S2) Ventilation and Lung Mechanics

Question 1

Breathing rate and depth is controlled to allow for certain processes.

Identify 5 processes

Answer 1

• Eating/drinking
• Speech
• Defecation
• Parturition
• Change in metabolic requirements (sleep/exercise)

Question 2

Which two structures both need to function to ensure healthy respiration?

Answer 2

• Lung parenchyma
• Respiratory airways

Question 3

Inspiration is an active form of tidal breathing.

Describe the processes involved

Answer 3

• Diaphragm contracts and moves down
• Ext. intercostals contract and elevate ribs
• Thoracic cavity expands
• Intrathoracic pressure falls below atmospheric pressure and air flows in

Question 4

Expiration is an passive form of tidal breathing.

Describe the processes involved

Answer 4

• Muscle contraction ceases
• Muscles relax
• Elastic recoil of the lungs results in return to the resting end-expiratory level
• Air flows out

Question 5

What is resting expiratory level?

Answer 5

Resting expiratory level refers to the state of equilibrium in the respiratory system before you breathe in and after you breathe out

Question 6

Identify and describe the forces acting on the lung at the equilibrium position at the end of quiet expiration

Answer 6

• Inward: lung’s elasticity and surface tension generate an inwardly directed force that favours small lung volumes
• Outward: elastic elements of muscles and various connective tissue associated with the rib favour the outward movement of the chest wall

Result = opposing forces balance each other and create a negative pressure gradient in intrapleural space

Question 7

What is tidal volume?

Answer 7

Tidal volume is the lung volume representing the normal volume of air displaced between normal inhalation and exhalation when extra effort is not applied

Question 8

What is inspiratory reserve volume?

Answer 8

Inspiratory reserve volume is the additional air that can be forcibly inhaled after the inspiration of a normal tidal volume

Question 9

What is expiratory reserve volume?

Answer 9

Expiratory reserve volume is the additional air that can be forcibly exhaled after the expiration of a normal tidal volume

Question 10

What is residual volume?

Answer 10

Residual volume is the volume of air still remaining in the lungs after the expiratory reserve volume is exhaled

Question 11

What is Inspiratory Capacity?

Answer 11

Inspiratory capacity is the maximum amount of air that can be inspired i.e. inspiratory reserve + tidal volumes

Question 12

What is Functional Residual Capacity?

Answer 12

Functional residual capacity is the volume of air in the lungs at the end of a passive exhalation

Question 13

What is vital capacity?

Answer 13

- Vital capacity is the total amount of air that can be expired after fully inhaling

- Vital capacity = inspiratory capacity + expiratory reserve OR inspiratory reserve volume + TV + expiratory reserve volume

Question 14

What is Total lung volume?

Answer 14

- Total lung volume is the maximum amount of air that can fill the lungs

- Total lung volume = vital capacity + residual volume

Question 15

What determines the functional residual capacity?

Answer 15

• The balance of elastic forces of the chest wall, favouring outward expansion
• The elasticity and surface tension of the lung, favouring a smaller lung volume

Question 16

What role do the pleural membranes have in ventilation

Answer 16

• The pleural membranes constitute the pleural seal which holds the lungs to the chest wall
• Hence, as the chest wall expands, the lung is forced to follow

Question 17

Describe the structure and function of the pleural membranes

Answer 17

• Structure: double-walled sacs enclosing each lung
• Function: slide over each other to enable smooth expansion of the lung

Question 18

What is the pleural space/cavity?

Answer 18

• The pleural space is the space between the visceral and parietal pleural membranes
• Contains 10-20 ml pleural fluid (lubricant)

Question 19

Identify the muscles of quiet inspiration and expiration

Answer 19

• Inspiration: diaphragm and external intercostal muscles
• Expiration: due to elastic recoil, no muscles used

Question 20

Describe the mechanism of quiet expiration and the role of elastic recoil

Answer 20

In quiet expiration, when muscle contraction ceases, the elastic recoil of the lung results in the thoracic cavity and the lung returning to the original equilibrium position (passive process)

Question 21

Identify the accessory muscles of forced inspiration

Answer 21

• Sternocleidomastoid
• Scalene
• Pectoralis major & minor
• Trapezius

Question 22

Identify the accesory muscles of forced expiration

Answer 22

• Internal intercostals
• Muscles of the abdominal wall

Question 23

In the older adult, resting end-expiratory level (FRC) is relatively higher than in the young person.

Why?

Answer 23

• The balance between the chest wall (recoiling out) and the lung (recoiling in) has changed
• Lung tissue has lost its elasticity

Question 24

What is compliance?

Answer 24

Compliance is the relationship between pressure and volume

C = Δvolume/Δpressure

Question 25

Describe how the stiffness/slackness of the lungs influences compliance

Answer 25

- **Stiff lungs** = low compliance (fibrosis) - **Slack lungs** = high compliance (emphysema – less elastin)

Question 26

Identify the factors which affect compliance of the lung

Answer 26

- **Elastic fibres** (elastin) - **Fibrosis** (disease) - **Surface tension**

Question 27

What effect does surface tension have?

Answer 27

- Makes inflation of alveolus harder - Makes smaller alveoli tend to collapse into larger ones

Question 28

Surfactant reduces surface tension. How does it do this?

Answer 28

- Allows the lung to inflate more easily (increased compliance) - Helps to regulate alveolar size - Prevents alveolar collapse

Question 29

Describe the structure of surfactant

Answer 29

- Complex mixture of lipids and proteins - Secreted by alveolar cells

Question 30

Describe the production and depletion of surfactant

Answer 30

- **Production** begins between 25-28 weeks gestational age - **Diminished** in Acute Respiratory Distress Syndrome (ARDS)

Question 31

Ventilation is measured through minute ventilation. What is the minute ventilation for 15 breaths per minute with a tidal volume of 0.5 L?

Answer 31

**Minute ventilation** = 15 x 0.5 L/min = 7.5 L/min

Question 32

Where in the bronchial tree is the main site of airways resistance?

Answer 32

Upper respiratory tract

Question 33

How can airways resistance be increased?

Answer 33

- Increased mucus - Hypertrophy of the smooth muscle and/or oedema - Loss of radial traction

Question 34

A small change in radius makes a big difference in resistance. How is resistance of airways calculated?

Answer 34

- **Resistance** = Pressure ÷ Flow (units kPa.L^-1.s) - **Resistance** α 1/r⁴

Question 35

Provide examples of clinical conditions which manifest the following: - Increased mucus - Hypertrophy of the smooth muscle and/or oedema - Loss of radial traction

Answer 35

- Increased mucus: **chronic bronchitis** - Hypertrophy of the smooth muscle and/or oedema: **asthma** - Loss of radial traction: **emphysema**

Question 36

Why does air flow into the pleural cavity during pneumothorax?

Answer 36

- Normally, a **negative pressure gradient** in the pleural space keeps the two pleurae together & facilitates ventilation - When punctured, **air flows from high→low pressure**, from the atmosphere into the space until it reaches equilibrium

Question 37

Why do the lungs recoil when air enters the pleural cavity (in pneumothorax)?

Answer 37

- If air enters the pleural space between the parietal and visceral pleura, the -4cm H₂O **pressure gradient disappears** - This gradient normally keeps the lung against the chest wall, hence, the **lung recoils and collapses**

Question 38

A tube is placed in the pleural space to drain the air in pneumothorax. How could damage be minimised during this procedure?

Answer 38

- Tube is inserted through the **intercostal space** under ultrasound guidance - Tube is inserted **above superior border of lower rib** as intercostal nerves and vessels run along the inferior border (4/5th intercostal space, mid-axillary line)

Question 39

Identify 4 intra-abdominal structures that could be damaged due to a stab wound to the chest

Answer 39

- Nerves (intercostal and phrenic) - Intercostal vessels (veins and arteries) - Diaphragm - Aorta

Question 40

Why would lack of surfactant cause difficulty breathing?

Answer 40

- Less surfactant **increases surface tension** in alveoli and they don't inflate properly - Lungs are stiffer and compliance is lower

Question 41

What is Respiratory distress of the newborn?

Answer 41

**- Neonatal respiratory distress syndrome** is a condition occurring in premature infants caused by developmental insufficiency of pulmonary surfactant production and structural immaturity in the lungs -There is an insufficient amount of surfactant to inflate and ventilate the lungs

Question 42

Why would a premature baby with Respiratory Distress Syndrome have intercostal recession?

Answer 42

- Intercostal muscles are **immature** not strong enough to expand the ribcage - Hence, soft tissues between the lungs **draw inwards** against the negative pressure