Mechanics of respiration

1. Dr.Sara Sadiq
MBBS, M.Phil

2. Breathing
• The movement of air into and out of the lungs
(ventilation) results from a pressure
difference between the thoracic cavity and the
atmosphere.
• This pressure difference is created by changing
the volume of the thoracic cavity.

3. Mechanics of respiration
• There are five steps to respiration:
– Muscles – contract/ relax to cause;
– Movement – ribs/ sternum/ abdomen to cause;
– Thoracic cavity volume – increase/ decrease causing
– Lung air pressure – increase/ decrease causing
– Inspiration/ Expiration – air in or out

4. Inspiration
• An active process requiring
muscle contraction
• Diaphragm and ext. Intercostal
muscles contract
• Draw the ribs upward and
outward
• Thoracic cavity expands
• Pleural cavity pressure
decreases
• Lung surface is pulled outward
• Alveolar pressure decreases
below atmospheric pressure
causing air to rush in

6. Expiration
• Expiration is a passive
process, no muscle
contraction required
• Diaphragm and external
intercostals relax
• Thoracic cavity decreases in
size
• Pleural pressure increases
• Alveolar pressure greater
than atmospheric pressure
• Air flows out

9. Dimensions of the Chest Cavity

10. Mechanics Of Respiration During
Exercise
• Inspiration:
Sternocleidomastoid
Scalene
Anterior serratous muscle
Pectoralis major and minor
• Expiration:
Internal intercostal
Rectus abdominis muscle

11. Pulmonary Ventilation
• The total amount of air moved in and out of
the lungs each minute is called Pulmonary
Ventilation
– depends upon 2 factors:
• The size of each breath (tidal volume)
• The number of breaths/minute (breathing rate)
Rest = 7.5 L/min. Exercise = 220 L/min

12. Involuntary Respiration
• The basic rhythm of breathing occurs without
conscious effort.
• The inspiratory center located in the medulla sets
the basic rhythm by automatically initiating
inspiration with a two second burst of nerve
impulses to the diaphragm and the external
intercostal muscles.
• Contraction of the diaphragm and the external
intercostal muscles draws air into the lungs.

13. The Expiratory Center:
located in the medulla
Functions during forced expiration stimulating the internal
intercostal and abdominal muscles to contract.

14. External Intercostals:
Outside of the ribcase => wrap around
from the back of the rib almost to the
end of the bony part of the rib in front. .
They elevate the ribs.
Internal Intercostals:
Inside of the ribcase => extend from the
front of the ribs, and go around back,
past the bend in the ribs . They depress
the ribs.

15. Perfect Lung
Action of the intercostal muscles

16. Diaphragm
• In a healthy adult, the diaphragm is the dominant muscle of
respiration at rest
• The diaphragm is a musculotendinous sheet separating the
thorax from the abdomen. It is attached to the thoracic cage
under the lower ribs.

18. Perfect Lung
Action of the abdominal muscles

19. Movement Of Thoracic Cage
During Inspiration:
• Increases in all diameters
• Anterioposterior diameter Elevation of ribs
• Transverse diameter
• Vertical diameter  Descent of diaphragm
During Expiration:
• Decrease in all diameters

20. Mechanics of respiration
Three important pressures:
1) Atmospheric
2) Intraalveolar
3) Intrapleural

24.  Inspiration:
Intraalveolar pressure > Atm pressure
 Expiration:
Intraalveolar pressure < Atm pressure

25. Intrapleural Pressure
Pressure inside pleural
sac
• Always negative
under normal
conditions
• Always less than Palv
Varies with phase of
respiration
• At rest, -5 mm Hg

26. Intrapleural Pressure
– Negative pressure due to elasticity in lungs
and chest wall
• Lungs recoil inward
• Chest wall recoils outward
• Opposing pulls on intrapleural space
• Surface tension of intrapleural fluid hold wall
and lungs together

27. Intra-alveolar Pressure
–Pressure of air in alveoli
–Varies with phase of respiration
• During inspiration = negative
(less than atmospheric)
• During expiration = positive
(more than atmospheric)

28. Transpulmonary Pressure
– Transpulmonary pressure = Palv – Pip
– Distending pressure across the lung wall
– Increase in transpulmonary pressure:
• Increase distending pressure across lungs
• Lungs (alveoli) expand, increasing volume

29. During Inspiration:
• Due to enlargment of thoracic cage
• Negative pressure increased in thoracic cavity
• causes expansion of lungs
During Expiration:
• Thoracic cavity decrease in size
• Pressure comes back to pre-inspiratory level

30. Collapsing Tendency Of Lung
Factors causes lung collapse:
• Elastic tissue of lung show constant recoiling
tendency
• Surface tension
Factors prevent lung collapse:
• Negative intrapleural pressure
• Surfactant