Respiratory Mechanisms

Question 1

Why do the lungs expand ?

Answer 1

To fill the thoracic cavity because interpleural pressure is negative compared to atmospheric pressure so air can enter the lungs

Question 2

Why do the lungs never collapse at rest ?

Answer 2

At rest the tendency for the lungs to collapse is opposed by the equal tendency for the chest wall to expand

Question 3

What is inspiration ?

Answer 3

• inspiration is an active process (requires energy)
• ventilation occurs when active muscle force is applied to the relaxed respiratory system

Question 4

Describe the process of inspiration

Answer 4

• 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

Question 5

What happens when the volume of the thorax increases ?

Answer 5

• diaphragm contracts and moves down
• external intercostal muscles contract and the rib cage moves up and out

Question 6

What is Boyle’s law ?

Answer 6

At constant temperature :
P1 x V1 = P2 x V2

Question 7

What is expiration ?

Answer 7

• a largely passive process
• elastic recoil of the lungs and the chest wall reduces the volume of the thorax

Question 8

Describe the process of expiration

Answer 8

• interpleural pressure rises
• alveoli recoil
• alveolar pressure is greater than atmospheric pressure
• air is expelled from the lungs

Question 9

How does air move ?

Answer 9

Air moves passively from a region of high pressure to low pressure down a pressure gradient

Question 10

What is compliance ?

Answer 10

• compliance describes the elasticity or distensibility of the system
• compliance = change in volume / change in pressure
• it is determined when the system is static

Question 11

What is hysteresis ?

Answer 11

Deviation from the compliance line
- inspiration - line deviates to the right
- expiration - line deviates to the left

Question 12

Describe compliance during inspiration

Answer 12

• 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

Question 13

What is airway resistance ?

Answer 13

Resistance to the movement of air

Question 14

What is pulmonary tissue resistance ?

Answer 14

Friction between the lungs and the chest wall
- it is slightly reduced by the fluid

Question 15

Describe compliance during expiration

Answer 15

• 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

Question 16

Summarise convection and diffusion

Answer 16

Convection occurs in the conducting zone
Diffusion of air into the body occurs in the respiratory zone

Question 17

What are the factors that affect airway resistance ?

Answer 17

• lung volume
• bronchial smooth muscle tone
• inflammation and mucus

Question 18

How does lung volume affect airway resistance ?

Answer 18

• radial traction during lung expansion decreases airway resistance
• dynamic compression during forced expiration increases airway resistance

Question 19

How does bronchial smooth muscle tone affect airway resistance ?

Answer 19

The airways dilate which decreases airway resistance or constrict which increases airway resistance
This happens in response to :
- nervous activity
- hormones
- external factors

Question 20

How does inflammation and mucus affect airway resistance ?

Answer 20

It increases airway resistance

Question 21

What is the relationship between increased airway resistance and lung diseases ?

Answer 21

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)

Question 22

Give some examples of obstructive lung diseases

Answer 22

• asthma
• COPD

Question 23

What are the factors that affect compliance ?

Answer 23

• the elastic properties of the tissues of the lung and chest wall including the diaphragm
• surface tension in the alveoli

Question 24

Describe surface tension in the alveoli

Answer 24

• 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

Question 25

What is the relationship between compliance and lung disease ?

Answer 25

• reduced compliance is associated with restrictive lung diseases
• lung volumes are limited

Question 26

Give some examples of restrictive lung diseases

Answer 26

• pulmonary fibrosis
• scoliosis