The document summarizes the mechanics of breathing. During inspiration, contraction of the diaphragm pulls the lungs downward while muscles elevate the ribs to increase the chest cavity. Expiration occurs passively as the elastic lungs and chest wall recoil, or actively through abdominal muscle contraction during heavy breathing. Inspiration increases the anteroposterior, transverse, and vertical diameters of the chest cavity through movements of the rib cage units and diaphragm descent. Dead space refers to air that does not participate in gas exchange and includes air in the conducting airways.

1. MECHANICS OF BREATHING

2. LUNG EXPANSION AND CONTRACTION
• The lungs can be expanded and contracted in
two ways:
1. By downward and upward movement of the
diaghragm to lengthen or shortern the chest
cavity
2. By elevation and depression of the ribs to
increase and decrease the AP diameter of the
chest cavity.

3. DURING INSPIRATION
Contraction of the diaghragm pulls the lower
surfaces of the lungs downwards.
DURING EXPIRATION
Diaphragm simply relaxes and the elastic recoil
of the lungs, chest wall, and abdominal structure
compresses the lungs and expels the air.

6. During heavy breathing, however the elastic forces recoil are
not powerful enough to cause the necessary rapid expiration,
extra force is achieved mainly by contraction of the abdominal
muscle.
MOVEMENT OF THE RIB CAGE
Natural resting position: Ribs slant downward, thus allowing
the sternum to fall backward toward the vertebral column.
Elevation of the ribcage
The ribs project away most directly forward, so that the
sternum also moves forward, away from the spine, making AP
thickness about 20% greater during max inspiration than
during expiration.

7. MUSCLES OF THE INSPIRATION
• Muscles that elevate the chest cage are
muscles of elevation.
• The most importantly inspiratory muscles is
external intercostals.
• Muscles that help in inspiration are:
1. Sternocleidomastoid muscles: lifts upward on
the sternum.
2. Anterior seratti: lift the first 2 ribs.

8. MUSCLES OF EXPIRATION
These are the muscles that pull the rib cage downward
during expiration.
These are:
1. Abdominal recti: It has powerful effect of the
pulling downward on the lower ribs at the same
time they and other abdominal muscle also
compress the abdominal contents upward against
the diaghragm.
2. Internal intercostals: It causes leverage to the ribs
to depress them downward, thereby causing
expiration.

10. MOVEMENT OF THORACIC CAGE
• Inspiration causes enlargement of thoracic cage.
Thoracic cage enlarges because of increase in all
diameters, viz. anteroposterior, transverse and
vertical diameters. Anteroposterior and
transverse diameters of thoracic cage are
increased by the elevation of ribs. Vertical
diameter is increased by the descent of
diaphragm.
• Change in the size of thoracic cavity occurs
because of the movements of four units of
structures: 1. Thoracic lid 2. Upper costal series 3.
Lower costal series 4. Diaphragm.

11. 1. Thoracic Lid
Thoracic lid is formed by manubrium sterni and
the first pair of ribs. It is also called thoracic
operculum. Movement of thoracic lid increases
the anteroposterior diameter of thoracic cage.
Due to the contraction of scalene muscles, the
first ribs move upwards to a more horizontal
position. This increases the anteroposterior
diameter of upper thoracic cage.

12. Upper Costal Series
• Upper costal series is constituted by second to
sixth pair of ribs. Movement of upper costal
series increases the anteroposterior and
transverse diameter of the thoracic cage.
Movement of upper costal series is of two
types:
• i. Pump handle movement
• ii. Bucket handle movement.

13. Contraction of external intercostal muscles
causes elevation of these ribs and upward and
forward movement of sternum. This movement
is called pump handle movement. It increases
anteroposterior diameter of the thoracic cage.

14. Bucket handle movement
• Simultaneously, the central portions of these
ribs (arches of ribs) move upwards and
outwards to a more horizontal position. This
movement is called bucket handle movement
and it increases the transverse diameter of
thoracic cage.

16. Lower Costal Series
Lower costal series includes seventh to tenth pair of
ribs. Movement of lower costal series increases the
transverse diameter of thoracic cage by bucket
handle movement.
Bucket handle movement
Lower costal series of ribs also show bucket handle
movement by swinging outward and upward. This
movement increases the transverse diameter of the
thoracic cage. Eleventh and twelfth pairs of ribs are
the floating ribs. These ribs are not involved in
changing the size of thoracic cage.

17. DEAD SPACE
Air that never reaches the gas exchange areas
but simply fills respiratory passages where gas
exchange does not occur, such as nose, pharynx
and trachea.
This air is called dead space air because it is not
useful for gas exchange.
It is of two types:
1. Anatomical dead space.
2. Physiological dead space.

18. • Anatomical dead space: conducting zone
• Physiologic dead space or total dead space:
conducting zone and volume of air in the
alveoli (i.e, wasted alveolar ventilation)
VARIATIONS:
1. Physiological variations:
i. Sex: DS is more in males
ii. Age: DS increases with age, because inflated
lungs pull the airways thereby increasing the
airways diameter.

19. iii. Body height: DS increases in proportion with increase
in body height.
PATHOLOGICAL VARIATIONS:
i. Emphysema: Loss of elasticity of lungs in emphysema
decreased elastic recoil, this produces hyperinflation
of lungs to cause increases in DS.
ii. Bronchiectasis: Causes DS to increase.
iii. Pulmonary embolism: Produces regional decrease in
pulmonary vascular bed.
Normal dead space volume: In young adults it is about
150 millilitres. This increases slightly with age.

21. Refrences
• K. Sembulingam, Prema sembulingam, jaypee
essentials of physiology, sixth edition, page no.
682- 683; 690- 692
• Alexandra Hough, Physiotherapy in respiratory
care, third editions, page no. 54-60
• Arthur C. Guyton, Textbook of medical
physiology, eleventh edition, page no. 471-
476

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