L7. Histology of the Respiratory System

Question 1

What are the major features of the nasopharynx?

Answer 1

Sinuses, olfactory epithelium (sense of smell), complex turbinate bones (for increasing the surface area).

Question 2

What is the main function (apart from conduction) of the nasopharynx? Why is this function important?

Answer 2

It is to condition and humidify the air coming into the respiratory tract. It needs to warm to body temperature and humidify to 100% to make sure that rapid dehydration doesn’t occur across the mucosa.

Question 3

Where are and what are the functions of the sinuses in the nasopharynx?

Answer 3

They are embedded in the wall of the skull, bones and face and are critical for conditioning the air and for phonation (echoing to give the tone of the voice).

Question 4

What are the main functions of the epiglottis and the larynx?

Answer 4

The epiglottis prevents food and other substances that aren’t air from entering the RT. The larynx is very important to phonation, muscles squeeze the air passing through it to cause sounds.

Question 5

What is the respiratory epithelium? Where is it located and what are its main histological features [4]?

Answer 5

Respiratory epithelium lines the upper RT and the nasopharynx.

1. They are pseudostratified, columnar cells
2. The cells are ciliated cells
3. containing secretory cells (goblet cells and deeper glands) which secrete mucous.
4. There are also sensory cells to initiate coughing in response to irritants.

Question 6

What is meant by pseudostratified epithelium? What cells make it up? What are the major functions of each cell type?

Answer 6

1. Ciliated columnar cells (30%) - move mucous
2. Goblet cells (30%) - secrete mucous in a ‘shot gun firing’ expulsion in one hit
3. Basal (stem) cells (30%) in the base of the epithelium that renew both epithelial and goblet cells
4. Brush cells with microvilli (3%)
5. Serous cells (3%)
6. Small granule cells of endocrine function (1%)

Question 7

What are the protective mechanisms of the lungs?

Answer 7

The mucous conditions and coats the airways so pathogens and particulates stick to the walls and don’t make their way down. The cilia beats actively and moves the mucous out of the tract against gravity. Often these mucous boluses are swallowed into the stomach where they can be sterilised.

Question 8

How does smoking affect the lining of the airways?

Answer 8

Smoking functionally destroys cilia and renders them dysfunctional. This means that there is little ability for them to beat out mucous and this is the purpose of the smokers cough.

Question 9

What are the features of the trachea

Answer 9

A single tube (12 x 2 cm diameter) that is vertical and relatively short that is reinforced by hyaline cartilage C shaped rings with the opening facing backwards. The cartilage gives the trachea flexibility but ensures it doesn’t collapse down under the negative pressures.
At the openings is smooth muscle which is able to change the diameter of the trachea

Question 10

Describe the smooth muscle layer in the trachea

Answer 10

Three layers:

1. Mucosa (respiratory epithelium + lamina propria). There are also ducts spanning to the lumen allowing submucosal gland secretions.
2. Submucosa: broad with layers of CT and glands. The cartilage is technically part of the submucosa
3. Adventitia: cartilage and outer CT

Question 11

What are the main features of the Bronchi?

Answer 11

The trachea splits into bronchi. The primary bronchi split into 5 branches (3 Right and 2 Left). They are initially structured like the trachea but eventually lose the cartilage rings to interspersed cartilage space and the smooth muscle becomes a continuous ring (between the LP and submucosa). - Glands are still present and lymphoid nodules in the submucosa

Question 12

How does branching occur in the bronchi?

Answer 12

There are around 23 rounds of dichotomous branching getting smaller and smaller.

Question 13

What determines the change from bronchi to bronchioles and around what level does this occur?

Answer 13

The disappearance of cartilage altogether and this is around the 10th-15th dichotomous branch.

Question 14

What are the main features of the bronchioles?

Answer 14

They are around 1-2mm in diameter.
The respiratory epithelium changes: loses the cilia and then the goblet cells along the length.
The smooth muscle layers thin but very important to keeping the airways open.
Gains specialised clara cells

Question 15

What are the clara cells?

Answer 15

Take the shape of the epithelium they are living in
Have microvilli (no cilia) that don’t move
Secrete surfactant to pull airway open
Granules contain glycoproteins
May be able to neutralise toxins
Become more and more common moving down the tract

Question 16

What are the terminal bronchioles?

Answer 16

These are the FINAL level of the conducting system.
Lose goblet cells
Epithelium becomes cuboidal and with few cilia
Only 1-2 layers of smooth muscle: enough to constrict and relax the airways
Gives rise to the respiratory bronchioles

Question 17

What are the respiratory bronchioles?

Answer 17

The first respiratory structures with alveoli appearing intermittently in them as thin walled pouches off them. They are squamous and give rise to alveolar ducts.

Question 18

What are the features of the alveolus?

Answer 18

Connected, sponge-like network of air sacs
There are about 300 million of them meaning a large SA.
It is lined by simple squamous epithelium
Respiratory capillaries in the septa (tissue between alveoli)

Question 19

What are the function of the pores in the alveolar septa?

Answer 19

To allow for gas to flow through as many alveoli as possible and to allow for gas equilibration

Question 20

What is the main function of the alveolar septum?

Answer 20

Contains an abundance of reticular and elastin fibres that mechanically pull and holds the alveoli open, this force is aided by the positive air pressure keeping them open.

Question 21

What are the features of the pulmonary capillaries?

Answer 21

A dense anastomosing network of vessels for gas exchange lying under the epithelium. They are very small diameter (< RBC diameter).

Question 22

What are the features of the type 1 pneymocytes?

Answer 22

Type 1 are present in the alveoli, simple squamous and form most of the exchange surface that makes up 95% of the SA.

• tight junctions between them to limit ECF leakage
• prominant basal lamina

Question 23

What are the features of the type 2 pneymocytes?

Answer 23

Type 2 are more numerous but are smaller and don’t make up the exchange surface.
Make up only 5% of the surface area of the alveoli
Cuboidal cells clustering in the angles in the septa of alveoli
Short microvilli
Contain lamellar bodies: contain and release surfactant
Are the stem cells giving rise to both type 1 and type 2

Question 24

What components of the blood-gas barrier can affect gas exchange?

Answer 24

Surfactant, Type 1 pneumocytes, the basal lamina thickness, thickness of connective tissue, endothelial plasma cell tissues.

Question 25

What protects the alveolar component of the RT from partlcles and pathogens?

Answer 25

Resident macrophages sitting in the alveoli (outside the body) that take up load and either move up to the bronchioles to be beaten upwards by cilia or move back into the alveolar septa and stay there until death of the person.

Question 26

What are the histological layers of the pleura?

Answer 26

An outer surface, the mesothelium (squamous epithelium), thin layer of CT supported by a basal lamina and a fibrous CT.

Question 27

What are the histological features of the visceral pleura?

Answer 27

The CT contains blood vessels and lymphatics (some of which drain into the pleural space)
The mesothelium has microvilli on the surface that traps hyaluronic acid to aid for lubrication