Breathing Mechanism:
Breathing is a continuous physical process consisting of two main phases: Inspiration
(inhaling) and Expiration (exhaling).
The movement of air into and out of the lungs is entirely driven by pressure differences
between the inside of the lungs and the outside atmosphere:
Air moves IN when the air pressure inside the lungs is lower than atmospheric pressure.
Air moves OUT when the air pressure inside the lungs is greater than atmospheric pressure.
The Two Phases of Breathing
A. Inspiration (Inhalation) – Taking Air In
During inspiration, the body actively works to increase the volume of the chest cavity.
Because a larger space lowers air pressure, air is sucked into the lungs.
How it happens:
Diaphragm Contracts: The dome-shaped diaphragm muscle contracts and flattens out,
moving downward.
Intercostal Muscles Contract: The muscles between the ribs contract, pulling the ribcage
upward and outward.
Result: The overall volume of the chest cavity increases. This causes the air pressure inside
the lungs to drop below atmospheric pressure, forcing outside air to rush into the lungs
through the airways.
B. Expiration (Exhalation) – Letting Air Out
Expiration is largely a passive process where the chest cavity relaxes, decreasing its volume
and squeezing the air out.
How it happens:
Diaphragm Relaxes: The diaphragm muscle relaxes and returns to its natural, dome-
shaped position, moving upward.
Intercostal Muscles Relax: The rib muscles relax, causing the ribcage to drop back down
and inward to its original position.
Breathing is a continuous physical process consisting of two main phases: Inspiration
(inhaling) and Expiration (exhaling).
The movement of air into and out of the lungs is entirely driven by pressure differences
between the inside of the lungs and the outside atmosphere:
Air moves IN when the air pressure inside the lungs is lower than atmospheric pressure.
Air moves OUT when the air pressure inside the lungs is greater than atmospheric pressure.
The Two Phases of Breathing
A. Inspiration (Inhalation) – Taking Air In
During inspiration, the body actively works to increase the volume of the chest cavity.
Because a larger space lowers air pressure, air is sucked into the lungs.
How it happens:
Diaphragm Contracts: The dome-shaped diaphragm muscle contracts and flattens out,
moving downward.
Intercostal Muscles Contract: The muscles between the ribs contract, pulling the ribcage
upward and outward.
Result: The overall volume of the chest cavity increases. This causes the air pressure inside
the lungs to drop below atmospheric pressure, forcing outside air to rush into the lungs
through the airways.
B. Expiration (Exhalation) – Letting Air Out
Expiration is largely a passive process where the chest cavity relaxes, decreasing its volume
and squeezing the air out.
How it happens:
Diaphragm Relaxes: The diaphragm muscle relaxes and returns to its natural, dome-
shaped position, moving upward.
Intercostal Muscles Relax: The rib muscles relax, causing the ribcage to drop back down
and inward to its original position.
