An Overview of the Respiratory System for the EMT Exam

In a sense, the respiratory system serves a very simple purpose: to bring oxygen in and get carbon dioxide out. The EMT exam will expect you to be familiar with the workings of the respiratory system. However simple it seems, the task is much more complex than that and requires a sophisticated set of structures:

• The upper airway consists of the nares (nostrils), mouth, nasopharynx, pharynx, and larynx. Combined, they work to not only channel air in and out of the body but to warm, humidify, and filter it as well.

• The lower airway begins at about the level of the vocal cords and includes the trachea, mainstem bronchi, and bronchioles, terminating in the alveoli. The bronchi, bronchioles, and alveoli comprise the lungs.

  The main function of the lower airway is to produce efficient gas exchange between the alveoli and capillaries surrounding each alveolus. Blood passing through the capillaries absorbs the oxygen onto red blood cells, and then circulates oxygen to the body’s cells. At the same time, carbon dioxide is released into the alveolus, which is then exhaled out of the body.

  

You need oxygen to produce adenosine triphosphate (ATP), the energy block used by the body, and you need to regulate carbon dioxide so just enough is available to the body and the rest is released to the atmosphere.

This process takes place in the alveoli, where the cell walls are thin enough to allow gases to diffuse freely from areas of high concentration to areas of low concentration. So, oxygen diffuses from the alveoli to the capillaries (into the blood), and carbon dioxide diffuses from the capillaries into the alveoli (out of the blood).

Of course, the concentration of gases would equalize quickly if the gases just stayed in the alveoli. Breathing, or ventilation, is the mechanical effort the body makes to move gases into and out of the lungs. Ventilation occurs with the use of the diaphragm and intercostal muscles:

• Inspiratory phase: When these muscles contract, the chest cavity increases in size as the diaphragm moves downward and the ribs are pulled outward by the intercostal muscles. This produces a slight negative pressure inside the cavity, causing the lungs to expand and drawing air in. This is the inspiratory phase of ventilation.

• Expiratory phase: During the expiratory phase, the reverse occurs. The diaphragm relaxes and moves upward, and the intercostal muscles relax as the chest returns to its resting position. The chest cavity shrinks, creating a positive pressure on the lungs. They return to their smaller resting state, pushing air out of the alveoli and back through the lower and upper airways into the atmosphere.

  

  Credit: Illustration by Kathryn Born, MA

Under normal resting conditions, this cycle of inspiration and exhalation occurs about 12 to 20 times a minute, every minute of your life. Chemoreceptors, found primarily in the brainstem, detect levels of carbon dioxide and oxygen. They send signals to the brain, which, in turn, trigger an increase or decrease in the work of breathing.

You might think oxygen levels in the body would be the primary driver for respiratory control. But carbon dioxide (CO₂) is the real mover and shaker. That’s because CO₂ is used to help create conditions in the body’s fluids that are best suited for various life functions. So, in a healthy person, the body depends on detecting a rise in CO₂ more than it depends on a drop in oxygen.

Patients who retain CO₂ all the time, such as those with chronic obstructive pulmonary disease (COPD), lose their ability to sense that gas. They end up depending upon hypoxic drive, using oxygen levels to regulate their breathing.

A 30-year-old male is hyperventilating after receiving very emotional news. His breaths are fast and deep. Which of the following statements is most accurate in this situation?

• (A)Inhaling too much oxygen will cause breathing to slow, allowing oxygen levels to fall to normal levels.

• (B)Inhaling too much oxygen will cause breathing to speed up.

• (C)Exhaling too much carbon dioxide will cause breathing to slow, allowing carbon dioxide levels to rise to normal levels.

• (D)Exhaling too much carbon dioxide will cause breathing to speed up.

The correct answer is Choice (C). Assuming everything else is normal, as carbon dioxide levels fall within the bloodstream, the brain will signal the respiratory system to slow down, allowing it to retain carbon dioxide and build it back to normal levels. Choice (A) may be true, but only if the patient has COPD and depends upon oxygen levels to control breathing. Neither Choice (B) nor (D) is true.