Respiratory Histology Flashcards
What are the two main components of the respiratory system?
The respiratory system is made up of a conducting part (nasopharynx, trachea, bronchi, bronchioles) which carries and conditions the air and a respiratory part (alveoli in lung) which exchanges oxygen for carbon dioxide.
Explain the histology of the nasopharynx.
Nasopharynx
- sinuses (phonation and conditioning air)
- olfactory epithelium (sense of smell)
- turbinates (warm and moisten air)
- failure to do this would lead to dehydration
What are the histological features of The epiglottis and larynx?
Epiglottis & Larynx
- epiglottis and larynx divert food and drink from airways
- larynx (uppermost part of the trachea) is an organ of phonation (sound production)
- air forced over vocal chords leads to vibrations (non-respiratory function of lungs)
What are the histological features of Respiratory epithelium?
Respiratory Epithelium
- The airways and much of the nasopharynx are lined with respiratory epithelium
- Pseudostratified epithelium –>
- Ciliated cells
Role
- mucus layer (with trapped particles) swept upwards by cilia to the epiglottis
- ends up in the stomach for sterilisation
- smoking destroys cilia, coughing shifts mucus upwards from the lungs
- Secretory cells (goblet cells and deeper glands) produce mucus
- Sensory cells to initiate coughing to expel irritants
- Ciliates columnar cells (30%) - move mucus
- Goblet Cells (30%) - secrete mucus
- Basal (stem) cells (30%) in base of epithelium renew the epithelium
- Brush cells with microvilli (3%)
- Serous cells (3%) - secretory but the product is unknown
- Small granule cells - endocrine
What are some protective mechanisms of respiratory epithelium?
Protective Mechanisms:
- air contains dust particles and fungal and bacterial spores
- Lungs are a perfect environment for pathogens
- Walls of airways are coated in mucus from goblet cells and glands.
What are the histological features of the trachea?
Trachea
- tube 12cm long, 2cm diameter
- reinforced wall (hyaline cartilage in 10-12 rings)
- cartilage is “C”- shaped; opening faces backward - bridges by smooth muscle
- Three layers (mucosa, submucosa and adventitia)
- mucosa = respiratory epithelium + lamina propria (LP)
- submucosa - glands and connective tissues
- glands (mucous and serous) keep the tracheal surface moist
- Adventitia contains cartilage and outer layer of connective tissue
What are the histological features of the bronchi?
Bronchi
- trachea splits into two primary, extrapulmonary bronchi in the thorax.
- primary bronchi split into 5 branches (3 right; 2left) which lead to different lobes of the lungs
- First 2 of around 23 dichotomous branchings
- The structure is initally like the trachea but with thinner walls
- Cartilage ring becomes cartilage plates in the intrapulmonary bronchi
- Smooth muscle at the boundary between the lamina propria and the submucosa
- glands still present
- lymphoid nodules present
- The microscopic structure is a cartilage plate (c) which reinforces the wall and the submucosa which contains glands (g) and smooth muscle (s) arranged circumferentially.
What are the histological features of bronchioles?
Bronchioles
- When the cartilage is gone, it is considered a bronchiole.
- Around a millimetre or two in diameter
- Around the level of the 10-15th dichotomous branch
- Over length of the bronchioles, respiratory epithelium loses goblet cells and ciliates columnar cells and gains Clara cells
- Clara cells are cuboidal cells with short microvilli that secrete surfactant to destroy surface tension
- the granules contain a glycoprotein and may also neutralise toxins
- more common in deeper bronchioles
- The structure shows radial connective tissue keeps airways open, but it still has smooth muscle
- Ciliated cells extend further down than the goblet cells
What are the histological features of Terminal bronchioles?
- Terminal Bronchioles are the final level of the conducting system and contain no goblet cells.
- They do contain clara cells plus some cuboidal epithelium with some cilia.
- They have one/two layers of smooth muscle and give rise to respiratory bronchioles
What are the histological features of respiratory bronchioles?
Respiratory Bronchioles
- First respiratory structures, alveoli, appear intermittently
- alveoli are thin walled pouches
- The epithelium of the respiratory chronchiole is cuboidal to squamous
- It gives rise to alveolar ducts, chains of connected alveoli
What are the histological features of the alveolus?
The Alveolus
- There are around 300 million alveoli (140m2 of exchange surface)
- Structure:
- around 200μm across
- lined with mainly simple squamous epithelium
- wall contains many pulmonary (respiratory)
capillaries
- Individual alveoli connected by pores
What are the histological features of the Interalveolar Septum?
Interalveolar Septum
- between alveoli is the interalveolar septum
- contain reticular fibres and elastin fibres
- pores allow air to equilirate
- elastin fibres keep alveoli from collapsing
- positive air pressure helps
What are the histological features of pulmonary capillaries?
Pulmonary Capillaries
- pulmonary capillaries form a dense, anastomosing network of vessels for exchange of gases
(capillaries shown in brown)
- pulmonary epithelium is very thin so that individual RBCs are visible through the wall of the alveolus
What are the histological features of Type I Pneumoctyes?
Type I Pneumoctyes
- simple squamous epithelium formed from Type I pneumocytes
- provide the majority of surface area of alveoli (95%)
- provide the exchange surface
- tight junctions limit ECF leakage
- Basal lamina is prominent
What are the histological features of Type II Pneumocytes?
Type II Pneumocytes
- More numerous than Type I, but only 5% of the area
- Cuboidal cells, often in the angle between alveoli
- Short microvilli and lamellar bodies
- Lamellar bodies contains surfactant that is secreted
- Surface tension would tend to collapse alveoli, surfactant prevents this.
What are the histological features of The Intra-alveolar Macrophage?
Intra-alveolar Macrophages:
- Ciliated epithelium and mucus trap most incoming particles
- any in alveoli are phagocytosed by macrophages that live in the alveoli
- when they are full, they migrate up the airways until they are carried off by the ciliated cells
- Some macrophages end up in interalveolar septum loaded with particles.
Give an indication of pneumocyte turnover.
Pneumocyte Turnover:
- Type I pneumocytes die and must be replaced
- Type II pneumocytes can divide and give rise to new type I or type II pneumocytes as required
What are some features of the BBB?
Blood-Gas Barrier:
- exchange of air takes place across the type I pneumocytes and the endothelial cell of the capillary
- The Blood-gas barrier consists of surfactant, type I pneumocytes, basal lamina, endothelial cells, plasma
- In the thinnest barrier, two basal lamina can fuse and there is no connective tissue.
