S1: Pulmonary Defence Mechanism - What can go wrong?

Question 1

Why are the lungs a potential site of immunological vulnerability due to adaptations for gas exchange?

Answer 1

• Fast movement of air from atmosphere to respiratory surfaces places limits of filtering/barrier structures possible
• Efficient gas exchange requires a thin membrane innervated by blood vessels and a warm moist environment (in which microorganisms also thrive)
• The air we breathe in is not sterile/pure gas and it contains a mixture of potentially harmful organisms and particles

Question 2

What are the three sizes of particles that air contains?

Answer 2

Large particulates
Fine particulate material
Microscopic pathogens

Question 3

Give an example of large particulates

Answer 3

Foreign body aspiration (e.g. food, liquid, choke hazards)

Question 4

Give examples of particulate material

Answer 4

• Pollution
• Dust
• Pollen

Question 5

Give examples of microscopic pathogens

Answer 5

• Fungal spores
• Bacteria
• Viruses

Question 6

List 4 mechanisms the respiratory system has to improve immunological defence

Answer 6

1. Physical Obstructions:
   - Macro (nasal hairs, turbinates, branching airways)
   - Micro (cilia, mucous)
2. Protective Reflexes
   
   • Coughing
   • Sneezing
   • Expiratory reflex
3. Immunological Defence System
   
   • Lung resident immune cells (alveolar macrophages)
   • Structural cells (epithelial cells)
   • Antimicrobial proteins
4. Biological symbiosis (commensals/microbiota)

Question 7

What do nasal hairs and turbinates do?

Answer 7

They filter air and prevent particles from reaching the airways

Question 8

What does the epiglottis do?

Answer 8

The epiglottis is the flap at the back of the tongue

It covers the airway during swallowing

Question 9

What is the glottis?

Answer 9

Space between vocal cords

Question 10

What is the role of cilia and mucus?

Answer 10

• Trap and remove microorganisms
• They are ever present
• Increased production of mucus when epithelium is irritated
• 90% of mucus from glands, 10% from epithelial goblet cells
• Contains antimicrobial peptides and lysozyme

Question 11

Explain the factors that affect the efficiency of cilia and mucus

Answer 11

• Air surface liquid depth which is a layer of liquid the mucus floats on ( if too shallow the mucus sticks, if too deep the cilia can’t reach to shift the mucus )
• Cilia beat in one direction in a coordinated manner and this moves the mucus along (mucocilary escalator)
• Ciliary beat rate and direction
• Mucus volume

Question 12

What is reflex mucus secretion?

Answer 12

• Under parasympathetic control
• Various stimuli activate sensory (afferent) neurones
• Local parasympathetic, post ganglionic neurones secrete mainly acetylcholine to induce mucus secretion
• Always involuntary

Question 13

What are the three reflexes from the airway that help remove irritant or harmful particles from the airways?

Answer 13

Nasal Cavity: Sneeze
Larynx and Large Airways: Cough
Vocal folds: Expiration reflex

Question 14

Explain the mechanism of coughing

Answer 14

Stimulation of mechano or chemoreceptors

Afferent impulses to cough center (medulla)

Efferent impulses via parasympathetic and motor nerves to diaphragm, intercostal muscles and lungs

Increased contraction of diaghramatic, abdominal and intercostal (ribs) muscles –> noisy expiration (cough)

Question 15

Compare coughs and sneezes

Answer 15

Both involve:

• Deep inspiration phase ‘ahh’
• Compression phase (glottis closed)
• Expiration phase ‘choo’
• Coughs can be voluntary
• Coughs involve bronchocontriction

Question 16

What is the expiration reflex?

Answer 16

• Immediate expiration
• Discrete stimulus: vocal cords
• No inspiration (or compression) phase
• Can be voluntary

Question 17

How does branching structure of the airway help filter particles?

Answer 17

The airflow becomes more turbulent so more air comes in contact with the mucus and more particles are filtered out

Question 18

What are alveolar macrophages (AM)?

Answer 18

• Large, round, mobile cells
• Crucial ‘sentry’ cells

They contribute to immunological defence and remove particles

Question 19

What part of the immune system is AM cells part of?

Answer 19

Innate immune system

Question 20

What do AM cells do?

Answer 20

• Recognise components of bacteria, viruses etc. without prior exposure
• Phagocytose (engulf) pathogens/particles
• Release inflammatory mediators to recruit other, specialised cell types

Question 21

What do structural cells and airway/lung microbiota do?

Answer 21

Structural cells also help detect pathogens and alert immune system
The airway/lung microbiota also help to maintain immunological balance

Question 22

SUMMARY (READ)

Answer 22

1. Inhaled air contains harmful particles of various sizes: fine particulates (e.g. pollution), microorganisms and allergens.
2. The respiratory system defensive adaptations include: physical barriers (hair, nasal turbinates), mucociliary clearance, protective reflexes (coughing, sneezing) and resident immune cells.
3. Mucus (secreted by goblet cells & submucosal glands) traps inhaled particles, contains antimicrobial peptides, and is removed from the lung via coordinated cilia movement.
4. Alveolar macrophages patrol the airspaces, removing particles, microorganisms and dead cells from respiratory surfaces via phagocytosis.