Lower respiratory tract
The lower respiratory tract consists of the trachea, bronchi and lungs. The air passages are lined with mucous membrane composed mainly of ciliated epithelium. Cilia constantly clean the tract and carry foreign matter upwards for swallowing or expectoration.
The trachea (windpipe) extends from the laryngopharynx at the level of the cricoid cartilage at the top to the carina (also called the tracheal bifurcation). C-shaped cartilage rings reinforce and protect the trachea to prevent it from collapsing. The carina is a ridge-shaped structure at the level of T6 or T7. The carina possesses sensory nerve endings which cause coughing if food or water is inhaled accidently.
The primary bronchi begin at the carina. The right primary bronchus - shorter, wider and more vertical than the left - supplies air to the right lung. The left primary bronchus delivers air to the left lung. Along with blood vessels, nerves, and lymphatics, the primary bronchi enter the lungs at the hilum. Located behind the heart, the hilum is a slit on the lung’s medial surface.
Each primary bronchus divides to form secondary bronchi. In each lung, one secondary bronchus goes into each lobe which means that the right lung has three secondary bronchi and the left lung has two.
Each lobar bronchus enters a lobe in each lung. Within its lobe, each of the lobar bronchi branches into segmental bronchi (tertiary bronchi). The segments continue to branch into smaller and smaller bronchi, finally branching into bronchioles. The larger bronchi consist of cartilage, smooth muscle and epithelium. As the bronchi become smaller, they lose cartilage and then smooth muscle. Ultimately, the smallest bronchioles consist of just a single layer of epithelial cells.
Each bronchiole includes terminal bronchioles and the alveolar sac - the chief respiratory unit for gas exchange. Within the acinus, terminal bronchioles branch into yet smaller respiratory bronchioles. The respiratory bronchioles feed directly into alveoli at sites along their walls.
The respiratory bronchioles eventually become alveolar ducts, which terminate in clusters of alveoli surrounded by capillaries (alveolar sacs). Gas exchange takes place through the alveoli.
Alveolar walls contain two basic epithelial cell types:
- Type I cells are the most abundant. It is across these thin, flat, squamous cells that gas exchange occurs.
- Type II cells secrete surfactant, a substance that coats the alveolus and reduces surface tension. This allows the alveoli to remain inflated so that gas exchange can occur by diffusion. Surfactant is formed relatively late in foetal life; thus premature infants born without adequate amounts experience respiratory distress and may die.
The cone-shaped lungs are located in the thoracic cavity and are surrounded by pleura. The right lung is shorter, broader and larger than the left. It has three lobes and handles 55% of gas exchange. The left lung has two lobes and contains a space for the heart (cardiac notch). Each lung’s concave base rests on the diaphragm; the apex extends about 1.5 cm above the first rib.
Pleura and pleural cavities
The pleura - the membrane that totally encloses the lung - is composed of a visceral layer and a parietal layer. The visceral pleura covers the entire lung surface, including the areas between the lobes. The parietal pleura lines the inner surface of the chest wall and upper surface of the diaphragm.
Serous fluid has serious functions
The pleural cavity - a potential space between the visceral and parietal pleural layers - contains a thin film of serous fluid. This fluid has two functions:
- It lubricates the pleural surfaces, which allows them to slide smoothly against each other as the lungs expand and contract.
- It creates a bond between the layers that causes the lungs to move with the chest wall during the mechanical breathing process.
Excerpted from Scott: Anatomy & Physiology Made Incredibly Easy! 1st UK Edition (ISBN: 978-1-901831-22-1)