Anatomy of the Lower Respiratory Tract

This follows the introduction to the respiratory system and the anatomy of the upper respiratory tract.

The lower respiratory tract consists of several layers. The main parts of the lower respiratory tract are shown below. This includes the information needed for many first-level courses in anatomy and physiology.

Diagram of the Structure of the Lower Respiratory Tract :

Most textbooks describe the larynx as part of the upper respiratory tract, it is also shown in the diagram above to make clear the position of the structures described here relative to those of the upper respiratory tract.

The trachea (also known as the windpipe) extends from below the larynx towards the lungs. It is reinforced by a column of C-shaped rings of hyaline cartilage that support the trachea in the open position when thoracic pressure falls. The trachea is also coated with mucous membranes and cilia (ciliated epithelium) that trap minute dust particles and sweep them upwards. The C-shaped cartilage merges with the submucosa.

Outer Structures of the Lower Respiratory Tract:

The external intercostal muscles contract to raise the rib cage upwards and outwards.
For more about the function of these muscles, see the page about external respiration.

Two continuous layers of epithelium called 'pleurae' cover the lungs, chest wall, and mediastinum*. The inner pleura (covering the lung) is called the visceral pleura and the outer pleura (covering the chest wall) is called the parietal pleura. These two pleurae are separated by a thin layer of liquid called the pleural effusion this fluid occupies the space between the pleurae (which is called the pleural cavity). The pleural effusion acts as a lubricant, allowing the surfaces of the two pleurae to slip over each other during breathing.
The pleural membranes are also described on the page about components of the respiratory system.

Inner Structures of the Lower Respiratory Tract:

Inside the pleural membranes are the structures that perform the functions of the lungs. Their main functions are the gaseous exchanges that oxygenate blood.

This involves 3 main groups of structures:

1. Structures delivering deoxygenated blood to the lungs:
   The pulmonary artery conveys blood to the lungs after it has passed around the body delivering oxygen and nutrients to tissues. This blood is therefore low in oxygen but high in carbon dioxide (CO₂) when it is returned to the lungs. The pulmonary artery divides into many arterioles that carry blood further into the structures of the lung. These continue to sub-divide until they eventually form capillaries.
2. Structures that re-oxygenate the blood
   by removing carbon dioxide (CO₂) from it and replacing the CO₂ removed with oxygen (O₂) :
   These structures perform the specialised functions that re-oxygenate blood.
   Only some of the bronchial structures are illustrated above - to show their position relative to other structures of the lower respiratory tract. The diagram on this page shows the main bronchus, an example of a terminal bronchiole (there are many in each lung), and some alveoli (sing. = alveolus), in each lung. These structures are all part of the main bronchial structure of each lung, which is called the tracheobronchial tree. For more detail see notes about the tracheobronchial tree.
3. Structures that return the re-oxygenated blood to the heart:
   The pulmonary vein and the network of venules that lead to it convey newly oxygenated blood from the capillaries of the lung to the heart (from which that oxygenated blood is pumped around the body for supply to the rest of the body, via the blood vessels of the systemic circulation system).

* The mediastinum is located in the midline of the body between the lungs. It contains the heart, trachea, oesophagus, major blood vessels to/from the heart (aorta and vena cava), phrenic and vagus nerves, and some lymph nodes.

Next: Also see the pages about the tracheobroncial tree then external respiration and internal respiration.