Ventilation

Question 1

What are the 3 phases of respiration?

Answer 1

External - ventilation (breathing)
Internal - pulmonary gas exchange, gas transport, systemic gas exchange
Cellular - metabolism

Question 2

What different types of cell are the alveolar membranes composed of?

Answer 2

Type I cells - simple flat epithelial cells where gas exchange happens

Type II cells - septal cells, specialised surfactant secreting cells, free surface has microvilli

Alveolar dust cells - wandering macrophages removing debris - defence

Pores of Kohn - permit collateral airflow between alveoli. Number varies - more in well ventilated areas

Question 3

What is alveolar gas exchange dependent on?

Answer 3

Fick’s law

Q (net diffusion rate) proportional to:

conc gradient x surface area of membrane x diffusion coefficient
\
thickness of membrane

The diffusion coefficient = membrane permeability / square root of MW of diffusing substance

Question 4

What are the two components associated with the stretchy behaviour of the lungs?

Answer 4

Compliance - ease with which lungs are stretched. A lung with normal compliance can be stretched easily with a small transmural pressure gradient. A poorly compliant (stiff) lung is not very stretchy - emphysema - destruction of elastic tissue

Elastic recoil - how readily the lungs recoil after stretching (responsible for quiet expiration) - a lung with normal elastic recoil will allow passive expiration - pulmonary fibrosis

Question 5

What do elastic recoil and compliance depend on?

Answer 5

• Alveolar surface tension created by the thin film of liquid lining each alveolus
• Surface tension pulls alveolus inwards because water molecules are involved in H-bonding strongly attached to each other
• Mesh of elastin fibres (connective tissue) also has role in recoil and compliance

Question 6

Describe alveolar surface tension

Answer 6

• Surface tension induced by H-bonding and strongly elastic fibres is very strong
• Unchecked these forces would collapse
• Surface tension decreased by pulmonary surfactant - secreted by T2 alveolar cells so decreased work required to inflate lungs (dec tendency to recoil, prevent collapse)

Question 7

What is La Place’s Law?

Answer 7

P=2T/r

P=inward directed collapsing pressure
T=surface tension
r=radius of alveoli (strictly a bubble)

i.e. the smaller the alveolus, the smaller the radius, the greater the tendency to collapse

Question 8

What are collapse forces overcome by?

Answer 8

Surfactant - reduces surface tension more in smaller alveoli

Surrounding alveoli - if one alveolus starts to collapse, the surrounding alveoli, joined by connective tissue, resist the collapse due to their own elasticity

Question 9

Surfactant and the new-born

Answer 9

• Premature babies (< 7 months) not able to produce surfactant so can’t overcome alveolar surface tension easily
• Lungs tend to collapse after exhalation
• Lots of effort required to inflate lungs - not compliant and baby has underdeveloped muscles
• May die due to exhaustion/lack of oxygen
• Thought that surfactant production has tole in triggering labour

Question 10

What are the 3 pressures involved in mechanics of breathing?

Answer 10

Atmospheric pressure
Intra-alveolar pressure (pressure within alveoli)
Intrapleural pressure (pressure in pleural sac - lower than other two)

Question 11

What are the different muscles involved in inspiration and expiration?

Answer 11

Accessory muscles of inspiration - contract only during forceful inspiration

Major muscles of inspiration - contract every inspiration - relaxation causes passive expiration

Muscles of active expiration - contract during active expiration

Question 12

Define Boyle’s law

Answer 12

At constant temp the volume of a given mass is inversely proportional to the pressure

V proportional to 1/P

Question 13

Talk through inspiration

Answer 13

Active process brought about by contraction of inspiratory muscles, the chest wall and lungs stretched, the increase in size of the lungs make the intra-alveolar pressure to fall,

This is because air molecules become contained in a larger volume - Boyle’s law

The air then enters down the pressure gradient into the lungs until the intra-alveolar pressure becomes equal to the atmospheric pressure

Question 14

Describe expiration

Answer 14

Passive process brought about by relaxation of inspiratory muscles. The chest wall and stretched lungs recoil to their preinspiratory size because of their elastic properties. The recoil makes the intra-alveolar pressure rise. This is because air molecules become contained in a smaller volume. Air leaves lungs down pressure gradient until intra-alveolar pressure equals atmospheric pressure

Question 15

What condition abolishes the transmural pressure gradient?

Answer 15

Pneumothorax

Air in pleural space

Question 16

Discuss spirometry

Answer 16

Look at lecture slides for graph

Question 17

What changes in obstructive lung disease?

Answer 17

Difficult to empty lungs

So…RV increases, FRC increases

Question 18

What changes in restrictive lung disease?

Answer 18

Lungs have reduced volume

VC and TLC are lower

Question 19

What is dead space and what are the two types?

Answer 19

Not all inspired gas gets to the site of gas exchange - some remains in the trachea - anatomical or airway dead space

We can measure the volume of lungs which do not eliminate Co2 - this is physiological dead space

If healthy both physio and anatom dead spaces similar. In lung disease patients the physio dead space likely to be higher

Question 20

What are pulmonary and alveolar ventilation?

Answer 20

PV is air breathed in/out in 1 min
Tidal vol x resp rate

AV is volume of air exchanged between atmosphere and alveoli per minute
(TV-dead space)x resp rate