Respiratory System

The respiratory system is comprised of two functional components:

1) the conducting portion consisting of a series of air passages

In addition to air transport its functions include:

  • humidification and temperature regulation (conditioning the air)
  • filtration and removal of particles
  • antibacterial and immunologic defense
  • the sense of smell (via specialised olfactory epithelium)
  • the production of sound (voice)

2) the respiratory portion where gaseous exchange takes place

1. Conducting components

The conducting components can be divided anatomically into the upper respiratory tract and the lower respiratory tract:

Each type of airway has its own characteristic structural features. Although the upper respiratory tract performs important functions and is involved in a number of illness (ie. the common cold; allergic rhinitis), we will restrict this overview mainly to the airways of the lower respiratory tract. With decreasing diameter of the airways, structural changes are gradual rather than abrupt transitions. Prior to discussion of the individual components and how these transitions occur, a description of 'respiratory epithelium' and the functionally important 'mucociliary escalator' system is necessary.

Respiratory Epithelium:
  • is the characteristic epithelium of the upper respiratory tract, trachea and large bronchi.
  • is typified by pseudostratifed columnar, ciliated epithelium with mucous secreting goblet cells.
  • As bronchi progressively decrease in size, there is a gradual transition to ciliated simple columnar and finally simple cuboidal epithelium. There is also a gradual decrease in the number of goblet cells.
  • In areas subjected to abrasion (eg. pharynx), respiratory epithelium is replaced by stratified squamous epithelium.
  • is called 'respiratory mucosa' when referred to in combination with its underlying lamina propria.
Photomicrograph 1
Respiratory epithelium

The Mucociliary Escalator:

Airways of the lower respiratory tract:

A. The Trachea

The trachea is a flexible tube made of fibroelastic tissue and cartilage. Its properties allow for expansion in diameter and extension in length during inspiration and elastic recoil during expiration.

The layers of the trachea are:

ABC Photomicrograph 2
Fetal trachea and bronchi:
longitudinal section A
transverse sections B and C.
AB Photomicrograph 3
A) Adult trachea; H&E
B) Adult trachea; alcian blue stain

B. Bronchi

Bronchi have the same basic structure as the trachea. A few differences along with decreasing size should be noted:

By the time the airways have divided into tertiary bronchi:

Photomicrograph 4
Bronchus, low magnification, H&E
Photomicrograph 5
Bronchus, high magnification
Photomicrograph 6
Bronchus; high magnification; H&E

Case 1

This seventy four year old woman went to see her GP complaining of a troublesome cough productive of whitish sputum which had been present for the last year. She had had two courses of antibiotics during the winter for chest infections. The photomicrographs shown illustrate a change that has occurred in this woman's bronchi (A. normal; B. abnormal from a similar case at autopsy).

AB Case 1
(Photomicrograph 7)
A - Normal
B - Abnormal

C. Bronchioles

Photomicrograph 8
Low magnification of a bronchiole
Photomicrograph 9
Bronchiole, higher magnification
Photomicrograph 10
Bronchiole, very high magnification

Due to the lack of supporting cartilage and their small size, bronchioles are vulnerable to blockage and collapse. In chronic bronchitis, hypersecretion of mucus may lead to bronchiolar blockage. During an asthma attack, contraction of the hypertrophied smooth muscle layer, combined with mucosal thickening due to inflammation and excess mucus production may result in narrowing and blockage of bronchioles with possibly fatal consequences. If pulmonary elastin is depleted (eg. in emphysema) bronchioles tend to compress fully on expiration leading to airway obstruction (increased airway resistance).

Respiratory bronchioles are the smallest bronchioles and are considered to be part of both the conducting and respiratory portions. They are characterised by interspersed alveoli in their walls (see below).

Table 1. A summary of the characteristics of the airways of the lower respiratory tract showing the progression of structural changes that occur.

  Trachea

Bronchus

Tertiary bronchus

Bronchiole

Respiratory bronchiole

Epithelium

Pseudostratified

è

Columnar

è

Cuboidal

Goblet cells

+++

++

++

+

Absent

Clara cells

Absent

Absent

Absent

+

+

Muscularis mucosae

Absent

+

++

+++

+++

Mucous glands

+++

++

+

Absent

Absent

Cartilage

+++

++

+

Absent

Absent

Alveoli

Absent

Absent

Absent

Absent

+

Case 2

This seventeen year old girl presented to A&E at 2.30 am complaining of sudden onset of shortness of breath and wheeze. Her peak flow was 120 l/min and she had a family history of eczema and hayfever.

AB Case 2
(Photomicrograph 11)

2. Respiratory components

The respiratory portions of the lungs are the structures that are directly involved in the transfer of oxygen from the air into the blood and carbon dioxide from the blood into the air.

The alveolus is the structural unit of gas exchange:

Respiratory bronchioles:

This diagram summarises the conducting and respiratory portions of the respiratory system. Approximately 23 generations of progressively smaller airways are formed between the trachea and alveolar sacs.

The pulmonary acinus (respiratory acinus) is the functional unit of the lung and is defined as all the components involved in respiratory exchange beginning at a first order respiratory bronchiole and ending in the sacs of alveoli. This definition becomes important in the characterisation of some respiratory diseases where the acinus as a whole becomes affected (eg. emphysema).

The alveolus and blood-air barrier

The histology of the alveolus is simple as it only contains three cell types:

Adjacent alveoli form sandwich-like interalveolar septa consisting of the epithelial lining of the alveoli on each side and a thin layer of loose connective tissue in between. This loose connective tissue contains the extensive network of pulmonary capillaries, a few fibroblasts, some reticular fibres and numerous elastic fibres. This part of the lung is also known as the interstitium, and is affected in a number of inflammatory and fibrosing conditions.

The blood-air barrier is formed by:

Photomicrograph 12
Lung; low magnification
Photomicrograph 13
Lung alveoli; high magnification
Photomicrograph 14
Lung alveoli and interstitial tissue

Case 3

This sixty five year old man was seen preoperatively for an elective hernia repair. The astute houseman noted in his examination : "Pursed lip breathing. Poor lung expansion. Chest x ray shows hyperinflated lung fields".

AB Case 3
(photomicrograph 15)
A Normal
B Abnormal

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Systems