Respiratory Mechanics

Question 1

Explain how the cyclic changes in intra-alveolar pressure facilitates breathing

Answer 1

• Intra-alveolar pressure must be lesser than atmospheric pressure during inspiration, where volume of air increases
• Intra-alveolar pressure must be more than atmospheric pressure during expiration, where volume of air decreases

Question 2

What occurs during the onset of inspiration (quiet breathing)

Answer 2

• Contraction of inspiratory muscles like diaphragm and External intercostal muscles
• Contraction of these muscles enlarge the thoracic cavity

Question 3

What occurs during inspiration?

Answer 3

• Thoracic cavity enlarges, lungs expand to fill thoracic cavity
• Intra-alveolar pressure drops to 759mmHg
• Air flows into lungs down pressure gradient until pressure equalises
• Intrapleural pressure drops to 754mmHg as highly stretched lungs pull away more from thoracic wall.

Question 4

What happens during the onset of expiration?

Answer 4

• Inspiratory muscles relax
• Elevated ribs fall due to gravity
• Diaphragm returns to its original position

Question 5

What occurs at the end of expiration?

Answer 5

• Chest wall and stretched lungs return to their pre-inspiratory size due to their elastic properties
• Results in lungs recoiling, becoming smaller in volume
• Intra-alveolar pressure rises to 761mmHg as the number of air molecules contained in a larger lung volume are now compressed into a smaller volume
• Air will leave the lungs down the pressure gradient until equilibrium is achieved

Question 6

What happens during forced/active expiration

Answer 6

• Contraction of expiratory muscles to empty the lungs more completely
• Intra-alveolar pressure must be increased even further above atmospheric pressure
• Expiratory muscles must contract to reduce the volume of thoracic cavity and lungs
• Lungs reduces in volume as they do not have to be stretched as much to fill the smaller thoracic cavity, allowing recoil
• The greater difference in pressure gradient between intra-alveolar and atmosphere facilitates movement of air down the pressure gradient.

Question 7

Describe the pressure changes that occur during breathing

Answer 7

1. During inspiration, intra-alveolar
2. During expiration, intra-alveolar>atmospheric pressure
3. At the end of inspiration and expiration, intra-alveolar=atmospheric
4. Throughout the respiratory cycle, intrapleural remains less than intra-alveolar pressure
5. Hence, a transmural pressure gradient always exists and lung is always stretched to a certain degree

Question 8

What are the physical factors that influence ventilation?

Answer 8

1. Airway Resistance
2. Lung compliance and elastic recoil
3. Alveolar surface tension

Question 9

What is the primary determinant of airway resistance?

Answer 9

• Radius of the conducting airways

Question 10

What causes adjustments in airway size?

Answer 10

• Parasympathetic stimulation: Low demand for airflow, increased airway resistance through bronchoconstriction
• Sympathetic stimulation: High demand for O2 uptake causes bronchodilation. This ensures that pressure gradients established by respiratory muscle activity can achieve maximum airflow rate with minimum resistance

Question 11

Define lung compliance

Answer 11

• How much effort is required to stretch or distend the lungs
• A measure of how much change in lung volume results from a given change in transmural pressure gradient.

Question 12

How does lung compliance affect ventilation?

Answer 12

• A highly compliant lung stretches further for a given increase in pressure difference
• The lower the lung compliance, the larger the transmural pressure gradient that must be created during inspiration for normal lung expansion
• A greater transmural pressure gradient is achieved by making intrapleural pressure more subatmospheric, through more vigorous contraction of inspiratory muscles

Question 13

Define elastic recoil

Answer 13

How readily the lungs respond to after having been stretched

Question 14

How does elastic recoil influence ventilation?

Answer 14

Responsible for lungs returning to preinspiratory volume when inspiratory muscles relax at the end of inspiration

Question 15

What causes alveolar surface tension?

Answer 15

• Alveoli are lined with a layer of water
• Water molecules are attracted to each other because of the hydrogen bond and the unequal attraction produces surface tension

Question 16

Effects of alveolar surface tension

Answer 16

• Liquid layer resists any force that increases surface area. This opposes the expansion of alveolus upon inspiration as water molecules oppose being pulled apart
• Liquid layer shrinks as small as possible as water molecules tend to be attracted to one another. This causes reduction in alveolus size, squeezing in on the air inside, causing the alveoli to collapse on expiration

Question 17

Explain the relationship between the magnitude of inward directed pressure and surface tension

Answer 17

The higher the surface tension, the higher the inward directed pressure

Question 18

Explain the relationship between the magnitude of inward directed pressure and alveolar radius

Answer 18

The smaller the radius, the larger the inward directed pressure

Question 19

What is the function of pulmonary surfactant

Answer 19

• Decreases surface tension and contributes to lung stability, prevents lung from collapsing
• Increases pulmonary compliance, reducing the work of inflating the lungs
• Reduces the lungs tendency to recoil so that they do not collapse as readily

Question 20

Characteristics of Pulmonary surfactant

Answer 20

• Mixture of lipids and proteins
• Produced by type 2 alveolar cells

Question 21

How does pulmonary surfactant lower surface tension?

Answer 21

Intersperses between water molecules in the fluid lining the alveoli and lowering the surface tension. This decreases the extent of hydrogen bonding between molecules.

Question 22

Why is pulmonary surfactant able to reduce surface tension in small alveoli more?

Answer 22

Surfactant molecules are crowded closer together in the smaller alveoli. The surfactant-induced lower surface tension of small alveoli offsets the effect of their smaller radius in determining inward-directed pressure.

Question 23

What are the forces keeping the alveoli open?

Answer 23

1. Transmural pressure gradient
2. Pulmonary surfactant

Question 24

What are the forces promoting alveolar collapse?

Answer 24

1. Elasticity of stretched elastin fibers in lung connective tissue
2. Alveolar surface tension