mechanism of respiration: inhalation & Exhalation

1. Ms.Anu Sebastian: B.Pharm; M.Pharm
Assistant Professor,
Dept. Of Pharmacology
Nirmala College Of Pharmacy
Muvattupuzha,Ernakulam,Keral

2. RESPIRATION
 The process of gaseous exchange in the body is called respiration.
 The process involves 3 steps:
1. Pulmonary ventillation/Breathing: It is the inhalation (inflow)
& exhalation(outflow) of air & involves the exchange of air
between the atmosphere & the alveoli of the lungs.
2. External/Pulmonary respiration: It is the exchange of gases
between the alveoli of the lungs & blood in pulmonary
capillaries across the respiratory membrane. In this process
,Pulmonary capillary blood gains O2 & loses CO2

3. 3. Internal /Tissue respiration: It is the exchange of gases
between blood in systemic capillaries & tissue cells. In this
step, blood loses O2 & gains CO2.
Pressure Changes During Pulmonary Ventillation
 Air moves into the lungs when the air pressure inside the
lungs is less than the air pressure in the atmosphere.
 Air moves out of the lungs when the air pressure inside the
lungs is greater than the air pressure in the atmosphere.

4. MECHANISM/PHYSIOLOGY OF RESPIRATION
INHALATION
 Breathing in is called inhalation.
 Boyle’s law states that the pressure in a closed container is
inversily proportional to the volume of the container.
 Just before inhalation the pressure inside each lungs is equal to the
atmospheric pressure 760 mmHg.
 For air to enter into the lungs, the pressure in the lungs should be
less than the atmospheric pressure.
 In order to decrease the lung pressure as per Boyle’s law, the
volume should be increased.
 The volume of lungs is increased by the contraction of diaphragm
& external intercostal muscles.

5.  Diaphragm is a dome shaped skeletal muscle that forms the
floor of the thoracic cavity.
 During inhalation,
 Contraction of diaphragm( 75%)
 Under normal conditions, diaphragm descends by 1 cm
creating a pressure difference of about 1-3 mmHg due to
which about 500ml of air enters into the lungs.
 Under stressful conditions, the diaphragm descends by
about 10cm which produces a greater pressure difference of
about 100 mmHg & thus inhalation of about 2-3 L of air
occurs.

6.  Contraction of External intercostal muscles(25%)
 When the external intercostal muscles contract, the ribs are pulled
upwards, thereby decreasing the intrathoracic pressure ,which
increases the volume of thoracic cavity.
 Contraction of the external intercostals is responsible for about
25% of the air that enters the lungs during normal quiet breathing.
 Intrapleural/ Intrathoracic pressure
 It is the pressure difference between the 2 pleural layers.
 Just before inhalation it is 756mmHg—Expansion of thoracic
cavity—Decreases the pressure to 754mmHg—the parietal pleura
is pulled outwards—along with that visceral pleura & lungs also
pulled out—increases the thoracic volume.

7.  So by all the above 3 ways the intrapulmonary pressure drops
from 760mmHg to 758mmHg.
 Due to this pressure difference between atmosphere & lungs, the
air diffuse into the alveoli until the pressure difference exists.
EXHALATION
 Breathing out is called exhalation /Expiration.
 It is a passive process(No muscle contraction)
 During exhalation the elastic recoiling of chest wall & lungs takes
place.
 Relaxation of diaphragm makes it acquire its characteristic dome
shaped structure while relaxation of the external intercostals
depresses the ribs.

8.  As a result , the volume of thoracic cavity decreases followed by a
decrease in the volume of lungs.
 This results in an increased pressure within the alveoli of the lungs
than the atmospheric pressure.
 As a result, air flows from the alveoli to the atmosphere
 During quiet breathing exhalation lasts for about 3 seconds.
 During forceful breathing exhalation becomes active due to the
contraction of abdominal & internal intercostal muscles.
Factors Affecting Pulmonary Ventillation
 Surface tension of the alveolar fluid
 Surface of the alveoli consists of a layer of fluid known as
alveolar fluid.

9.  The force exerted by alveolar fluid on the luminal surface of
alveoli is referred as surface tension.
 This surface tension should be overcome for expanding the lungs
during inhalation & exhalation.
 Compliance of the lungs
 The force or effort required to expand the lungs & chestwall is
known as compliance.
 In the lungs, compliance is related to two principal factors:
Elasticity & Surface tension.
 The lungs normally have high compliance & expand easily
because elastic fibres in lung tissue are easily stretched &
surfactants in alveolar fluid reduces surface tension.

10.  Airway Resistance
 The rate of airflow through the airways depends on both the
pressure difference & the resistance.
 The walls of the airways, especially the bronchioles offer
some resistance to the normal flow of air into & out of the
lungs.
 As the lungs expand during inhalation, the bronchioles
enlarge because their walls are pulled outward in all
directions.
 Larger diameter airways have decreased resistance.
 Airway resistance then increases during exhalation as the
diameter of bronchioles decreases.

11. THANK YOU