The conducting portion is made up of:

nasal cavities, nasopharynx, larynx, trachea, bronchii and bronchioles

The trachea branches to give rise to two primary (main) bronchii. These then branch successively to give rise in turn to secondary and tertiary bronchii.

These then branch to give rise to several orders of progressively smaller airways called bronchioles, the smallest of which are called terminal bronchioles. These are the last components of the conducting portion of the respiratory system.

Terminal bronchioles give rise to respiratory bronchioles, which ultimately lead to the alveoli.

Find out more about the respiratory portion.

Trachea

The trachea is a wide flexible tube, the lumen of which is kept open by 20 tracheal cartilages, which are C-shaped rings of hyaline cartilage. The gaps between the rings of cartilage are filled by the trachealis muscle - a bundle of smooth muscle, and fibroelastic tissue. Together these hold the lumen of the trachea open, but allow flexibility during inspiration and expiration.

The respiratory mucosa and submucosa are adapted to warm and moisten the air, and to trap particles in mucous.

Mucosa and sub-mucosa of Trachea

The respiratory mucosa is made up of the epithelium and supporting lamina propria). The epithelium is tall columnar pseudostratified with cilia and goblet cells. The supporting lamina propria underneath the epithelium contains elastin, that plays a role in the elastic recoil of the trachea during inspiration and expiration, together with blood vessels that warm the air.

The sub-mucosa contains glands which are mixed sero-mucous glands. The watery secretions from the serous glands humidify the inspired air. The mucous, together with mucous from the goblet cells traps particles from the air which are transported upwards towards the pharynx by the cilia on the epithlium. This helps to keep the lungs free of particles and bacteria.

This is a cross section through the trachea, showing the major layers.

This is a higher power image of the trachea showing the glands and epithelium in more detail. Can you classify the epithelium? There are lots of sero-mucous glands in the submucosa layer. The layer of cartilage is not seen here, but instead there is a layer of fibro-elastic connective tissue which runs between the rings of cartilage.

Bronchii

The trachea branches into two primary bronchii, which branch into secondary and then tertiary bronchii. In the tertiary bronchii, there is less cartilage, and it does not completely encircle the lumen, as shown diagramatically above.

Notice also how the mucosa is folded, and think about how this might change as you breathe in and out.

Bronchioles

The tertiary bronchii branch into bronchioles, which have a diameter of 1mm or less, and the wall structure changes.

The epithelium is made up of ciliated columnar cells in larger bronchioles, or non-ciliated in smaller bronchioles (difficult to see at this magnification). There are no goblet cells, but there are cells called Clara cells. These cells are secretory - they secrete one of the components of surfactant.

Asthma: because the diameter of the bronchioles is reliant on smooth muscle tone, these airways can almost completely shut if the smooth muscles contract strongly, which can happen in an asthmatic attack.

Terminal Bronchioles.

This picture shows a Terminal bronchiole (TB in the diagram). Note that this is at a lower magnification than the three pictures above.This is the last part of the conducting portion of the respiratory system, and has the smallest diameter of all (less than 1mm). There is no cartilage, or glands, some smooth muscle is still present, there are no goblet cells. The epithelium is either columnar or cuboidal.

The final branches of the bronchioles are called terminal bronchioles. These have a layer smooth muscle surrounding their lumens.

Stimulation of the vagus nerve (parasympathetic) causes the smooth muscle to contract, and reduce the diameter of the terminal bronchioles.

Small sacs are found extending from the walls of the terminal bronchii called respiratory bronchioles (R), that are lined by a ciliated cuboidal epithelium, and some non-ciliated cells called clara cells.

Click here to find out more about the respiratory portion of the lung.

The respiratory bronchii have a few single alveoli off their walls.