Respiratory: Anatomy of the Lower Respiratory Tract L12

Question 1

What are the fundamental functions of the lower respiratory tract?

Answer 1

1. Conduct air to and from the site of exchange (the alveoli).
2. Clean, warm and humidify air.
3. Supply the body with oxygen and remove carbon dioxide (carry out gas exchange at the alveoli).

Question 2

Describe the location, structure and function of the trachea.

Answer 2

The trachea is a tube that carries air from the bottom of the larynx to the primary bronchi (where the trachea splits in two). It is located anterior to the oesophagus (which carries food to the stomach) in the region of the neck.
The trachea consists of C-shaped rings of hyaline cartilage stacked on top of one another which keeps the trachea open (patent).
The ‘C’ is closed posteriorly by a strip of smooth muscle called trachealis.
The trachea is lined with respiratory mucosa which contains cilia and goblet cells that secrete mucus. This mucus helps to trap debris, and the cilia help to remove it by beating mucus up towards the pharynx for swallowing. Smoking can damage the cilia, and this means smokers must resort to coughing to clear mucus.
The trachea conducts air to the alveolar as well as helps warm, clean and humidify air.

Question 3

Describe the location, neighbouring structures of the lung.

Answer 3

Each lung lies in a cavity surrounded by a sack-line membrane. These are the pleural cavities and the pleura respectively. The top of the lung is found above the level of the clavicle and is called the apex.
The bottom lies on the diaphragm and is called the base. The diaphragm is a muscle that controls the movements of our lungs as we breathe in and out.
Like the heart, the lungs are protected by the rib-cage. The anterior surface of the lungs lies deep to the ribs (costals), and is thus referred to as the costal surface.
The medial depression on each lung is called the hilum (or hilus). This is where structures enter and leave the lungs. These structures include blood vessels, nerves, lymphatics and the primary bronchi.

Question 4

Describe the divisions of the left and right lung.

Answer 4

The right lung is divided into 3 parts, the superior, middle and inferior lobes. These lobes are separated by the transverse and oblique fissure, The left lung is divided into 2 parts by the oblique fissure, the superior and inferior lobes. The left lung only has 2 parts because it must make way for the heart.

Question 5

Name the structures before and after, and describe the primary bronchi path.

Answer 5

The trachea divides into the left and right primary bronchi at the carina, which is anatomically outside of the lungs. The left primary bronchus enters the left lung and the right enters the right lung.

Question 6

Name the structure before, and describe the secondary bronchi branching.

Answer 6

The primary bronchi divide again to form secondary bronchi. Secondary bronchi correspond to the lung lobes (‘lobar bronchi’), so there are three secondary bronchi on the right and only two on the left.

Question 7

Name the structures before and after, and describe the function tertiary bronchi.

Answer 7

The secondary bronchi then divide into tertiary bronchi. These carry air into the segments of the lobes (‘segmental bronchi’). Tertiary bronchi divide into bronchioles. These are less than 1mm in diameter.

Question 8

Describe bronchioles size and function and where they end.

Answer 8

Bronchioles continue to divide and decrease in size many times until they become terminal bronchioles. These are less than 0.05mm in diameter. After these, we move into the respiratory zone, which is no longer part of the bronchial tree. This is because the bronchial tree is defined by its function: it serves to conduct air to the lungs, but no gas exchange occurs across it.

Question 9

Describe the gradual change in wall support structure of the bronchial tree as air moves deeper into the lungs.

Answer 9

As we move down the bronchial tree, cartilaginous support progressively disappears and the lumen of the airways decreases in diameter.
From the level of the bronchioles down, the wall is supported by smooth muscle as opposed to cartilage.

Question 10

Describe the change in cells present as air goes deeper down into the lungs from the bronchi.

Answer 10

The epithelium also changes from the pseudostratified cilia columnar epithelium of the respiratory mucosa to a simple columnar epithelium (no cilia). Few goblet cells are present in the epithelium of the bronchioles, and they are replaced by Clara cells (tall columnar cells with apical secretory granules) in the terminal and respiratory bronchioles. Macrophages ingest pathogens which are present in this level.

Question 11

Define the respiratory zone.

Answer 11

All structures below the level of terminal bronchioles are part of the respiratory zone. Terminal bronchioles divide into respiratory bronchioles. Respiratory bronchioles feed into alveolar ducts that supply alveolar sacs.

Question 12

Describe the components of the alveolar sac.

Answer 12

An alveolar sac is a bunch of alveoli. The appearance of the alveoli surrounding an alveolar duct can be likened to that of a bunch of grapes.

Question 13

What are the advantages of using alveoli in gas exchange?

Answer 13

Gas exchange occurs across the alveoli. The sheer number of alveoli (approx 300million) as well as their spherical shape means that gas exchange is able to occur across a huge surface area and is very efficient.

Question 14

Describe the structure of the alveolar wall.

Answer 14

In most of an alveolus, the alveolar wall consists of a thin basement membrane underlying a single layer of epithelium. this epithelium is made up of 2 types of cells:

1. Large squamous cells called type 1 pneumocytes.
2. Occasional cuboidal cells that secrete surfactant called type 2 pneumocytes.

Question 15

What is surfactant?

Answer 15

A substance that helps reduce surface tension of the alveolar fluid, and stops the alveoli from collapsing.

Question 16

What structures reach the alveolus to exchange gases?

Answer 16

Pulmonary capillaries (venous and arterial) run down with the respiratory bronchioles. The capillaries branch many times and form networks that surround each alveolus.

Question 17

Define the air-blood barrier.

Answer 17

The basement membrane of the endothelium cells of these capillaries fuses with the basement membrane of the type 1 pneumocytes of the alveolar wall to form the ‘air-blood barrier’.

Question 18

Describe the structure of the air-blood barrier and relate to it’s function.

Answer 18

Includes:
1. Type 1 pneumocytes
2. Fused basement membranes( of type 1 pneumocytes and capillaries).
3.Endothelium of capillaries.
This barrier is thin enough to allow for gas exchange by diffusion. Oxygen moves in the blood across this barrier to be carried to tissues. Carbon dioxide moves into the alveoli across this barrier to be breathed out of the lungs.