Respiratory Mechanisms Flashcards
Why do the lungs expand ?
To fill the thoracic cavity because interpleural pressure is negative compared to atmospheric pressure so air can enter the lungs
Why do the lungs never collapse at rest ?
At rest the tendency for the lungs to collapse is opposed by the equal tendency for the chest wall to expand
What is inspiration ?
- inspiration is an active process (requires energy)
- ventilation occurs when active muscle force is applied to the relaxed respiratory system
Describe the process of inspiration
- volume of the thorax increases
- interpleural pressure falls
- alveoli expand
- atmospheric pressure is greater than alveolar pressure
- air flows into the lungs until the alveolar pressure is equal to the atmospheric pressure
What happens when the volume of the thorax increases ?
- diaphragm contracts and moves down
- external intercostal muscles contract and the rib cage moves up and out
What is Boyle’s law ?
At constant temperature :
P1 x V1 = P2 x V2
What is expiration ?
- a largely passive process
- elastic recoil of the lungs and the chest wall reduces the volume of the thorax
Describe the process of expiration
- interpleural pressure rises
- alveoli recoil
- alveolar pressure is greater than atmospheric pressure
- air is expelled from the lungs
How does air move ?
Air moves passively from a region of high pressure to low pressure down a pressure gradient
What is compliance ?
- compliance describes the elasticity or distensibility of the system
- compliance = change in volume / change in pressure
- it is determined when the system is static
What is hysteresis ?
Deviation from the compliance line
- inspiration - line deviates to the right
- expiration - line deviates to the left
Describe compliance during inspiration
- greater pressures are required to achieve a given volume
- the line deviates to the right because of the resistive forces which oppose airflow :
• airway resistance
• pulmonary tissue resistance
• inertia of the air and tissues
What is airway resistance ?
Resistance to the movement of air
What is pulmonary tissue resistance ?
Friction between the lungs and the chest wall
- it is slightly reduced by the fluid
Describe compliance during expiration
- curve deviates to the left because the resistive forces assist airflow :
• elastic recoil of the lungs and the chest wall
• surface tension in the alveoli
Summarise convection and diffusion
Convection occurs in the conducting zone
Diffusion of air into the body occurs in the respiratory zone
What are the factors that affect airway resistance ?
- lung volume
- bronchial smooth muscle tone
- inflammation and mucus
How does lung volume affect airway resistance ?
• radial traction during lung expansion decreases airway resistance
• dynamic compression during forced expiration increases airway resistance
How does bronchial smooth muscle tone affect airway resistance ?
The airways dilate which decreases airway resistance or constrict which increases airway resistance
This happens in response to :
- nervous activity
- hormones
- external factors
How does inflammation and mucus affect airway resistance ?
It increases airway resistance
What is the relationship between increased airway resistance and lung diseases ?
increased airway resistance is associated with obstructive lung diseases
- greater pressure is required to move air in and out of the lungs
- work of breathing is increased (expiration is especially difficult)
Give some examples of obstructive lung diseases
- asthma
- COPD
What are the factors that affect compliance ?
- the elastic properties of the tissues of the lung and chest wall including the diaphragm
- surface tension in the alveoli
Describe surface tension in the alveoli
- attractive forces in the liquid oppose expansion by inspired air
- this promotes the collapse of the smaller alveoli
- this causes transudation of fluid from the capillaries
- this is minimised by surfactant
