The human respiratory system allows for gas exchange between the blood and air through breathing. Air enters through the nasal cavity and is warmed and moistened before passing into the trachea and branching bronchi and bronchioles that lead to millions of tiny alveoli in the lungs. The alveoli are adapted for efficient gas exchange with very thin walls, a large surface area, and moist inner surfaces surrounded by blood capillaries. During inhalation, the diaphragm and intercostal muscles expand the ribcage to decrease pressure and allow air into the lungs. Exhalation is a passive process where the muscles relax and pressure increases to force air back out.

2. The Human Breathing Mechanism
•The human respiratory system provides an area for gas
exchange between the blood and the environment. It
allows oxygen acquisition and the carbon dioxide
elimination.
•The gas exchange in lungs is called breathing (external
respiration). This exchange involves the taking in and
letting out of air.

3. Structure of Human Respiratory System
•Air enters the respiratory system through the nasal
cavity.
•As air passes the nasal cavity, it is warmed and
moistened. Dust is trapped by the hairs.
•After the air leaves the nasal cavity, it enters the
trachea.
•The wall of the trachea contains C – shaped rings
of cartilage which give it support and prevent it
from collapsing during inhalation.
•The trachea branches into two bronchi (singular:
bronchus)

4. •One bronchus enters the right lung, the others enters
the left lung.
•Each bronchus branches into smaller tubes called
bronchioles.
•The bronchioles end in millions of tiny air sacs called
alveoli (singular: alveolus).
• the lungs are made up of alveoli, bronchioles and blood
capillaries.
•The lungs are located in a space called the thoracic
cavity.
•The lungs are protected by the ribcage which consists of
ribs and intercostal muscles.
•The sheet of muscle called the diaphragm separates the
thoracic cavity from the abdomen.

5. The human respiratory system

6. Adaptation of the alveoli for efficient gas exchange
•Large surface area – the lungs have millions of alveoli to
provide a large surface area for gas exchange.
•Very thin walls – the wall of each alveolus is one cell –
thick to provide a very short distance for the diffusion of
gases.
•Moist inner surface – the inner surface of each alveolus
is moist so that oxygen can dissolve in the moisture
before diffusing across the alveolar wall.
•Surrounded by numerous blood capillaries – the blood
capillaries help to transport gases to and from the alveoli
quickly.

7. Detailed structure of alveolus

8. Mechanism of human breathing
•The breathing mechanism consists of two phases. The
taking in of air is called inhalation. The letting out of the
air is called exhalation.
Inhalation

9. • during breathing, the lungs expand and return to
their normal size to pump air into and out of them.
•The movements of the ribs and the diaphragm
bring about breathing.
Exhalation

10. Comparison of the mechanisms of
inhalation and exhalation
Inhalation Exhalation
•Diaphragm muscles contract.
•Diaphragm flattens out
•Diaphragm muscles relax.
•Diaphragm arches upwards
•Intercostal muscles contract. •Intercostal muscles relax.
•Ribcage moves upwards and
outwards.
•Ribcage moves downwards
and inwards.
•Volume of thoracic cavity
increases
•Volume of thoracic cavity
decreases.
•Air pressure surrounding the
lungs decreases.
•Air pressure surrounding the
lungs increases.
•Air flows into the lungs •Air is forced out of the lungs.

11. Composition of air in respiration
Content Inhaled air Exhaled air Alveolar air
Oxygen 21 % 16 % 14 %
Carbon
dioxide
0.04 % 4 % 6 %
Nitrogen 79 % 79 % 80 %
Water vapour Variable Saturated Saturated
Temperature Variable 34 ° C 37° C

12. •The changes in the content of air entering the lungs
are due to gas exchange in the alveoli.
Common mistake
-There is more carbon dioxide than oxygen in
exhaled air.
The Truth
- Exhaled air contains 16 % oxygen, 4 % carbon
dioxide and 79 % nitrogen.