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Respiratory System Anatomy and Function Explained

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Trachea: Structure and Function

The trachea, or windpipe, measures approximately 12 cm in length and 2.5 cm in diameter.

Location and Relations

  • Location: Situated in the lower neck and upper chest (vertebral levels C6 to T4), connecting the larynx and the bronchi.
  • Relations:
    • The esophagus lies directly behind it.
    • The thyroid gland crosses the first four tracheal rings.

Tracheal Structure and Role

The primary function of the trachea's cartilaginous rings is to maintain a permanently open airway, preventing its walls from collapsing (unlike the esophagus).

  • Structure: Formed by a series of C-shaped cartilage rings.
  • These rings are united by fibrous tissue, with the opening of the 'C' oriented posteriorly.
  • The interior is lined by ciliated columnar epithelium and mucus-secreting cells.

Bronchi: The Airway Branches

The bronchi originate at the Carina, the bifurcation point of the trachea. Each bronchus leads to a lung:

  • Right Bronchus (BD): Thicker, shorter, and more vertical.
  • Left Bronchus (BI): Thinner, longer, and more horizontal.

Bronchial Structure and Branching

Bronchi are formed by incomplete rings of hyaline cartilage and are lined by ciliated mucous epithelium. They also contain smooth muscle tissue, which is under involuntary innervation.

The bronchi branch extensively, similar to the branches of a tree, leading eventually to the bronchioles. Bronchioles are distinguished by the absence of cartilaginous rings and the presence of a significant amount of smooth muscle tissue.

Branching Pattern

  • Right Lung: 3 Lobar Bronchi → Segmental Bronchi → Subsegmental Bronchi.
  • Left Lung: 2 Lobar Bronchi → Segmental Bronchi → Subsegmental Bronchi.

Alveolar Sacs and Gas Exchange

Each bronchiole terminates in an irregular sac, known as an Alveolar Sac, which consists of numerous small air bags (alveoli).

These air sacs are covered with a thin layer of flattened epithelial cells and are surrounded by numerous capillaries. Gas exchange takes place across these thin walls.

Capillary blood reaches the lungs via the pulmonary artery.

Lungs: Anatomy and Lobes

The lungs are a pair of conical organs, wrapped in the pleura. They occupy most of the chest cavity and are separated by the mediastinum.

Parts of a Lung

  • Apex (Vertex): Rises approximately 2.5 cm above the clavicle (at the base of the neck).
  • Base: Concave, resting on the upper surface of the diaphragm.
  • Costal Surface: Contacts the pleural cavity wall, formed by the ribs and intercostal muscles.
  • Mediastinal Surface: Features the hilum (or root of the lung).

The Hilum (Root of the Lung)

The hilum is the entry and exit point for structures connecting the lung to the mediastinum:

  • Entering: The bronchus and the pulmonary artery.
  • Exiting: The pulmonary veins.
  • Also Present: Lymph nodes (which may enlarge in diseases such as cancer) and bronchial vessels.

Lung Lobes

  • Right Lung: Divided into three lobes by two fissures:
    • Upper Lobe
    • Middle Lobe (separated by the Minor Fissure)
    • Lower Lobe (separated by the Major Fissure)
  • Left Lung: Divided into two lobes by one fissure:
    • Upper Lobe
    • Lower Lobe (separated by the Major Fissure)

Internal Structure of the Lungs

Lungs have a spongy texture and float if placed in water due to the air they contain. Each lobe is composed of many small lobules, which consist of:

  • Bronchioles and alveoli.
  • Bronchioles uniting to form larger ones.
  • An interstitial tissue framework through which blood and lymph vessels pass.

Pleura: The Protective Membrane

The pleura is a serous membrane consisting of two layers:

  1. Visceral Pleura: Surrounds the lung surface and reflects at the hilum.
  2. Parietal Pleura: Lines the inner surface of the chest wall.

Divisions of the Parietal Pleura

  • Apical Pleura
  • Diaphragmatic Pleura (forming the costophrenic sulcus)
  • Costal Pleura (related to the costomediastinal recesses)
  • Mediastinal Pleura

The Pleural Cavity is the potential space between these layers, which can sometimes fill with fluid or air (e.g., pneumothorax).

Respiratory Movements: Mechanics of Breathing

Inspiration (Inhalation)

Inspiration increases the volume of the thoracic cavity through two primary actions:

  1. Rib Elevation: Increases the anterior-posterior (AP) and transverse diameters.
  2. Diaphragm Descent: Increases the vertical diameter.

Expiration (Exhalation)

Expiration decreases the volume of the thoracic cavity through:

  1. Rib fall (depression).
  2. Diaphragm relaxation (ascending).

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