• Pulmonary ventilation, or breathing, is the exchange of air between the atmosphere and the lungs.
• As air moves into & out of the lungs, it travels from regions of high to of low air pressure
Boyle's Law: Relationship Between Pressure and Volume
• Gas molecules striking the walls of a container cause pressure.
o The pressure exerted by the gas molecules is related to the volume of the container.
o A large sphere contains the same number of gas molecules as a small sphere.
Notice that in a larger volume, the gas molecules strike the wall less frequently, thus
exerting less pressure.
In a small sphere, the gas molecules strike the wall more frequently, thus exerting more
• Illustrates Boyle's Law
o States that the pressure of a gas is inversely proportional to the volume of its container.
Thus, if you increase the volume of a container, the pressure will decrease
If you decrease the volume of a container, the pressure will increase
Quiet Inspiration: Muscle Contraction
• The volume of the thoracic cavity is changed by muscle contraction and relaxation.
• During quiet inspiration, the diaphragm and the external intercostal muscles contract, slightly enlarging
the thoracic cavity.
o As we learned from Boyle's Law, increasing the volume decreases the pressure within the
thoracic cavity and the lungs.
• The diaphragm flattens and moves inferiorly while the external intercostal muscles elevate the rib cage
and move the sternum anteriorly
o These actions enlarge the thoracic cavity in all dimensions.
• As we learned from Boyle's Law, increasing the volume decreases the pressure within the thoracic cavity
and the lungs.
Quiet Expiration: Muscle Relaxation
• Quiet expiration is a passive process, in which the diaphragm and the external intercostal muscles relax,
and the elastic lungs and thoracic wall recoil inward.
o This decreases the volume and therefore increases the pressure in the thoracic cavity.
• As the diaphragm relaxes, it moves superiorly
o As the external intercostal muscles relax, the rib cage and sternum return to their resting
o These actions decrease the size of the thoracic cavity in all dimensions, and therefore increase
the pressure in the thoracic cavity.
Muscles of Deep Inspiration and Expiration
• Deep breathing uses forceful contractions of the inspiratory muscles and additional accessory muscles to
produce larger changes in the volume of the thoracic cavity during both inspiration and expiration.
• During deep inspiration, the diaphragm and the external intercostal muscles contract more forcefully than
during quiet breathing
o Additionally, the sternocleidomastoid and scalenes contract, lifting the rib cage higher
o These actions further increase the volume
o As we learned from Boyle's Law, this decreases the pressure within the thoracic cavity.
1 Module X
• Deep or forceful expiration is an active process
o The internal intercostal muscles depress the rib cage, and the external oblique, internal oblique,
transversus abdominis and rectus abdominis muscles compress the abdominal organs, forcing
them superiorly against the diaphragm
o These actions can dramatically decrease the volume, and further increase the pressure within the
thoracic cavity, producing forceful expiration.
Intrapulmonary Pressure Changes
• Specific pressure changes that occur in the lungs during breathing.
o For reasons described later, the lungs closely follow the movements of the thoracic wall.
• The pressure within the lungs is called the intrapulmonary, or intraalveolar, pressure.
o Between breaths, it equals atmospheric pressure, which has a value of 760 millimeters of
mercury at sea level
o When discussing respiratory pressures, this is generally referred to as zero.
• During inspiration, the volume of the thoracic cavity increases, causing intrapulmonary pressure to fall
below atmospheric pressure
o This is also known as a negative pressure
o Since air moves from areas of high to low air pressure, air flows into the lungs
o Notice that at the end of inspiration, when the intrapulmonary pressure again equals atmospheric
pressure, airflow stops.
• During expiration, the volume of the thoracic cavity decreases, causing the intrapulmonary pressure to rise
above atmospheric pressure.
o Following its pressure gradient, airflows out of the lungs, until, at the end of expiration, the
intrapulmonary pressure again equals atmospheric pressure.
• Intrapleural pressure is the pressure within the pleural cavity.
o Intrapleural pressure is always negative, which acts like a suction to keep the lungs inflated.
• The negative intrapleural pressure is due to three main factors:
o The surface tension of the alveolar fluid.
The surface tension of the alveolar fluid tends to pull each of the alveoli inward and
therefore pulls the entire lung inward
Surfactant reduces this force.
o The elasticity of the lungs.
The abundant elastic tissue in the lungs tends to recoil and pull the lung inward.
As the lung moves away from the thoracic wall, the cavity becomes slightly larger.
The negative pressure this creates acts like a suction to keep the lungs inflated.
• The elasticity of the thoracic wall.
o The elastic thoracic wall tends to pull away from the lung, further enlarging the pleural cavity
and creating this negative pressure
o The surface tension of pleural fluid resists the actual separation of the lung and thoracic wall.
Intrapleural Pressure Changes During Breathing
• As the thoracic wall moves outward during inspiration, volume of pleural cavity increases slightly
o Decreasing intrapleural pressure.
• As the thoracic wall recoils during expiration, the volume of the pleural cavity decreases