3 - Lung Mechanics

Question 1

What is ventilation and what is it initiated by?

Answer 1

• Process of inspiration and expiration
• Neurones in respiratory centre of the brain generate automatic rhythmic impulses that go to inspiratory muscles
• Normal involuntary

Question 2

What is the definition of the following terms:

• Tidal Volume
• Inspiratory Reserve Volume (IRV)
• Expiratory Reserve Volume (ERV)

Answer 2

TV: volume of air which enters and leaves the lungs on each quiet breath

IRV: Additional amount of air that can be inhaled after a normal inspiration

ERV: Amount of extra air — above anormal breath — exhaled during a forceful breath out

IRV and ERV can be used in exercise

Question 3

What is residual volume?

Answer 3

Can never empty lungs so amount of air left in lungs after forced expiration

Question 4

What are the different types of lung capacity, as lung capacity is not fixed, it is relative to a point?

Answer 4

Question 5

How do muscles allow inspiration and expiration?

Answer 5

Expansion of thoracic cavity through EIM and diaphragm, reduced pressure which pushes air inwards

Question 6

What is anatomical and alveolar dead space?

Answer 6

Anatomical: Volume of the conducting airways from nostils to terminal bronchioles as this air doesn’t participate in gas exchange

Alveolar: Air in alveoli which is not perfused or are damaged so do not take part in gas exchange so ventilating this alveoli is wasted

Physiological Dead Space = Alveolar + Anatomical

Question 7

What are the equations for pulmonary and alveolar ventilation?

Answer 7

Question 8

What is the resting expiratory level?

Answer 8

At the end of quiet expiration just before breathing in the lung is subject to two equal opposing forces, so an equilibrium with no chest wall movement.

Creates negative pressure within intrapleural space as two forces on pleura

Outward: elasticity of chest wall

Inward: elasticity and surface tension of lung

Question 9

The lungs have a natural elastic recoil, especially with increased stretch, so during inspiration what keeps the lungs open and against the chest wall?

Answer 9

Pleural seal

Surface tension between molecules of pleural fluid allows chest wall and lungs to move together

Question 10

Summarise the volume and pressure changes in the ventilation cycle.

Answer 10

Contraction of muscles overcomes the equilibrium of resting expiratory level by overcoming force of the lungs elastic recoil

Question 11

How does the pleural pressure change throughout respiratory cycle?

Answer 11

At rest it is negative, during inpiration it becomes more negative due to expansion of thoracic cavity

The intrapleural pressure is always negative to help maintain proper inflation of the lungs and to help prevent a pneumothorax (i.e. collapsed lung).
The negative pressure of the pleural cavity acts as a suction to keep the lungs from collapsing.

Question 12

What are the accessory muscles of respiration?

Answer 12

- Inspiration: scalene, SCM, serratus anterior, pectoralis major

- Expiration: internal intercostal muscles and abdominal wall muscles

Used in diseases of the lung and during exercise

Question 13

Why is energy expended in inspiration?

Answer 13

• Needed to overcome the airways resistance and to stretch the lungs
• Most effort is overcoming elastic recoil not the resistance

Question 14

What would happen if the pleural seal was broken? e.g pneumothorax from a stabbing

Answer 14

Negative pressure in the intrapleural space would draw air in from outside the chest wall and collapse the lung

Atelectasis

Question 15

What is lung compliance?

Answer 15

• Stretchiness of the lungs
• Volume change per unit pressure change

Question 16

What limits the expansion of the lungs and how has the body adapted to overcome this?

Answer 16

• Fluid lining the alveoli, needed for gas exchange, creates surface tension decreasing the compliance

- Surfactant from type II pneumocytes disrupts interation between fluid molecules, decreasing surface tension

• Without surfactant, wouldn’t overcome inward pull

Question 17

What is the content of surfactant?

Answer 17

• Phospholipids and proteins with detergent properties
• Hydrophillic ends in alveolar fluid
• Hydrophobic ends in alveolar gas
• Surfactant floats on surface of the alveolar fluid disrupting fluid interactions, lowering surface tension

Question 18

Which type of alveoli is surfactant more effective in disrupting surface tension in, and why is this?

Answer 18

- Smaller alveoli due to molecules being closer together

• Surface tension increases as alveoli get larger
• Reducing surface tension in smaller alveoli prevents pressure rising due to smaller volume so smaller don’t collapse into bigger

Question 19

Why is forced inspiration harder than quiet inspiration?

Answer 19

As as the alveoli get larger, the surface tension of the alveoli increases so more energy needed to overcome this

Quiet, or unforced, breathing uses only the Diaphragm and the External Intercostals for inhalation with no accessory muscle contractions.

Question 20

How does surfactant stabilise the lungs and prevent smaller alveoli collapsing into larger alveoli to form bullae?

Answer 20

Law of Laplace: Pressure = 2ST/R

As alveolus expands, both ST and R increase due to surfactant. Keeps the pressure the same in any size alveoli

Question 21

What are the two roles of surfactant?

Answer 21

Stabilise alveolis especially smaller ones
increase compliance

Question 22

What is respiratory distress syndrome? What is the treatment?

Answer 22

• Surfactant absent from alveoli until fetus is 24 weeks
• Born less than 28-30 weeks old can get this
• Apnea, cyanosis, rapid shallow breathing, drawing in of lungs
• Corticosteroid therapy and ventilator

Question 23

How does resistance of airway tubes vary with diameter?

Answer 23

Pouiselle’s Law - smaller radius, increasing resistance sharply

Question 24

If a smaller airway tube confers more resistance then why are the bronchioles so small?

Answer 24

• Tubes are connected in parallel so combined resistance of all these is smaller than one big tube
• 1/R + 1/R ETC…

Question 25

Where is the biggest resistance to breathing?

Answer 25

Upper respiratory tract (trachea and large bronchi) , except when small airways are compressed during forced expiration

Question 26

Why does resistance in the small bronchioles increase during forced expiration?

Answer 26

• Contain no cartilage so airways collapse
• Tubes get smaller so there is a high resistance to flow making forced expiration hard
• Harder in asthma to expire as bronchioles even smaller than healthy person