Asthma Basics

Asthma Basics

Chapter One: Airway Anatomy - Oropharynx to Alveoli

Welcome to Chapter One. In this chapter, we follow the path of air from the oropharynx to the alveoli.

Air enters the oropharynx - a shared passage for air and food - lined with stratified squamous epithelium for protection.

It then moves into the laryngopharynx, just above the laryngeal inlet, where air is directed anteriorly into the larynx and food is guided posteriorly into the esophagus.

Next is the larynx, or voice box. Functions: air conduction, airway protection, and phonation. The epiglottis closes during swallowing to prevent aspiration.

Below lies the trachea - supported by C-shaped cartilage rings - ending at the carina, where it splits into right and left main bronchi.

The right main bronchus is wider, shorter, and more vertical - the most common site for aspiration. The left is longer and more angled.

Each main bronchus divides into lobar bronchi (three right, two left), then segmental bronchi supplying bronchopulmonary segments.

Smaller bronchioles have no cartilage and more smooth muscle; the last conducting portion is the terminal bronchiole.

Gas exchange begins in respiratory bronchioles, continues through alveolar ducts, and culminates in alveolar sacs and alveoli.

In the alveoli: Type One pneumocytes enable gas exchange; Type Two pneumocytes produce surfactant; alveolar macrophages clear debris.

Chapter Two: Asthma Definition and Epidemiology

Asthma is a heterogeneous disease with chronic airway inflammation, variable symptoms, and reversible airflow obstruction.

It often starts with epithelial injury and release of alarmins: IL-25, IL-33, and TSLP which is Thymic stromal lymphopoietin. TSLP is primarily produced by airway epithelial cells in response to environmental triggers including allergens, viruses, bacteria, and air pollutants.

Clinically, patients report wheeze, shortness of breath, chest tightness, and cough with these symptoms varying over time. Episodes result due to acute airway narrowing caused by swelling (edema), increased mucus, and bronchial smooth muscle constriction.

In the U.S., prevalence is about seven percent; heritability ranges thirty-five to ninety-five percent, with higher risk from maternal history.

Phenotypes include cough-variant, exercise-induced, allergic, eosinophilic (Type Two), pediatric or adult onset, steroid-resistant, aspirin-induced, and obesity-related.

Spirometry demonstrates variable and reversible airflow limitation in asthma where FEV one increases greater than twelve percent and greater than two hundred milliliters after bronchodilator.

Peak Expiratory Flow is used for monitoring variability.

Bronchoprovocation, or methacholine challenge with a greater than twenty percent drop in FEV one at low concentration supports diagnosis of asthma.

Fractional exhaled nitric oxide indicates Type Two eosinophilic and is a direct marker of IL-thirteen. Blood eosinophils greater than one hundred fifty to three hundred is an indirect marker of airway eosinophilia. Allergen-specific IgE indicates allergic asthma.

COPD differs by being typically neutrophilic and poorly reversible with bronchodilators.