Structure and Function of the Airways Flashcards
Describe the structure of the airways and lungs
The right lung contains 3 lobes: superior lobe, middle lobe and inferior lobe while the left lung contains 2 lobes: superior and inferior. The trachea is surrounded by cartilage which provides mechanical stability as are the larger branches of the trachea. The trachea branches out into 2 primary bronchi which form the secondary bronchi and tertiary segmental bronchi. Airways formed through dichotomous branching.
Describe structure of the alveolar region
The terminal bronchiole gives rise to the respiratory bronchioles which form the alveolar duct from which the alveolar sac emerges containing multiple alveoli.
Describe alveolar type 1 and 2 cells
A type 1 cell is a squamous thin cell which forms a delicate barrier in the alveolar epithelium and facilitates gas exchange from the alveoli. A type 2 cell can replicate to form a type 1 cell and function is to secrete surfactant to reduce surface tension and antiproteases. Carry out xenobiotic metabolism - digest any chemicals or particles which get into lungs.
Type 1 cell covers 95% of epithelial surface while type 2 more abundant but covers only 5%
What cells are found in the alveolar membrane?
Type 1 and 2 alveolar cells, macrophages, fibroblasts and capillary endothelial cells. Macrophage ingests and breaks down any unnecessary substances inhaled in the alveoli. Fibroblasts produce the matrix.
What are the basic functions of the respiratory airways and what facilitates this?
Getting air efficiently to the gas exchange region and keeping the pipework clear. Conduits allow for oxygen passage to alveoli and conduct CO2 out of the lungs allowing gas exchange. This is facilitated by mechanical stability (cartilage), control of calibre (smooth muscle), protection and ‘cleansing’.
Describe the transverse section of the airway
Cartilage lines the outside but not continuously. A layer of smooth muscle can then be found and associated with it is the submucosal gland. The contraction of the smooth muscle may trigger mucosal secretion by the submucosal gland. Blood vessels can be found between the airway lumen and smooth muscle layer. Lining the inside of the airway lumen are goblet cells which secrete mucus and cilliated cells which waft this mucus.
What are the categories of airway cells and what cell types does it refer to?
- Lining cells - Ciliated, intermediate, brush, basal
- Contractile cells - Smooth muscle (airway, vasculature)
- Secretory cells - Goblet (epithelium), mucous, serous (glands)
- Connective tissue - Fibroblast, interstitial cell (elastin, collagen, cartilage)
- Neuroendocrine - Nerves, ganglia, neuroendocrine cells, neuroepithelial bodies
- Vascular cells - Endothelial, pericyte, plasma cell (+ smooth muscle)
- Immune cells - Mast cell, dendritic cell, lymphocyte, eosinophil, macrophage, neutrophil
How does the goblet cell release mucus?
Goblet cell contains numerous mucin granules and when stimulated, granule rises to apical surface of goblet cell and fuses with it forming an omega profile which is a double pore. The granule contains mucin in a highly condensed form so when it forms the double pore, airway fluid rushes into it and expands the mucin. Hence, there is a massice release of mucin onto the epithelial surface.
What do submucosal cells produce?
Mucous cells secrete mucus. Serous cells secrete anti-bacterial enzymes (e.g. lysozyme). Glands also secrete water and salts (e.g. Na+ and Cl-). These cell types are bunched together in bundles known as acini, such as mucosal acini and serous acini. Serous acini are distal to mucous acini - not clear why, but seen in other animals as well.
Describe cilliary structure
Contains anchoring proteins which are intracellular, connecting cillia to cell. The axoneme emerges from that, consisting of microtubles in a 9+2 arrangement (this arrangement of cillia and flagella is preserved through plant+animal kingdom). Microtubules connected by nexin linkages, each doublet with an inner and outer arm of dynein. This allows microtubules to move in unison and carry out cilliary beat. Outer doublets connected to centre by radial spoke. Axoneme has apical hooks at top surface which engages with mucus.
Describe beating pattern of cillia
Metachronal beating.
What are the functions of the airway epithelium?
- Secretion of mucins, water and electrolytes
- Movement of mucus by cilia – mucociliary clearance
- Providing physical barrier
- Production of regulatory and inflammatory mediators
What regulatory and inflammatory mediators does the airway epithelium produce?
- Nitric oxide (NO - via nitric oxide synthase, NOS) - increases cilliary beat
- Carbon monoxide (CO - via hemeoxygenase, HO)
- Arachidonic acid metabolites (e.g. prostaglandins – via COX)
- Chemokines (e.g. interleukin (IL)-8)
- Cytokines (e.g. GM-CSF)
- Proteases
What are the functions of the airway smooth muscle?
- Provide structure to airway
- Tone - contraction and relaxation
- Secretion of mediators, cytokines and chemokines
However, when there is inflammation present, airway muscle can hypertrophy and proliferate e.g. in asthma where smooth muscle enlarges enormously. May potentially also contract more but definitely produce a lot more mediators.
How does airway smooth muscle secretory function change during conditions of inflammation?
Bacterial products and cytokines are markers of inflammation which triggers nitric oxide synthase to produce more nitric oxide, cyclooxygenase produces prostaglandins and chemokines, cytokines plus adhesion molecules act to recruit inflammatory cells.
