Lower Respiratory Tract Part 1

Anatomy and Organization of the Lower Respiratory Tract

• The lower respiratory tract is defined as starting at the larynx.

• The respiratory pathway serves two primary functions:

  • Phonation: The process of making sounds and speech.

  • Airway Passage: Providing a route for air to enter the lungs for respiration.

Structural Components of the Larynx

• Hyoid Bone:

  • This bone is freely suspended at the top of the laryngeal structure.

  • It is connected to the larynx via connective tissue.

• Thyroid Cartilage:

  • This is the first major cartilage of the larynx.

  • It is named after the thyroid gland due to their physical resemblance (or vice versa), specifically a central band of tissue where the structure narrows.

  • Structure: It does not form a complete circle; it stops short of wrapping around to the posterior side.

  • It acts as a protective shield for the internal structures of the voice box.

• Cricoid Cartilage:

  • Located inferior to the thyroid cartilage.

  • Unlike the thyroid cartilage, the cricoid cartilage forms a complete ring, wrapping all the way around to the posterior aspect.

• Arytenoid Cartitages:

  • These are A-shaped cartilages that sit on top of the posterior aspect of the cricoid cartilage.

  • They are situated deep within the lumen of the larynx, protected anteriorly by the thyroid cartilage.

• Corniculate Cartilages:

  • These small cartilages sit directly on top of the arytenoid cartilages.

• Tracheal Cartilages:

  • These are located inferior to the cricoid cartilage.

  • They are described as half-rings of cartilage that only partially wrap around to the posterior.

Vocal Ligaments and Phonation

• Vocal Ligaments (True Vocal Cords):

  • Two ligaments that originate from the base of the arytenoid cartilages and extend anteriorly.

  • They vibrate within the larynx to produce sound.

  • The "true cords" consist of these vocal ligaments and their surrounding mucosa.

• Manipulation of Sound:

  • Sound is controlled by rotating the arytenoid cartilages.

  • This rotation makes the ligaments more taut or changes the expanse of the opening.

• False Vocal Cords (False Folds):

  • These are additional folds located along the walls of the larynx, superior to the true vocal cords.

Laryngeal Musculature

• The movement and tension of the vocal cords are managed by a collective effort of several muscles:

  • Arytenoarytenoid Muscles: Muscles connecting the arytenoid cartilages to one another.

  • Arytenocricoid Muscles: Muscles extending from the arytenoids down to the posterior side of the cricoid cartilage.

  • Thyrocricoid Muscles: Muscles that wrap across the front or around the back between the thyroid and cricoid cartilages.

  • Thyrohyoid Muscles: Muscles connecting the thyroid cartilage to the hyoid bone.

Airway Protection and Swallowing

• The larynx is the final checkpoint to prevent food from entering the lower respiratory tract.

• The Epiglottis: A structure that seals the airway during swallowing.

• Mechanism of Closure:

  • When the thyrohyoid muscle is engaged, the hyoid bone pulls the entire laryngeal mechanism and the lumen (pathway) upward.

  • This action pulls the opening into the epiglottis, effectively sealing it against food particles.

• During Respiration: The mechanism remains relaxed, allowing air to pass the vocal cords and move into the trachea.

The Trachea and the Carina

• Trachea: The main airway leading down from the larynx into the lungs.

• The Carina:

  • A highly sensitive ridge of cartilage located at the bifurcation (division) of the trachea into the left and right main bronchi.

  • Function: It is highly innervated and, when touched by a foreign object, triggers a neurological cough reflex designed to propel the object back out of the airway.

• Clinical Significance for Smokers:

  • Cigarette agents paralyze the ciliary escalator (ciliated pseudostratified columnar epithelium) of the respiratory mucosa.

  • Because the cilia cannot clear debris effectively, the patient relies almost entirely on the cough reflex triggered by the carina.

  • Warning: Patients who smoke should never be placed on drugs that inhibit the cough reflex, as they lose their primary means of clearing the airway.

Respiratory Histology and Cancer Detection

• Epithelial Transitions:

  • Stratified Squamous Epithelium: Found in the oral cavity, oropharynx, hypopharynx, and the superior part of the larynx (above the vocal ligaments). This tissue is designed to resist friction.

  • Respiratory Mucosa (Ciliated Pseudostratified Columnar Epithelium): Found in the nasal cavity and returning below the level of the larynx to line the lower airway.

• Clinical Application for Dental Hygienists:

  • The presence of the "wrong" tissue type in a specific location can indicate metastatic cancer.

  • Example: Finding ciliated respiratory epithelium in the oral cavity or stratified squamous epithelium in the nasopharynx or lower airway indicates cancer cells that have spread.

  • Early detection of these irregular tissue bits on a macroscopic level can lead to a significantly better prognosis.

The Main Bronchi and Aspiration Risks

• Right Main Bronchus:

  • Wider and more vertical than the left.

  • Due to this anatomy, inhaled foreign objects (aspirated objects) are much more likely to fall into the right main bronchus.

• Left Main Bronchus:

  • Narrower and more horizontal.

• Filtration Limit:

  • The vocal cords and respiratory mucosa typically filter out particles larger than approximately $7\,\mu m$.

  • Note: A red blood cell is approximately $7.5\,\mu m$; thus, particles larger than a single cell generally do not reach the lower respiratory tract.

Gross Anatomy of the Lungs

• Right Lung:

  • Volume: Larger overall volume than the left lung.

  • Shape: Wider and shorter (due to the presence of the liver beneath it).

  • Lobes: Consists of three lobes: Superior, Middle, and Inferior.

  • Fissures: Separated by two fissures: a transverse fissure and an oblique (longitudinal) fissure.

• Left Lung:

  • Shape: Taller and skinnier (to accommodate the heart's position).

  • Lobes: Consists of two lobes.

  • Fissures: Separated by a single oblique fissure.

  • Lingula: A tongue-like projection of tissue that may be a remnant of a third lobe, similar to the right lung.

  • Cardiac Impression: An indentation where the heart sits.

• Common Lung Landmarks:

  • Apex: The top point of the lung.

  • Base: The bottom surface, also called the diaphragmatic surface.

  • Costal Surfaces: The surfaces facing the ribs (front and back).

  • Mediastinal Surfaces: The surfaces facing the heart, esophagus, and central structures.

The Bronchial Tree

• The airway divides into progressively smaller branches:

  • Primary Bronchi: Also known as the main bronchi (one for each lung).

  • Secondary Bronchi: Also known as lobar bronchi (three on the right, two on the left).

  • Tertiary Bronchi: Also known as segmental bronchi.

• There are approximately $23$ orders of magnitude of branching as the airways work deeper into the lobes of the lungs.

The Hilum and Identification (RALS Acronym)

• Hilum: The region on the mediastinal surface where blood vessels and bronchi enter or exit the lung. It contains:

  • Pulmonary Arteries: Thick-walled and patent (open).

  • Pulmonary Veins: Thinner-walled and often collapsed.

  • Bronchi: Identified by the presence of broken cartilage.

• RALS Acronym: Used to distinguish the right lung from the left lung based on the position of the pulmonary artery relative to the bronchus:

  • Right Anterior: In the right lung, the pulmonary artery is anterior to the bronchus.

  • Left Superior: In the left lung, the pulmonary artery is superior to the bronchus.

• This identification is critical in clinical settings, such as organ recovery for lung transplants, to ensure the correct organ is identified despite potential surgical alterations or pathologies.