BIOM1001 MOD4 LEC3 - The Process of Breathing

What are the Pressures in the Thoracic Cavity

Intrapulmonary (Intra-alveolar) pressure

• the pressure inside lungs (alveoli)

• Atmosphere and alveoli are linked by conducting airways → inta-alveolar pressure quickly becomes same as atmospheric pressure

• 760 mm Hg/ 0 mm Hg

Intrapleural pressure

• Pressure inside pleural cavity

• Less than atmospheric pressure (sub-atmospheric)

• -3 to -4 mm Hg

What are the causes of negative intrapleural pressure

Forces pulling the thorax outward:

• during development, chest wall grows more quickly than lungs

• Chest wall elasticity - naturally wants to expand, pulling lungs outward

Forces pulling lung inward

• lung recoil: elastic tissue in alveoli walls causes lungs to want to collapse to their smallest size

• Alveolar surface tension: fluid lining alveoli pulls alveolar walls inward.

How does intrapleural Pressure help lungs stay inflated

Opposing forces create a negative pressure:

• pressure is inversely proportional to volume

• Volume created in pleural cavity by opposing forces → reduced pressure

Helps lungs stay inflated

• creating suction effect of lungs to chest walls

• Counteracts elastic recoil of lungs inwards and chest wall outwards, keeping them together

What is the pressures in the respiratory system

Transpulmonary pressure

• difference between intra-alveolar and intra-pleural pressure

• Reflects pressure that is keeping lungs open relative to atmosphere

• Higher transpulmonary pressure = more expansion

How does air flow through the respiratory system and what affected the amount of airflow.

• Air flows from regions of high pressure to low pressure (pressure gradient)

Amount of airflow determined by:

• pressure gradient

• Resistance in airways

How do lungs change their volume?

Lungs are “stuck” to thoracic wall by surface tension and pressure gradient across pleural membranes.

Muscles move thoracic wall during breathings, lungs move with thoracic wall.

Pressure gradients are created by volume change, driving air flow.

What are the forces involved in inspiration

Muscle Contraction

• diaphram

• External inter coastal

• Scalenes (forceful inspiration)

• Sternocleidomastoids (forceful inspiration)

Works against elastic recoil of lungs.

What happens during passive inspiration.

1. Diaphragm and external intercoastal muscles contract.

2. Lung volume increases

3. Intra-alveolar and intra-pleural pressures drop

4. Air moves into the lungs - high pressure in atmospher to lower pressure in lungs.

What are the Forces involved in passive expiration

Muscle relaxation

• passive relaxation of inspiratory muscles

Works with elastic recoil of lungs

How does Passive expiration work?

1. Diaphragm and external intercostals relax, helped with the elastic recoil of lungs

2. Lung volume decreases

3. Intra-alveolar and intrapleural pressures drop Air increases

4. Air moves out of lungs - from high lungs pressure to lower atmospheric pressure.

What is needed in forced (active) inspiration

Forced inspiration involves:

• greater contraction of diaphragm

• Greater use of external intercostals

• Recruitment of scalenes and Sternocleidomastoids (draws ribcage up and out more)

Results in:

• greater increase in volume

• Greater decrease in intrapulmonary pressure

• Greater inflow of air

What is needed for Forced (active) expirations

Forced expiration involves:

• recruitment of internal intercostals (draws ribcage inwards)

• Recruitment of abdominal muscles (pushes diaphragm upwards to further reduce volume in lungs)

Results in:

• greater decrease in volume

• Greater increase in pressure

• More air flow out

What are the three pressure terminology used in respirations

Atmospheric pressure (Patm)

• pressure of air surrounding body

• At sea level: 760mm Hg = 0 atm

Negative Pressure

• lower than atmospheric pressure

Positive Pressure

• higher than atmospheric

Zero Pressure

• equal to atmospheric