Anatomy of the Lower Respiratory Tract

Lower Respiratory System Anatomy

The lower respiratory system includes the trachea, main bronchi, bronchioles, alveoli, lungs, and pleura.

The Trachea

The trachea is a fibrocartilaginous duct that runs from the larynx down to its division into the major bronchi in the chest. This division occurs at a point marked by semi-rigid cartilage called the carina. The trachea is composed of 15 to 20 incomplete hyaline cartilage rings, which are open at the back and connected by fibrous tissue. It is lined by respiratory mucosal tissue.

Bronchi and Bronchioles

The trachea primarily forks into the main bronchi, which enter the lungs through the pulmonary hilum. The right bronchus is longer and more oblique than the left, partly due to the position of the heart. Within each lung, the bronchi branch into progressively smaller caliber tubes called bronchioles, eventually leading to respiratory bronchioles and terminal alveolar sacs.

Pulmonary Hilum

The pulmonary hilum is the region on the medial surface of each lung where the bronchi, pulmonary arteries, and pulmonary veins enter or exit the lung.

Function of Cartilage

The function of the cartilage plates in the trachea and primary bronchi is crucial: they keep the walls of the air passages open, preventing them from collapsing and allowing them to expand properly to facilitate the passage of air.

The Lungs

Lung Structure and Location

The lungs are a pair of organs located inside the thoracic cavity. They are separated by the mediastinum, the central space that houses the heart, trachea, esophagus, and parts of the aorta and vena cavae. The lungs are roughly prism-shaped, with an apex at the top and a base that rests on the diaphragm.

Lung Lobes

Each lung is divided into lobes. The right lung has three lobes (superior, middle, and inferior), while the left lung has two lobes (superior and inferior). The left lung is slightly smaller than the right because a portion of its space is occupied by the heart.

The Pleura

Pleural Structure

The pleura is a double-layered serous membrane. One layer, the visceral pleura, surrounds the lungs, while the other layer, the parietal pleura, is attached to the chest wall.

Pleural Function

The pleura serves to cushion respiratory movements. Between the visceral and parietal pleura is a potential space, the pleural cavity, which contains a small amount of pleural fluid. This fluid acts as a lubricant, allowing the layers to slide smoothly against each other during breathing. An alteration in the pleura, such as the entry of air into the pleural cavity (pneumothorax), can cause the lung to collapse.