Histology of the respiratory tract

Question 1

What does the respiratory system consist of?

Answer 1

The lungs, the site of gas exchange, a system of conducting passage connected by the pharynx.

Question 2

What does the upper respiratory tract consist of and what are the main functions of this?

Answer 2

Nasal cavity
Paranasal sinuses
Nasopharynx

Functions:
Filtration, moistening and warming on inhaled air
Smell

Question 3

What does the lower respiratory tract consist of?

Answer 3

Larynx - sphincter, phonation 
Trachea 
Primary bronchi - lungs
Secondary bronchi - lobes of lungs 
Tertiary bronchi - bronchopulmonary segments 
Bronchioles 
Alveolar duct 
Alveolar sacs (where gas exchange takes place)

Question 4

What does respiratory epithelium line?
What type of epithelium is it?
What does it contain?

Answer 4

Trachea, bronchi and larger bronchioles.
Pseudostratified columnar, ciliated epithelium with goblet cells.

Contains - tall columnar ciliated cells, goblet cells, neuroendocrine cells, basal cells

Question 5

What is the function of the respiratory epithelium?

Answer 5

Mucus from goblet cells (G) and submucosal glands traps dust
Coordinated beating of cilia propels the mucus towards the pharynx (muso-cillary escalator). This moves it away from there lungs and do the pharynx and disposed of through the digestive tract.
Serous secretion from submucosal glands humidifies inspired air (these glands are the in trachea). Mucus cells are also present in these glands.

Question 6

How many micrometres is cilia?

How are they arranged?

Answer 6

Motile structures, 7-10 micrometres long
Core of 20 microtubules arranged as 9 doublets around a central pair (9+2)
Microtubules grow out from a basal body: structurally identical to a centriole and acts as a template.

Question 7

Give details on trachea

Answer 7

• C shaped rings of cartilage to prevent collapse during inspiration and expiration (right of cartilage is over half the diameter)
• Contraction of trachealis muscle (connecting the free ends) reduces diameter, raising intrathroacic pressure. This muscle is smooth muscle. This allows some ability to modulate diameter of trachea. Longitudinal strands of smooth muscle also.
• Tall respiratory epithelium (E)
• Rests on a highly cellular and vascular lamina proper (LP), rich in elastin. LP is connective tissue.
• Submucosa (SM) contains mucoserous glands, located mainly between the cartilage rings

Question 8

How many bronchi are there?

Explain the layers and structure of the bronchi

Answer 8

One goes into each lung.

Respiratory epithelium less tall; fewer goblet cells
Discontinuous smooth muscle layer separating lamina propria and submucosa
Plates of cartilage rather than C - shaped rings. Stops the bronchus from being displaced.

Question 9

Give info on the structure of the tertiary bronchus?

• epithelium type
• smooth muscle
• glands
• cartilage

Answer 9

Respiratory epithelium tall columnar = little pseudostratification; fewer goblet cells. Still has cilia.
Complete layer of smooth muscle below lamina propria
parasympathetic NS = contraction, opposed by sympathetic NS
Few mucoserous glands
Irregular plates of cartilage
Bronchi have cartilage (primary, secondary and tertiary). Bronchioles do not have any cartilage.

Question 10

How large is a bronchiole?
Does it have cartilage?
Does it have submucosal glands?
Any other info

Answer 10

Airways <1mm diameter
Lack cartilage
Discrete bundles of smooth muscle (M) - implicated in asthma
No submucosal glands (no mucus produced)
Respiratory epithelium columnar gradually changed to cuboidal ciliated cells; a few goblet cels in larger bronchioles only
Smooth muscle cells here respond to allergic reaction e.g. asthmatic attack. Smooth muscle cells are released by antihistamine in inhalers.
Primary response generates the respond.

Question 11

Where do terminal bronchioles terminate?
Do they have a gas exchange role?
What type of cells are they?
What are goblet cells replaced by here?

What happens if there is a lack of chloride ions in surfactant?

Answer 11

Respiratory bronchioles

No

Cuboidal ciliated cells (squamous, no stratification)

Clara cells (secrete components of surfactant to reduce surface tension)

Increases the surface tension

Question 12

Explain respiratory bronchioles and alveolar ducts

Answer 12

RB - first tubules involved in gas exchange, have single alveoli in walls, terminate in alveolar duct

AD- passageway make entirely of alveoli, supported by spring-like spiral of smooth muscle, collagen, elastin, terminate in alveolar sacs

Question 13

What are the two prominent cells in alveoli?

What are alveoli lined by?

Answer 13

Lining cells and surfactant secretion cells.

Extremely flattened type 1 pneumocytes and more rounded type 2 pneumocytes which secrete surfactant

Question 14

Give details on type 2 pneumocytes

Answer 14

• coat the surface of the cells with surfactant
• contain lamellar bodies in phospholipids and cholesterol
• phospholipids released by exocytosis and combine with secreted surfactant proteins to form a tubular, lipoprotein lattice
• overcomes surface tension, allowing the alveolar walls to separate

Question 15

What does the alveolar wall contain?
How far is the diffusion distance?
What do endothelial cells share here?

Answer 15

An extensive capillary plexus

0.2um

A common basal lamina with the type 1 pneumocytes.

Question 16

Ciliated cells and mucus are —- from alveolar ducts.

Inhaled particulate material that escaped the —- —- is removed by alveolar macrophages, preventing its accumulation in alveoli.

Duct-laden alveolar macrophages crawl back to the —- part of the tract and hence get carried to the —- or they travel to lymph nodes via lymphatics.

Answer 16

Absent

Mucocillary escalator

Ciliated
Pharynx

Question 17

What is the purpose of alveolar pores?

How many micrometres are they?

What do they allow equilibrium of between adjacent alveoli?

Answer 17

Means air can still move in and out if the main entrance is blocked (allows for side entrances to other alveoli).

8-10um

Pressure

Question 18

What walls are supported by bundles of elastin condensed around alveoli openings?

What does this elastin come together with to form?

What is the importance of elastin?

What does destruction of elastin lead to?

Answer 18

Alveolar

Comes together with some collagen, forms a 3-D network, supporting the lung parenchyma

Passive recoil

Breakdown of alveolar walls leading to emphysema.

Question 19

Explain anatomical and physiological dead space

Answer 19

Anatomical - alveoli are located at the end of long conducting airways, not all inspired air reaches them. The purely conducting airways constitute anatomical dead space. Typically around 150ml.

Physiological - no gas exchange due to damage or disease. Any alveoli with a poor blood supply with contribute little to gas exchange.

Question 20

Explain the blood supply to the lungs

Answer 20

The pulmonary arteries (made from elastin and muscle) from the right side of the heart supply de-oxygenated blood to lungs.
Bifurcate along with adjacent bronchioles.
They are elastic in character up to the transition between bronchi and bronchioles. It is the tunica media which contains many lamellae of elastin - less smooth muscle here as pulmonary system is under less pressure than systemic system.

Question 21

What amount of times is systolic pressure less in the pulmonary system than the systemic?
What happens to the arteries at the bronchioles level?

What happens to blood vessels at the subdivisions of the respiratory tract? Why?

Answer 21

5 X

The elastic laminae in the tunica media are reduced and the arteries become more muscular in character.

The blood vessels also branch.
The respiratory epithelium and the endothelium use the same proteins to know where to branch and grow.

Question 22

Apart from the pulmonary circulation, where else does the respiratory system receive blood?

Give some details

Answer 22

Bronchial arteries.

These are branches of the aorta and are typical muscular arteries. They supply oxygenated blood to the walls of the bronchi and bronchioles and the pleura.

These connect with the distal pulmonary arteries at the level of the respiratory bronchioles.

Question 23

Explain drainage of blood from the lungs

Answer 23

Alveolar capillaries drain into the pulmonary veins. These return blood to the left side of the heart and lack valves.

A small portion of blood from bronchial arteries drains back to the right side of the heart via the azygous and hemi-azygous veins.

Question 24

Are there any lymphatics in alveoli?
Where does via the intersistium drain?

Where does this drain to?

Answer 24

No

To small lymphatics around respiratory bronchioles.

Drain to larger lymphatics which flow the airways back to the hilum (where major blood vessels enter the lung). passing through the hilar lymph nodes.