22) Pulmonary Defence Mechanisms

Question 1

Why are lungs vulnerable to infections?

Answer 1

• Everyday humans have to inhale large amount of air to meet oxygen demand.
• To do this they have fast movement of air from the atmosphere to respiratory surfaces. As a result this places limits on the filtering/barrier structures possible
• Furthermore efficient gas exchange requires a thin membrane innervated by blood vessels and a warm moist environment. It is in this type of environment that microorganisms can survive.
• In addition to this the air we breathe is not sterile and contains a mixture of harmful organisms and particles

Question 2

What size/ types of particles can pose a threat to the respiratory system?

Answer 2

• Large particles: Foreign body aspiration/inhaling (e.g. food, liquid, choke hazards)
• Fine particulate material: Pollution, dust or pollen
• Microscopic pathogen: Fungal spores, bacteria or viruses

Question 3

What is the relevance of size of particles entering the airway tract?

Answer 3

• Smaller particles are more likely to get deposited/filtered later on in the lungs (e.g. closer to the alveoli
• Bigger particles on the other hand are more likely to get filtered/deposited early on in the respiratory tree

Question 4

What are the different mechanisms of the respiratory system in immunological defence?

Answer 4

• Physical obstructions: A physical barrier of filtering consisting of macro (e.g. nasal hairs, turbinates, branching airways) and micro (e.g. cilia and mucous) structures
• Protective reflexes: Coughing, sneezing, expiratory reflex
• Immunological defence system: Involving lung resident cells (e.g. alveolar macrophages), structural cells (e.g. PRR on epithelial cells) which trigger the innate immune response and antimicrobial proteins
• Biological symbiosis

Question 5

How do nasal hairs and turbinates help improve immunological defence?

Answer 5

• Hair in the nose filters the air from large particles
• Turbinate (ridge) structures bring air entering the nose into contact with tissue that is lined with mucous/fluid in which these particles can get caught in

Question 6

How do cilia and mucus help improve immunological defence of the respiratory system?

Answer 6

• The airways are lined with ciliated cells along with goblet cells and mucus glands that secrete mucus
• This results in a layer of mucus that lines the airways which traps and filters out particles
• This mucus is pushed towards the throat, by the cilia, from the airways where it is either expelled out the mouth or swallowed into the stomach (killed by the acid)
• The mucus also contains antimicrobial peptides that are able to kill/digest microorganisms which may be trapped in the mucus
• Any particles trapped in the mucus are expelled this way

Question 7

How do cilia only move mucus in one direction?

Answer 7

• Cilia have an adaptation in which they curl/ pivot during reverse stroke so the depth in which it penetrates into the mucus decreases.
• This means the cilia remain in the periciliary layer and do not penetrate the mucus gel layer
• However during forward stroke it extends fully into the mucus gel layer which allows it to push the mucus forward
• This causes a net movement of mucus forward

Question 8

What are the effects of loss/impairment of mucociliary function in cystic fibrosis?

Answer 8

• In some immunological problems (e.g. cystic fibrosis) we experience mucus dehydration and a shallower airway surface fluid/ periciliary layer
• This means that even during reverse stroke the cilia come into contact with the mucus
• Therefore expulsion of mucus is not efficient which means it is no longer effective as a form of immunological defence

Question 9

What are the effects of loss/impairment of mucociliary function in smoking?

Answer 9

• Repeated smoking of cigarettes damages the airways and causes inflammation
• This inflammation causes damage to the cilia
• This decrease in cilia means mucus clearance is hindered

Question 10

What is the effect of impaired mucus clearance in the body?

Answer 10

• They can lead to increased vulnerability to respiratory infections as this physical barrier is no longer effective against pathogens
• The presence of these infections along with the presence of mucus build up leads to progressive coughing
• There is also airway dysfunction and obstruction as there is a lot of thick mucus building up in the airway

Question 11

What are protective reflexes?

Answer 11

• Reflex actions carried out by the body to expel pathogens from the respiratory system out of the body
• Examples include coughing, sneezing and laryngeal expiratory reflex (coughing without inhaling)

Question 12

Explain the mechanism of a protective reflex?

Answer 12

• First there is a stimulation of mechanoreceptors or chemoreceptors which are found in the throat, respiratory passages and stretch receptors in the lungs which are activated by irritant particles
• They send afferent impulses (towards the CNS) via sensory neurones to cough centres found in the medulla in the brainstem
• It then triggers a reflex/efferent impulse to the relevant muscles in the respiratory system which contract to bring about the desire manoeuvre which eventually results in the protective reflex

Question 13

How does branching help improve immunological defence of the respiratory system?

Answer 13

• The respiratory system contains many branching points/ bifurcations where the diameter of the airways also decreases
• This many different branch points causes air to become turbulent from a laminar flow
• Turbulent air moves in multiple directions and so is more likely to come into contact with the mucus layer on the surface of the airways
• The particles are more likely to be trapped in this mucus and so wont be able to move further into the respiratory tract

Question 14

How do resident immune cells help improve immunological defence of the respiratory system?

Answer 14

• They provide the respiratory system with the last line of denfence.
• There are immune cells which are resident (live in) the alveoli
• These resident immune cells deal with any pathogens that make it past the defence mechanisms found in the respiratory system.
• It starts with alveolar macrophages finding and phagocytosing foreign pathogens to destroy the pathogen and present their antigen to the adaptive immune system cells (e.g. B-cells, T-cells, etc) to mount an immune response

Question 15

How do structural cells help improve immunological defence of the respiratory system?

Answer 15

• Structural cells help detect pathogens and alert other parts of the immune system
• There are Pathogen Recognition Receptors (PRRs) which are present on the epithelial cells which detect specific constituents of the pathogens (e.g. a polysaccharide found in bacterial cell wall)
• This triggers a response in which cytokines are released by the epithelial cells
• Cytokine release causes the activation and proliferation of immune cells which results in leukocyte (white blood cell) activation and migration to the site of the stimulus

Question 16

How does the microbiome in the lungs help improve immunological defence of the respiratory system?

Answer 16

• There are bacterial cells in the lungs that help protect against infection from more harmful microorganisms