Carina

1. Overview

The carina is a prominent internal ridge located at the site where the trachea bifurcates into the right and left main (primary) bronchi. It is a key anatomical and clinical landmark within the lower respiratory tract, involved in airflow direction and cough reflex initiation. The carina plays an important role in airway defense and is often evaluated during bronchoscopy for signs of inflammation, malignancy, or foreign bodies.

2. Location

The carina is situated within the thoracic cavity, specifically:

• At the inferior end of the trachea, at the level of the T4–T5 vertebrae (sternal angle, or Angle of Louis).

• Posterior to the manubriosternal joint and anterior to the esophagus.

• Approximately at the level where the ascending aorta begins to arch.

It marks the transition from the trachea to the bronchial tree, dividing airflow between the two lungs.

3. Structure

The carina is formed by:

• Cartilage: A keel-shaped cartilage ridge, part of the last tracheal ring, which is widened and deviated posteriorly and inferiorly.

• Mucosa: Lined with pseudostratified ciliated columnar epithelium, rich in goblet cells and nerve endings.

• Highly innervated: The mucosa is extremely sensitive, playing a key role in triggering the cough reflex.

In a normal bronchoscopy, the carina appears as a sharp, well-defined ridge between the openings of the right and left bronchi.

4. Function

The carina serves several important functions:

• Airflow division: Directs incoming air into the right and left lungs.

• Cough reflex initiation: Sensitive to mechanical stimulation (e.g., by mucus, foreign bodies, or instruments).

• Airway protection: Prevents aspiration and assists in clearing lower airway irritants.

5. Physiological Role(s)

The carina contributes to respiratory physiology through:

• Reflex defense: Its rich innervation activates strong cough reflexes upon irritation, which expel mucus or foreign objects.

• Flow regulation: As a branching point, it minimizes turbulent flow and distributes inspired air efficiently to both lungs.

• Sensory feedback: Plays a role in autonomic regulation of airway tone and response to noxious stimuli.

6. Clinical Significance

Tracheal Intubation Landmark

During endotracheal intubation, the carina is used as a depth reference. The tube should rest 2–3 cm above the carina to ensure:

• Proper bilateral lung ventilation

• Avoidance of endobronchial (usually right-sided) intubation

Bronchoscopy Marker

The carina is routinely examined during fiberoptic bronchoscopy to assess:

• Airway anatomy

• Tumor invasion or compression

• Inflammation or ulceration

A blunted, widened, or deviated carina may suggest pathology (e.g., mediastinal mass, bronchogenic carcinoma).

Carinal Tumors

Primary or metastatic tumors may involve the carina, leading to:

• Hemoptysis (coughing up blood)

• Airway obstruction

• Chronic cough or wheezing

These often require complex surgical intervention, including carinal resection and reconstruction.

Foreign Body Aspiration

The carina is a common site where aspirated foreign bodies may lodge or be redirected. Due to the more vertical and wider right main bronchus, aspirated objects often pass into the right lung, just beyond the carina.

Cough Reflex Suppression Risks

Suppression of the carina’s sensory input—due to anesthesia, neurological injury, or disease—may impair cough reflex and increase risk for:

• Aspiration pneumonia

• Secretion accumulation

Post-Surgical and Transplant Relevance

During procedures like lung transplantation or tracheobronchial resections, precise identification and preservation of the carina are critical to maintaining airway patency and function.