1.2 Lower Airway Anatomy & Physiology

The lower airway is where the real work of respiration happens. Understanding the path from the trachea to the alveoli and the physiology of gas exchange lets an EMT recognize when oxygenation and ventilation are failing and intervene before the patient deteriorates.

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Anatomy of the Lower Airway

Trachea

The trachea (windpipe) is roughly 10-12 cm long, supported by 16-20 C-shaped cartilage rings. The open part of each C faces posteriorly so the esophagus can expand during swallowing. The trachea runs from the larynx to the carina, where it splits into the right and left main bronchi.

Bronchi and Bronchioles

At the carina, the trachea divides into:

• Right main bronchus — shorter, wider, and more vertical, which is why aspirated objects most often lodge in the right lung.
• Left main bronchus — longer, narrower, and more horizontal.

The main bronchi branch into secondary (lobar) bronchi, then tertiary (segmental) bronchi, then progressively smaller bronchioles. Unlike the cartilage-supported bronchi, bronchioles are surrounded by smooth muscle — the structures that constrict in asthma (bronchospasm) and dilate in response to a bronchodilator like albuterol. The smallest, the terminal bronchioles, lead into the respiratory zone.

Alveoli

The alveoli are tiny, grape-like air sacs at the ends of the respiratory bronchioles — roughly 300 million of them, providing about 70 square meters of surface area for gas exchange. Each alveolus is wrapped in a dense net of pulmonary capillaries and coated internally by surfactant, a substance that lowers surface tension and keeps the sacs from collapsing on exhalation.

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Gas Exchange

Gas exchange occurs at the alveolar-capillary membrane by diffusion — the passive movement of molecules from high to low concentration:

• Oxygen (O2) moves from the alveolus (high O2) into the capillary blood (low O2).
• Carbon dioxide (CO2) moves from the capillary blood (high CO2) into the alveolus (low CO2) to be exhaled.

Three Requirements for Effective Gas Exchange

1. Adequate ventilation — air must actually reach the alveoli.
2. Adequate perfusion — blood must flow through the pulmonary capillaries.
3. An intact, thin alveolar-capillary membrane — the diffusion barrier must be functional.

Disease impairs one or more of these. Pneumonia and pulmonary edema flood the alveoli or thicken the membrane (diffusion problem); a pulmonary embolism blocks perfusion; COPD and asthma obstruct ventilation. Recognizing which component is failing helps an EMT anticipate the patient's needs, even though field treatment (oxygen, ventilation, positioning, and, where authorized, albuterol) is similar across many causes.

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Respiratory Rates by Age

A rate outside these ranges — too fast or too slow — is abnormal and, combined with poor depth or effort, may require intervention.

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Tidal Volume, Dead Space, and Minute Volume

• Tidal volume (VT): air moved in one normal breath, about 500 mL in an average adult.
• Dead space: the ~150 mL of each breath that fills the conducting airways and never reaches the alveoli, so it does not participate in gas exchange.
• Minute volume (MV): total air moved per minute.

Minute Volume = Tidal Volume x Respiratory Rate

Example: 500 mL x 16/min = 8,000 mL/min (8 L/min). A key clinical insight follows from dead space: shallow, fast breathing is inefficient. A patient breathing 600 mL at 10/min moves more useful air than one breathing 250 mL at 30/min, because in the rapid-shallow pattern a large fraction of each small breath only fills dead space. That is why an EMT assists ventilations for shallow breathing even when the rate looks adequate or high.

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Respiratory Distress vs. Failure vs. Arrest

Recognizing the progression from distress to arrest drives the timing of EMT intervention:

The most dangerous transition is from distress to failure, because it can be subtle — a patient who was anxious and working hard becomes quiet, sleepy, and confused as CO2 rises and oxygen falls. That calming is not improvement; it is a red flag that the patient is tiring and approaching failure. When in doubt, support ventilation early rather than waiting for the patient to stop breathing.

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Oxygenation Is Not the Same as Ventilation

A high-yield exam concept is the difference between oxygenation (getting O2 into the blood) and ventilation (moving air in and out to remove CO2). A pulse oximeter measures oxygenation (SpO2) but tells you nothing directly about ventilation. A patient can show a normal SpO2 for a short time while ventilating poorly, because oxygen-rich blood lingers before saturation drops — meanwhile CO2 climbs. This is why an EMT does not rely on the pulse oximeter alone: a patient with adequate SpO2 but shallow, slow, or labored breathing still needs assisted ventilation.

Treat the patient's work of breathing, mental status, and chest rise, not just the number on the monitor.