Key Takeaways
- The lower airway begins at the trachea and includes the bronchi, bronchioles, and alveoli.
- The trachea is approximately 10-12 cm long and bifurcates at the carina into the right and left main bronchi.
- The right main bronchus is shorter, wider, and more vertical, making it the more common site for aspiration.
- Alveoli are the primary site of gas exchange, where oxygen enters the blood and carbon dioxide is removed.
- Gas exchange occurs by diffusion across the alveolar-capillary membrane, moving from high to low concentration.
- Normal adult respiratory rate is 12-20 breaths/min, child is 15-30 breaths/min, and infant is 25-50 breaths/min.
- Tidal volume is approximately 500 mL in adults; minute volume = tidal volume x respiratory rate.
- Respiratory distress involves increased work of breathing with adequate oxygenation, while respiratory failure indicates the body can no longer compensate.
Lower Airway Anatomy & Physiology
The lower airway is where the real work of respiration takes place. Understanding the anatomy of the trachea through the alveoli and the physiology of gas exchange is essential for every EMT.
Anatomy of the Lower Airway
Trachea
The trachea (windpipe) is a tube approximately 10-12 cm long supported by 16-20 C-shaped cartilage rings. The open portion of the C-rings faces posteriorly, allowing the esophagus to expand during swallowing. The trachea extends from the larynx to the carina, where it bifurcates (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 than the left. This is why aspirated objects most commonly lodge in the right lung.
- Left main bronchus — longer, narrower, and more horizontal.
The main bronchi further subdivide into smaller secondary (lobar) bronchi, then tertiary (segmental) bronchi, and finally into progressively smaller bronchioles. The smallest bronchioles are called terminal bronchioles, which lead to the respiratory zone.
Alveoli
The alveoli are tiny, grape-like air sacs at the ends of the respiratory bronchioles. There are approximately 300 million alveoli in the lungs, providing a massive surface area (about 70 square meters) for gas exchange. Each alveolus is surrounded by a dense network of pulmonary capillaries.
Gas Exchange
Gas exchange occurs at the alveolar-capillary membrane through a process called diffusion — the movement of molecules from an area of high concentration to an area of low concentration.
- Oxygen (O2): Moves from the alveolus (high O2) into the pulmonary capillary blood (low O2).
- Carbon dioxide (CO2): Moves from the pulmonary capillary blood (high CO2) into the alveolus (low CO2) to be exhaled.
Requirements for Effective Gas Exchange
- Adequate alveolar ventilation — air must reach the alveoli.
- Adequate perfusion — blood must flow through the pulmonary capillaries.
- Intact alveolar-capillary membrane — the membrane must be thin and functional.
Conditions like pulmonary edema, pneumonia, or COPD can impair one or more of these requirements.
Respiratory Rates by Age
| Age Group | Normal Rate (breaths/min) |
|---|---|
| Adult | 12-20 |
| Child (1-10 years) | 15-30 |
| Infant (0-1 year) | 25-50 |
A rate outside these ranges (either too fast or too slow) is abnormal and may require intervention.
Tidal Volume and Minute Volume
- Tidal volume (VT): The volume of air moved in and out of the lungs in one normal breath. In an average adult, this is approximately 500 mL.
- Dead space: The portion of tidal volume that does not participate in gas exchange (fills the conducting airways). Approximately 150 mL in adults.
- Minute volume (MV): The total volume of air moved in and out of the lungs per minute.
Minute Volume = Tidal Volume x Respiratory Rate
Example: 500 mL x 16 breaths/min = 8,000 mL/min (8 L/min)
A decrease in either tidal volume (shallow breathing) or respiratory rate will decrease minute volume and may lead to hypoxia.
Respiratory Distress vs. Failure vs. Arrest
Understanding the progression from distress to arrest is critical for timely EMT intervention:
| Stage | Description | Signs | EMT Action |
|---|---|---|---|
| Respiratory Distress | Increased work of breathing; body is compensating | Tachypnea, accessory muscle use, anxiety, tripod position, nasal flaring | Oxygen, positioning, monitor closely |
| Respiratory Failure | Body can no longer compensate; oxygenation is failing | Altered mental status, cyanosis, inadequate tidal volume, bradycardia (late) | Assist ventilations with BVM, high-flow O2 |
| Respiratory Arrest | Breathing has stopped completely | No chest rise, no air movement, unresponsive | Immediate BVM ventilations, prepare for CPR |
An aspirated peanut is most likely to lodge in which anatomical structure?
An adult patient has a respiratory rate of 18 breaths per minute and a tidal volume of 400 mL. What is the minute volume?
Gas exchange in the lungs occurs primarily by which process?
A 7-year-old child has a respiratory rate of 28 breaths per minute. This rate is:
An EMT is treating a patient who is breathing very rapidly and shallowly with a declining mental status and central cyanosis. This patient is most likely in: