Structure and Function Of airways

Question 1

Organisation and interrelations:

What allow for efficient inhalation and exhalation of air?

Answer 1

Nasal passages and airways form a smart pipe work for efficient inhalation and exhalation of air

Question 2

Airways, lungs and nasal passages:

What protects nasal passages and airways from inhaled ‘insult’?

Answer 2

Nasal passages and airways have homeostatic mechanisms to protect themselves from inhaled ‘insult’

Question 3

Airway cell types

Control of airway function

Answer 3

Many cell types contribute to making the pipework smart

The pipework is under precise control
Loss of control leads to respiratory diseases

Question 4

Organisation and physical interrelationships between the various main airway structures:

airway and lungs
What type of branching do airways have?

What provides mechanical stability?

Where does pipework lead to?

Answer 4

Airways: dichotomous branching (every tube/every airway divides into two)

Mechanical stability - cartilage

Pipework leads to alveolar region - where gas exchange takes place

Question 5

Alveolar region (gas exchange): alveolar unit

What are the two cell types that make up the alveoli, and describe their structure and functions

What are other cells/structures that lie close to the alveolar region

Answer 5

Type I cells:
Very thin, delicate barrier (facilitates gas exchange). Cover ~95% of alveolar surface

Type II cells: 
Replicate to replace type 1 cells if they are damaged. Greater number than type I cells but only cover ~5% of alveolar surface 
Secrete surfactant (reduces surface tension), antiproteases 
Xenobiotic metabolism (due to the surfactant)

In the alveolar region there are macrophages, capillary endothelium (for gas exchange), RBCs, fibroblasts (produce matrix)

Question 6

What are the basic functions of the respiratory airways?

What are they conduits to?
What facilitates their function?

Answer 6

Getting air efficiently to the gas exchange region. Keeping pipework clear

Conduit (pipes) to:
Conduct O2 to the alveoli
Conduct CO2 out of the lung
Gas exchange

Facilitated by:
Mechanical stability (cartilage)
Control of calibre (smooth muscle)
Protection and cleansing

Question 7

The nasal passages:

What are purposes of the following:
Pharynx
Conchae
Nasal hairs

Answer 7

Pharynx - common passageway for food, liquids and air

Conchae: highly vascular - contribute to warming and humidification of intra nasally inhaled air

Nasal hairs filter out large particles

Question 8

Organisation of airway structures

What are the structures in the airway?

Answer 8

Airway lumen 
Cartilage 
Smooth muscle (tracheal/bronchial) - if this contracts its trying to stop something going down your airway 
Submucosal gland 
Blood vessel (tracheal/bronchial) 
Goblet cell mucus 
Ciliated cell

Question 9

Describe the structure of an airway wall

Answer 9

Three layers (going from top to bottom):
Mucus layer
Cilia
Epithelial cells

Question 10

List the airway cell types (7)

Answer 10

Lining cells 
Contractile cells 
Secretory cells 
Connective tissue 
Neuroendocrine 
Vascular cells 
Immune cells

Question 11

Human airway epithelium:

What do the cilia contain?

How does intra-granular mucin expand?

Answer 11

Cilia contain lots of mitochondria - energy needed for wafting + mucin granules

Mucin granules contain mucin in a highly condensed form
Granules come to the apical surface of the cell and fuse w it -> double pour -> airway liquid comes into granule and makes highly condensed mucin expand -> massive release of mucin onto airway surface

Question 12

Airway submucosal glands:

What cell types do they contain? Function of these cells?

Answer 12

Mucous cells secrete mucus

Serous cells secrete anti bacterial enzymes (eg lysozymes)

Glands also secrete water and salts ( eg Na and Cl)

Question 13

Airway epithelium: list functions

Answer 13

Secretion of mucins, water and electrolytes
-components of mucus (+plasma, mediators etc)

Movement of mucus by cilia - mucociliary clearance

Physical barrier

Production of regulatory and inflammatory mediators:
Nitric oxide (NO - via nitric oxide synthase, NOS)
Carbon monoxide (CO - via hemeoxygenase, HO)
Arachidonic acid metabolites (eg prostaglandins - via COX)
Chemokines (eg IL-8)
Cytokines (eg GM-CSF)
Proteases

Question 14

NOS expression in human airway epithelium

Answer 14

Brown straining = nitric oxide synthase (NOS) - produces nitric oxide (NO)

Question 15

List the functions of airway smooth muscles

Answer 15

Inflammation —> airway smooth muscle cell

Airway smooth muscle cell functions:

Structure - hypertrophy, proliferation

Tone (airway Caliber) - contraction, relaxation

Secretion - mediators, cytokines, chemokines

Question 16

Airway smooth muscle: secretory functions

Answer 16

Inflammation: bacterial products, cytokines

NOS -> NO

COX -> prostaglandins + cytokines

Cytokines

Adhesion molecules

Inflammatory cell recruitment

Question 17

Airway vasculature:

What type of circulation?

What % of cardiac output?

How much blood flow to airway mucosa?

What sites do bronchial arteries arise from?

Via which veins does blood return to:
Tracheal circulation
Bronchial circulation

Answer 17

Trachea-bronchial circulation (systemic):

1-5% cardiac output

Blood flow to airway mucosa = 100-150 mL/min/100g tissue (amongst the highest to any tissue)

Bronchial arteries arise from many sites on: aorta, intercostal arteries + more

Blood returns from tracheal circulation via systemic veins

Blood returns from bronchial circulation to both sides of heart via bronchial and pulmonary veins

Question 18

List the functions of tracheo-bronchial circulation

Answer 18

Good gas exchange (directly between airway tissues and blood)

Contributes to warming of inspired air

Contributes to humidification of inspired air

Clears inflammatory mediators

Clears inhaled drugs (good/bad depending on drug)

Supplies airway tissue and Lumen w inflammatory cells

Supplies airway tissues and lumen w proteinaceous plasma (plasma exudation)

Question 19

Control of airway function:

Which nerves?

Which regulatory and inflammatory mediators?

Answer 19

Nerves :
Parasympathetic (cholinergic)
Sympathetic- adrenergic ?
Sensory

Regulatory and inflammatory mediators: 
Histamine 
Arachidonic acid metabolites (eg prostaglandins, leukotrienes) 
Cytokines 
Chemokines 
Proteinases/proteases 
Reactive gas species

Question 20

List the types of regulatory inflammatory cells in airways

Answer 20

Eosinophils

Neutrophils

Macrophages

Mast cells

T lymphocytes

Structural cells (eg airway smooth muscle)

Question 21

Respiratory disease:

What happens during respiratory diseases?

List some respiratory diseases

Answer 21

Loss of homeostatic control leads to pathophysiology and clinical symptoms of respiratory disease

Asthma, chronic obstructive pulmonary disease (COPD) and cystic fibrosis (CF)

All are common conditions
Asthma ~5% of population
COPD - 4th cause of death in uk and USA
CF - lethal autosomal recessive gene defect (~1:20 gene frequency; affects ~1:2000 Caucasians)

Airway inflammation, airway obstruction