric air
 Pulmonary ventilation: is the volume of air
breathed in or out per minute
 Inspired air is distributed to distinct spaces in
the lungs


Anatomical dead space:Occupies the air-conducting system down to the
terminal bronchioles
 No gas exchange
 Volume  150ml


Respiratory zone:Occupies the space distal to the terminal bronchioles
down to the alveolar sacs
 Gas exchange takes place
 Volume  350ml/min


Gas exchange in the lungs (Diffusion)

Exchange of gases (O2 and CO2) between the alveoli
and blood
FACTORS AFFECTING THE RATE
OF DIFFUSION
1. Alveolar capillary membrane (ACM):
 Semipermeable membrane
 Separates alveolar air from pulmonary capillary
blood
 Formed of several layers:
Fluid film lining the alveoli
 Alveolar membrane
 Interstitial fluid
 Capillary wall

Change in the thickness of alveolar capillary
membrane will affect the rate of gas diffusion
 In pulmonary edema  thickness of the ACM 
 rate of diffusion
 During exercise  thickness of ACM   rate of
diffusion

2. Partial pressure gradient of gases across the
alveolar capillary membrane:
 Partial pressure of O2 in mixed venous blood
40mmHg, the P–P of O2 in alveolar air is
100mmHg. So O2 diffuses from the alveolar space
to the capillary blood along partial pressure
gradient of about 60mmHg
 P–P of CO2 in venous blood is 46mmHg, while in
the alveolar air is 40mmHg, so CO2 diffuses from
the capillary to the alveolar space along partial
pressure gradient of about 6mmHg
3. Physical properties of gases:
 Solubility and M-W of gases affect the rate of
diffusion
 CO2 is 20 times more soluble than O2
 Diffusion failure affects O2 before CO2 is affected
4. Surface area:
 As the surface of the alveolar capillary
membrane increases, the total volume of gas
exchanged will be increased. The surface area
of ACM = 70m2 in adult male
5. Ventilation/blood flow ratio:
 Ventilation/blood flow ratio = alveolar
ventilation/ blood flow = 4/5 = 0.8
 The bases of the lung perfused more than the
apices
6. Temperature:
 The rate of diffusion of gases is normally
dependent on temperature
7. Chemical reactions:
 Each 1 gram of Hb, when fully saturated,
combines with 1.34ml of O2 at STP (at partial
pressure of O2 of 100mmHg)
 O2 dissolved in 100mL of blood at O2 partial
pressure of 100mmHg is 0.003 x 100 = 0.3mL
8. Diffusion capacity:
 CO2 has greater diffusion capacity than O2
about 20 times more (CO2 has greater
solubility than O2)
TRANSPORT OF OXYGEN
O2 is transported in the blood in
two forms:Oxyhaemoglobin > 98%
2. Dissolved oxygen < 2%
1.


In arterial blood 0.003 x 100 = 0.3mL of
O2
In venous blood 0.003 x 40 = 0.12mL of
O2
OXYHAEMOGLOBIN
Hb has great affinity for O2. It combines loosely,
fast and reversibly with O2
 It is the function of the P–P of O2 in relation with
saturation of Hb with O2 (oxyhaemoglobin
dissociation curve)

Oxygen-Haemoglobin dissociation curve:

The percentage saturation of Hb with O2
against partial pressure of O2

Sigmoid in shape (S-shaped)

Hb has 4 haem units attached to polypeptide
chains

Oxygenation of one haem unit leads to changes
in the configuration of the Hb molecule which
increase the affinity of the 2nd unit and so on

Steep rise in the percentage saturation of Hb
between PO2 of 0mmHg and 75mmHg, then
slow rise in the curve, becoming more or less
flat at PO2 of 80mmHg or above

Depends on PO2 and independent on Hb
concentration

At zero P–P of O2  %saturation is zero

At PO2 100mmHg (arterial blood) 
%saturation  97%

At PO2 40mmHg (venous blood) 
%saturation  75%

It is used for measurement of O2 content in
venous and arterial blood

When 1gm of Hb is fully saturated with O2 
it binds up to 1.34mL of O2

If Hb concentration = 150gm/L blood

O2 content of arterial blood (97% saturation) =
1.34 x 150 x 97/100 = 195mL/L of blood

O2 content of venous blood (75% saturation) =
1.34 x 150 x 75/100 = 150mL/L blood

O2 uptake by tissues = arterio-venous
difference = 195 – 150 = 45mL/L of blood
Factors affecting O2-Hb dissociation
curve:

Shift to the right indicate lower affinity of
Hb for O2

Shift to the left indicate increased affinity of
Hb for O2
1. Partial pressure of carbon dioxide:


 PCO2   affinity of Hb for O2 and shifts the
curve to the right (Bohr effect)
PCO2 is high in the tissues, while it is lower in
the lung
2. pH:

 pH ( H+ concentration)   Hb affinity to
O2 and shifts the curve to the right
3. Temperature:
Rise of temperature also shifts the curve to the
right
 In active tissues, heat is generated mainly due to
oxidation

4. 2,3-diphosphoglycerate (2,3-DPG):
2,3-DPG is found in RBCs bound to Hb
  2,3-DPG   Hb affinity to O2 and shifts the
curve to the right
 It helps the release of O2 from the tissue ( in
hypoxia at high altitude)

DISSOLVED OXYGEN




Less than 2%
It is at equilibrium with the O2 combined with
Hb
The dissolved O2 transferred to tissues then
replaced from O2 carried by Hb
It is essential for tissues that don’t have blood
supply, like the cornea and cartilage





Can be increased by breathing pure or
hyperbaric O2
O2 is poorly soluble in blood
In 100mL blood at body temperature, 0.003mL
O2 dissolved at PO2 1mmHg
In arterial blood  0.3mL/100mL O2 is
dissolved
In venous blood  0.12mL/100mL
TRANSPORT OF CO2
CO2 produced by active cells diffuses by
concentration gradient into the tissue fluid
to reach the plasma
CO2 IS TRANSPORTED BY:



The plasma (this reaction proceed very slowly
due to the absence of carbonic anhydrase
enzyme)
The red blood cells (this reaction proceed very
quickly due to presence of carbonic anhydrase
enzyme)
In 3 forms:



Dissolved  10% (0.4mL)
Bicarbonate  70% (2.8mL)
Carbamino compounds  20% (0.8mL)
CONTROL OF VENTILATION
Several mechanisms are involved which
can be grouped into two main categories
which are closely integrated:
Nervous control mechanism

Chemical control mechanism
THE RESPIRATORY CENTER
 Composed
of several groups of neurons
 Located in the entire length of the
medulla and pons
 Can be divided into four major groups of
neurons:Dorsal respiratory group
 Ventral respiratory group
 The apneustic center
 The pneumotoxic center

1. The dorsal respiratory group –
located in the entire length of the dorsal
aspect of the medulla. It comprises
inspiratory neurons which discharge
rhythmically during resting and forced
inspiration, so that it is called the
rhythmicity center. They are almost
responsible for inspiration.
2. The ventral respiratory group – lies
ventrolateral to the dorsal respiratory
group along the entire length of the
medulla. They are inactive during quiet
breathing but they are activated during
forced breathing as in exercise and they
are mainly expiratory neurons with some
inspiratory neurons. They are inactive
at rest or passive expiration and they
become activated when expiration is an
active process.
3. The apneustic center – it is situated in
the lower pons. It sends excitatory impulses
to the dorsal respiratory groups to
potentiate the inspiratory drive. It receives
inhibitory impulses from the sensory vagal
fibers of the Hering-Breuer inflation reflex
and inhibitory impulses from the
pneumotaxic center.
4. The pneumotaxic center – located in
the upper pons. It transmits inhibitory
impulses to the apneustic center and to
the inspiratory area to switch off
inspiration.
NERVOUS CONTROL OF
VENTILATION
 The
rhythmicity center received impulses
from:
Higher brain centers
 Centers in the brain stem (medulla and pons)
 Special receptors (respiratory reflexes)

 The
rhythmicity center sends excitatory
impulses via the intercostal and phrenic
nerves to the external intercostal muscles
and diaphragm
CHEMICAL CONTROL OF
VENTILATION
 The
rhythmicity center is affected by
chemical changes in the blood via two
types of chemoreceptors:

Peripheral chemoreceptors
Central chemoreceptors
PERIPHERAL CHEMORECEPTORS
Located mainly in the carotid and aortic bodies,
but may be found anywhere in the circulatory
system
 When stimulated, send excitatory impulses to the
rhythmicity center (via glossopharyngeal and
vagus nerves)
 Highly sensitive to changes in arterial PO2 and to
a lesser extent to PCO2 and pH
 Fall of PO2, rise in PCO2 and fall of pH, stimulate
the chemoreceptors to increase ventilation

Normal PO2, PCO2 and pH, low grade of tonic
activity in the nerves
  PCO2 and  pH causes low tonic activity  
ventilation
 In metabolic acidosis  pH causes  ventilation to
wash out CO2 and to bring pH to normal
 In metabolic alkalosis  pH causes  ventilation,
CO2 retained in the blood to compensate

CENTRAL CHEMORECEPTORS
 Most
probably located on the ventrolateral
surface of medulla oblongata (which is
bathed with cerebrospinal fluid)
 Highly sensitive to the hydrogen ion
concentration of the CSF evoked by
arterial PCO2 (CO2 can freely cross the
blood-brain barrier into CSF, while BBB
is relatively impermeable to H+ and HCO-3
ions)