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Unit 1A&PObjective A&P9 min read

The Respiratory System

Unit 1: Anatomy and Physiology. Lesson 3 of 60 in CBET Academy.

Ventilators, oximeters, capnographs, spirometers, and oxygen equipment all serve one system. Know how breathing works and what each number means.

The airway

Air enters through the nose and mouth, passes the pharynx and larynx, and goes down the trachea, which splits into the right and left bronchi, then smaller bronchioles, ending in the alveoli, tiny sacs wrapped in capillaries where gas exchange happens. The right lung has three lobes and the left two (leaving room for the heart); a pleural membrane around each lung reduces friction; cilia lining the airway sweep debris and microbes back toward the mouth. The lungs sit in the thoracic cavity, protected by the ribs, with the diaphragm and the intercostal muscles underneath and between the ribs.

How breathing works

Normal breathing is negative pressure: the diaphragm contracts and moves down, the chest expands, pressure inside falls below atmospheric, and air flows in. Exhalation is passive as the diaphragm relaxes. A ventilator does the opposite: it pushes gas in under positive pressure, which is why airway pressures and their alarms are central to ventilator care.

Gas exchange

Oxygen diffuses from the alveoli into the blood and binds to hemoglobin; carbon dioxide diffuses out and is exhaled. Three separate things can fail:

  • Ventilation: moving air in and out. Measured by respiratory rate, tidal volume, and end-tidal CO2.
  • Oxygenation: getting oxygen into the blood. Measured by SpO2 and arterial blood gases.
  • Perfusion: blood flow to carry it. A patient can ventilate fine and still be hypoxic if perfusion fails.

Breathing also keeps acid-base balance: blowing off CO2 raises blood pH, slowing breathing lowers it.

Volumes and rates

  • Tidal volume: one normal breath, about 500 mL in an adult; ventilators are set at 6 to 8 mL per kg of ideal body weight.
  • Minute volume: tidal volume times rate, about 6 liters per minute.
  • Vital capacity: the most air that can be exhaled after a full breath, about 4.5 liters.
  • Residual volume: air that stays in the lungs after full exhalation.
  • Spirometry reports FVC (forced vital capacity) and FEV1 (the volume exhaled in the first second); a normal FEV1/FVC ratio is roughly 0.7 to 0.8.
  • Respiratory rate 12 to 20 per minute; apnea is no breathing.

The numbers monitors report

  • SpO2 95 to 100 percent: how saturated hemoglobin is with oxygen.
  • EtCO2 35 to 45 mmHg: the CO2 at the end of exhalation, a direct sign that ventilation and circulation are working.
  • FiO2: the fraction of oxygen being delivered, 0.21 in room air up to 1.0.

When it goes wrong

Asthma and COPD narrow the airways and trap air. Pneumonia fills alveoli with fluid. Hypoxia is too little oxygen; hypercapnia is too much CO2. Respiratory arrest is the reason ventilators, bag-valve masks, and oxygen exist.

Exam tip: ventilation is air movement (EtCO2, tidal volume, rate); oxygenation is oxygen in the blood (SpO2). Normal breathing is negative pressure; a ventilator is positive pressure.
Remember
Trachea, bronchi, bronchioles, alveoli; diaphragm drives negative-pressure breathing, ventilators push positive pressure. Tidal volume 500 mL, rate 12 to 20, SpO2 95 to 100, EtCO2 35 to 45. Ventilation moves air; oxygenation loads blood.
Deeper explanation

The Respiratory System, explained slowly

Bellows, not a pump

The lungs do not push air. The diaphragm and chest wall expand the chest like bellows, pressure inside drops below the room, and air falls in. That single fact explains why ventilators are dangerous in a way that breathing is not: a ventilator pushes, and pushing too hard injures lungs. It is why every ventilator has pressure alarms.

The path of air

nose and mouth -> pharynx -> larynx (voice box) -> trachea -> right and left bronchi
   -> bronchioles (smaller and smaller) -> alveoli (about 300 million tiny sacs)
                                              wrapped in capillaries: the exchange surface

An endotracheal tube goes through the larynx into the trachea; a tube that slips too far usually enters the right bronchus because it branches more steeply. Capnography would still show CO2; a chest X-ray or breath sounds would show one lung ventilating.

Three things that can fail separately

This is the concept that makes monitors make sense.

Ventilation    moving air in and out                 measured by rate, tidal volume, EtCO2
Oxygenation    getting oxygen onto hemoglobin        measured by SpO2, arterial blood gas
Perfusion      circulating that blood to tissues     measured by blood pressure, cardiac output

A patient on a perfectly working ventilator can still be hypoxic if the lungs are full of fluid (oxygenation failure) or if the heart is not pumping (perfusion failure). Conversely, EtCO2 dropping to zero with a working ventilator means either the circuit disconnected or circulation stopped, because CO2 only reaches the lungs if blood is flowing. That is why capnography is used to confirm CPR is effective.

The volumes

Tidal volume (VT)       one normal breath              about 500 mL; ventilator 6 to 8 mL/kg ideal weight
Minute volume (VE)      VT x rate                      500 mL x 12 = 6 L/min
Vital capacity          max exhale after max inhale    about 4.5 L
Residual volume         what stays after full exhale   about 1.2 L; lungs never fully empty
FVC                     forced vital capacity          the spirometry number
FEV1                    volume out in the first second normal FEV1/FVC about 0.7 to 0.8;
                                                       lower means obstruction (asthma, COPD)

Worked example: a ventilator set to a tidal volume of 490 mL at a rate of 14 delivers a minute volume of 6.9 L/min. If the rate is raised to 20, minute volume rises to 9.8 L/min and EtCO2 falls, because more CO2 is being blown off.

The gas numbers

  • SpO2 is the percentage of hemoglobin carrying oxygen, 95 to 100 normal. It measures oxygenation, not ventilation; a patient on supplemental oxygen can have a fine SpO2 while barely breathing.
  • EtCO2 is the CO2 concentration at the end of exhalation, 35 to 45 mmHg normal. It measures ventilation (and perfusion). High means underventilated; low means overventilated, disconnected, or not perfusing.
  • FiO2 is the fraction of inspired oxygen: 0.21 in room air, up to 1.0. Oxygen analyzers verify it.

Positive pressure and its price

A ventilator delivers gas under pressure through the circuit. Peak pressure rises when the airway is narrowed, secretions block, the patient coughs, or the tube is kinked: the high-pressure alarm. Pressure falls when the circuit leaks or disconnects: the low-pressure alarm. PEEP, a few centimeters of water held at the end of exhalation, keeps alveoli from collapsing.

When it goes wrong

Asthma, COPD      narrowed airways, air trapping; wheezing; high peak pressures on a ventilator
Pneumonia         alveoli fill with fluid; oxygenation fails while ventilation may be fine
Apnea             no breathing; the ventilator's apnea alarm; sleep apnea treated with CPAP
Hypoxia           too little oxygen; hypercapnia: too much CO2; both show on the monitor

How the exam asks it

  • "Which measurement reflects ventilation rather than oxygenation?" EtCO2.
  • "Normal tidal volume for an adult?" About 500 mL, 6 to 8 mL/kg on a ventilator.
  • "What is minute volume?" Tidal volume times respiratory rate.
  • "Normal breathing is which kind of pressure?" Negative; ventilators use positive.

What to memorize

  • Airway: trachea, bronchi, bronchioles, alveoli. Diaphragm drives negative-pressure breathing.
  • Ventilation (EtCO2, VT, rate) versus oxygenation (SpO2) versus perfusion (BP).
  • VT 500 mL, rate 12 to 20, SpO2 95 to 100, EtCO2 35 to 45, FEV1/FVC about 0.7 to 0.8.