The Respiratory System

The respiratory system moves air between the environment and the lungs, where oxygen is transferred to the blood and carbon dioxide is removed. It is the organ system that the entire airway fundamentals topic describes and manages.

Anatomy

The respiratory system is divided into upper and lower airways:

Upper airway — from the nose and mouth to the larynx:

Nasal cavity — filters, warms, and humidifies inspired air. Air passing through the nose is cleaner and warmer than air entering through the mouth.
Pharynx — the shared passage for air and food; divided into nasopharynx (behind the nose), oropharynx (behind the mouth), and hypopharynx (above the larynx). The pharynx is collapsible — loss of muscle tone during unconsciousness or sedation can cause the tongue and soft tissues to obstruct the airway, which is why airway positioning (head tilt, jaw thrust) is the first intervention in airway management.
Larynx — contains the vocal cords. The larynx protects the lower airway from aspiration (food and liquid entering the trachea) by closing the epiglottis during swallowing. The vocal cords are the anatomical landmark that distinguishes upper from lower airway — and the landmark that a supraglottic airway sits above while an endotracheal tube passes through.

Lower airway — from the trachea to the alveoli:

Trachea — the rigid tube (supported by C-shaped cartilage rings) that carries air from the larynx to the bronchi. It divides at the carina into the right and left main bronchi.
Bronchi — the trachea’s first branches; right and left main bronchi enter the respective lungs and subdivide into progressively smaller lobar and segmental bronchi.
Bronchioles — smaller airways without cartilage support; their walls contain smooth muscle that can constrict (bronchospasm, producing wheeze) or dilate (bronchodilation, relieving wheeze). Bronchospasm in asthma and COPD is smooth muscle contraction in the bronchiolar walls.
Alveoli — microscopic air sacs (approximately 300 million in both lungs) where gas exchange occurs. Each alveolus is wrapped in capillaries. The alveolar wall is one cell thick; the capillary wall is one cell thick; oxygen and carbon dioxide diffuse across this minimal barrier. The total surface area of the alveoli is approximately 70 square meters — the size of a tennis court — providing an enormous area for gas exchange.

Breathing mechanics

Breathing is driven by the diaphragm — a dome-shaped muscle separating the chest from the abdomen.

Inspiration: the diaphragm contracts and flattens, increasing the volume of the thoracic cavity. This creates negative pressure inside the chest relative to the atmosphere, drawing air in through the airways. During labored breathing, accessory muscles (intercostals, sternocleidomastoid, scalenes) assist by further expanding the ribcage. The use of accessory muscles is a clinical sign of respiratory distress — the patient is working harder than normal to breathe.

Expiration: at rest, expiration is passive — the diaphragm relaxes, the elastic recoil of the lungs compresses the air, and it flows out. During forced expiration (coughing, exercise, respiratory distress), abdominal muscles actively compress the thoracic cavity. In obstructive lung disease (asthma, COPD), air trapping occurs because the narrowed airways resist outflow — the patient can get air in but struggles to get it out.

Breathing is regulated by brainstem respiratory centers that respond primarily to blood CO2 levels (detected by central chemoreceptors) and secondarily to oxygen levels (detected by peripheral chemoreceptors in the carotid bodies). Rising CO2 stimulates breathing. This is why capnography — monitoring exhaled CO2 — is the most direct bedside measure of ventilation adequacy.

Gas exchange

Gas exchange occurs in the alveoli by diffusion — molecules move from areas of high concentration to areas of low concentration across the alveolar-capillary membrane:

Oxygen: inhaled air has high oxygen concentration; pulmonary capillary blood has low oxygen concentration (having just returned from delivering oxygen to the tissues). Oxygen diffuses from alveolus into blood, binds to hemoglobin in red blood cells, and is carried to the tissues.
Carbon dioxide: pulmonary capillary blood has high CO2 concentration (picked up from metabolically active tissues); alveolar air has low CO2 concentration. CO2 diffuses from blood into alveolus and is exhaled.

Efficient gas exchange requires three things: adequate ventilation (fresh air reaching the alveoli), adequate perfusion (blood flowing through the pulmonary capillaries), and matching between the two (ventilation-perfusion matching). When areas of lung are ventilated but not perfused (dead space — as in pulmonary embolism) or perfused but not ventilated (shunt — as in pneumonia or atelectasis), gas exchange is impaired and oxygenation fails.