The Lungs and Breathing
Unit 2: Anatomy, Physiology, and Medical Terminology. Lesson 6 of 53 in BMET Academy.
Breathing moves two gases in opposite directions, and a whole shelf of your equipment exists to help with it or to measure it. The lungs are also the system where a device failure becomes an emergency fastest.
The path air takes
Air enters the nose or mouth, passes the throat, and goes down the trachea, the stiff ringed tube in the front of the neck. The trachea splits into two bronchi, one per lung, and those split again and again like an upside-down tree until the smallest branches end in alveoli: millions of tiny sacs, each wrapped in capillaries. The wall between a sac and a capillary is thin enough for gas to cross. That is where the actual work happens.
Below the lungs sits the diaphragm, a dome of muscle. When it contracts and flattens, the chest gets bigger, pressure inside drops, and air flows in. Relax it and air flows out. Normal breathing is pulled in, not pushed, which is exactly backwards from how a ventilator works: a ventilator pushes air in under positive pressure.
Two gases, two directions
Oxygen crosses from the alveoli into the blood, and carbon dioxide crosses out of the blood into the alveoli to be breathed away. Carbon dioxide is the waste product of every cell, and the body is fussier about getting rid of it than people expect.
This gives you two different ideas that sound the same:
- Ventilation is moving air in and out. The measure of it is breathing rate and volume, and the number that reflects it is the exhaled carbon dioxide.
- Oxygenation is getting oxygen onto the blood. The measure of it is the oxygen saturation.
A patient can be oxygenated and still not ventilating, which is why a monitor watches both.
The numbers
Respiratory rate 12 to 20 breaths per minute (adult, at rest) SpO2 95 to 100% EtCO2 35 to 45 mmHg Tidal volume about 500 mL per breath at rest
SpO2 is the oxygen saturation a pulse oximeter reads. EtCO2 is the carbon dioxide measured at the very end of a breath, which is the closest a sensor at the mouth can get to what is in the blood. Tidal volume is the size of one ordinary breath.
The devices on this system
- Ventilator: pushes measured breaths into a patient who cannot breathe enough alone.
- Oxygen concentrator: pulls nitrogen out of room air to deliver oxygen-rich gas without a cylinder.
- Suction: negative pressure to clear fluid out of an airway.
- Capnograph: measures the carbon dioxide in exhaled gas and reports EtCO2 with a waveform.
- Pulse oximeter: a red and infrared light clipped on a finger or ear to report SpO2 and pulse rate.
- Nebulizer: turns liquid medication into a mist to be breathed in.
- Spirometer: measures how much air a patient can move and how fast, used for testing rather than treatment.
The Lungs and Breathing, explained slowly
The tree and the sacs
The airway is a branching tree that gets smaller and more numerous at every split: trachea, two bronchi, bronchioles, and finally the alveoli, hundreds of millions of sacs with a combined surface area about the size of a tennis court, roughly 70 square meters. Each sac is wrapped in capillaries, and the barrier between air and blood is thin enough that oxygen and carbon dioxide simply diffuse across it, each from where it is plentiful to where it is scarce. Nothing pumps the gas across; concentration does the work.
Breathing moves fresh air to those sacs. The diaphragm flattens, the chest cavity enlarges, pressure inside drops below the air outside, and air flows in. That is negative pressure breathing. A ventilator cannot do that, so it pushes gas in instead, under positive pressure, which is why ventilated patients need their airway pressures watched and why too much pressure can injure a lung.
Ventilation against oxygenation
This distinction answers a surprising number of exam questions.
Idea What it is The number The device ventilation moving air in and out rate, tidal volume, EtCO2 ventilator, capnograph oxygenation loading oxygen onto the blood SpO2 pulse oximeter, oxygen supply
A patient on a high oxygen flow can show a beautiful SpO2 while barely breathing, because the blood that does reach the lungs is saturated. What gives that away is a rising carbon dioxide, because clearing carbon dioxide depends entirely on moving air. This is exactly why a monitor watches both and why a capnograph is standard where patients are sedated.
What to memorize
Measurement Normal adult at rest Instrument Respiratory rate 12 to 20 breaths/min monitor impedance channel, capnograph SpO2 95 to 100% pulse oximeter EtCO2 35 to 45 mmHg capnograph Tidal volume about 500 mL per breath ventilator display, spirometer
The devices, and the one thing each gets wrong
- Ventilator: delivers measured breaths. Checks are delivered volume and pressure, the alarms, the oxygen blend, and the battery. The most common service call is a circuit leak or water in the tubing.
- Oxygen concentrator: takes room air and removes nitrogen, so there is no cylinder to change. Checks are oxygen purity, flow, and filters; a dirty intake filter is the usual complaint.
- Suction: a regulator and a canister on wall vacuum, set in continuous or intermittent mode. Checks are gauge accuracy against a reference, leak tests, and the filter. Wall outlets deliver about 50 psi for gases, and vacuum is a negative pressure rather than a supply.
- Capnograph: measures carbon dioxide in exhaled gas, either in the breathing circuit (mainstream) or by drawing a sample down a thin tube (sidestream). Water in that sample line is the top failure, and a patient breathing through the mouth on a nasal cannula reads falsely low.
- Pulse oximeter: red and infrared light through the tissue. Motion, cold fingers, nail polish, and bright ambient light all produce the same complaint.
- Nebulizer: turns liquid drug into an inhalable mist, driven by gas flow or an ultrasonic element.
- Spirometer: measures how much and how fast a patient can move air. It is a measuring instrument, so it needs a calibration syringe and a documented verification, not just a function check.