2. Mechanics of ventilation

Question 1

what causes air to flow in and out of lungs during respiration

Answer 1

• inspiration: alveolar pressure decreases to -2 mmHg (due to increased volume) - lower than atmospheric pressure so air flows in
• expiration: alveolar pressure increases to +2 mmHg (due to decreased volume) - higher than atmospheric pressure so air flows out

Question 2

what causes the changes in pulmonary volume (and thus alveolar pressure) during respiration

Answer 2

• inspiration: increased volume due to contraction of diaphragm and external intercostals increasing thoracic cavity
• expiration: decreased volume due to elastic recoil of lungs

Question 3

what are tidal volume and total pulmonary ventilation

Answer 3

tidal volume = volume of air that enters and leaves the lungs with each breath

total pulmonary ventilation = tidal volume x RR

Question 4

why is alveolar ventilation different to total pulmonary ventilation

Answer 4

alveolar ventilation = total pulmonary ventilation - dead space

Question 5

what is lung compliance

Answer 5

• stretchiness of lungs, i.e. ability to overcome elastic recoil
• volume change (of lung) per unit pressure change

Question 6

name the 2 sources of lung elastic recoil

Answer 6

1. elastic tissue in the lungs

2. surface tension forces of fluid lining alveoli

Question 7

how does surfactant affect lung compliance

Answer 7

surfactant increases lung compliance by reducing surface tension

Question 8

why is surfactant important in stabilising the lungs

Answer 8

• Air flows from regions of high pressure to low pressure. According to Laplace’s law, smaller alveoli have higher pressure than large alveoli (pressure = (2 x surface tension) / radius). This would cause air to flow from small alveoli to large ones, collapsing the small alveoli and forming a few large bullae.
• BUT surfactant acts to equalise pressure between alveoli. Surfactant acts to reduce surface tension (and thus pressure), and is more effective in smaller alveoli than large ones as the molecules are closer together.

Question 9

what prevents the elastic recoil of the lung causing lung collapse inwards

Answer 9

1. pleural seal is pulled in opposite directions: inward elastic recoil of lung balanced by outward recoil of chest wall - creates sub-atmospheric pressure, and thus transpulmonary pressure difference of approx. -3 mmHg
2. cohesive force of pleural fluid

Question 10

how does intrapleural pressure change during respiration

Answer 10

becomes more negative (-8 mmHg) during inspiration as thorax expands

Question 11

why does a lung recoil and collapse in a pneumothorax

Answer 11

air entering pleural space… disrupts negative pressure (i.e. vacume) in pleural seal… visceral pleura no longer pulled to parietal pleura along chest wall

Question 12

explain the pathophysiology of respiratory distress syndrome of the newborn

Answer 12

Lack of surfactant production… high surface tension in alveolar sacs… collapse of small alveoli… decreased surface area for gas exchange

Question 13

how can RDS of newborn be managed and prevented

Answer 13

Management:

1. positive pressure ventilation
2. exogenous surfactant

Prevention:
1. maternal intake of corticosteroids (to hasten dev. of foetal resp. system)