The Respiratory System

Three Words That Are Not the Same Thing

Students use ventilation, respiration and oxygenation as if they were synonyms. They are three separate events, and a patient can be succeeding at one while failing the others.

• Ventilation is air physically moving in and out of the lungs. It is mechanical.
• Respiration is gas crossing a membrane. External respiration happens at the alveolus, between air and blood. Internal respiration happens at the tissue, between blood and cell.
• Oxygenation is oxygen actually arriving at the cells that need it.

A patient with a rising chest is ventilating. Whether he is respiring depends on his lungs, and whether he is oxygenating depends on his blood and his circulation as well. Chest rise is one piece of evidence, not a conclusion.

The Upper Airway

Air enters through the nose or the mouth. The nose warms, humidifies and filters, which is why it is the better route when there is a choice.

Behind both sits the pharynx, divided into three parts stacked top to bottom: the nasopharynx behind the nose, the oropharynx behind the mouth, and the laryngopharynx where the tube splits.

At that split, the larynx leads forward into the airway and the esophagus runs behind it into the stomach. The epiglottis is the leaf shaped flap that folds down over the larynx during swallowing, sending food into the esophagus rather than the trachea.

The larynx holds the vocal cords, and the space between them is the narrowest point of the adult airway. Landmarks you can feel on the outside include the thyroid cartilage (the laryngeal prominence), the cricoid cartilage below it, which is the only complete ring in the entire airway, and the cricothyroid membrane between them.

Pediatric Airway Differences

A child is not a small adult, and the airway is where that is most true.

• The head is proportionally larger with a prominent occiput, so a supine child's neck flexes and the airway narrows. Padding under the shoulders keeps it neutral.
• The tongue is proportionally larger and is the most common cause of obstruction.
• The airway itself is narrower at every point, so a small amount of swelling costs a large percentage of the opening.
• The epiglottis is floppier and more U shaped.
• The narrowest point in a young child is at the cricoid ring rather than the vocal cords.
• The trachea is softer and more easily compressed by over extending the neck.
• Infants under about a month breathe primarily through the nose, so a blocked nose is a real problem.

The Lower Airway

Below the larynx the trachea runs down the front of the neck, held open by C shaped cartilage rings that are open at the back where the esophagus sits.

The trachea divides at the carina into the right and left mainstem bronchi. The right mainstem comes off at a straighter angle, which is why an aspirated object or a tube inserted too far usually ends up on the right.

Each bronchus branches into smaller bronchi, then bronchioles, which have no cartilage and are wrapped in smooth muscle. That smooth muscle is what constricts in asthma and what a bronchodilator relaxes.

The bronchioles end at the alveoli, roughly 300 million tiny sacs where all gas exchange happens. Each alveolus is one cell thick, wrapped in a capillary that is also one cell thick, so oxygen and carbon dioxide have two thin membranes to cross and nothing else.

Surfactant is a soapy substance coating the inside of each alveolus. It lowers surface tension and stops the sac from collapsing on itself at the end of every breath. A premature infant who has not made enough of it cannot keep the alveoli open, which is the basis of infant respiratory distress syndrome.

The Lungs and the Pleura

The right lung has three lobes, the left has two, because the heart occupies space on the left. The lungs sit inside the thoracic cavity and each is wrapped in two membranes.

The visceral pleura clings to the lung. The parietal pleura lines the inside of the chest wall. Between them is a thin film of fluid in a space that carries negative pressure, and that negative pressure is what holds the lung expanded against the chest wall.

Break that seal and the lung collapses, because nothing is holding it open any more. That is a pneumothorax, and it is a mechanical failure of the pleural space rather than a disease of the lung tissue.

The Mechanics of a Breath

Breathing is a pressure problem. Air moves from higher pressure to lower pressure, and the body creates the difference by changing the size of the chest.

Inhalation is active. The diaphragm contracts and flattens downward. The external intercostal muscles pull the ribs up and out. The chest gets bigger, the pressure inside drops below atmospheric, and air flows in.

Exhalation is normally passive. The muscles relax, the elastic tissue of the lungs and chest wall recoils, the chest gets smaller, pressure rises above atmospheric, and air flows out. No effort is required.

That asymmetry explains a great deal. A patient working hard to exhale, using abdominal muscles and taking a long time doing it, has an obstructive problem, because exhaling should not require work. Asthma and COPD both produce that picture.

Volumes Worth Knowing

Dead space is the number that changes how you think. Shallow rapid breathing moves the same amount of total air as slower deeper breathing, yet far less of it reaches an alveolus, because the fixed 150 mL is paid on every single breath. A patient breathing 40 times at 200 mL is barely ventilating at all despite an impressive looking minute volume.

Tidal volumes for ventilation are calculated from ideal body weight, derived from height, never from actual weight. Gaining weight does not make the lungs larger.

What Drives You to Breathe

The primary stimulus to breathe is rising carbon dioxide.

Carbon dioxide dissolves in blood and forms carbonic acid, which lowers pH. Central chemoreceptors in the medulla read that pH change in the cerebrospinal fluid and adjust rate and depth to correct it. The system is fast, sensitive and running constantly.

Low oxygen is a secondary stimulus, read by peripheral chemoreceptors in the carotid bodies and the aortic arch. It is far weaker than the carbon dioxide drive and only becomes influential at quite low oxygen levels.

Two points that follow from this and matter clinically.

Hyperventilation lowers carbon dioxide, which is why someone who has been breathing fast then has no urge to breathe for a while.

Never withhold oxygen from a hypoxic patient. A patient with COPD who is hypoxic gets oxygen, titrated toward a target of 88 to 92 percent, which is that patient's own normal baseline rather than a reason to give less than they need. Treat the hypoxia in front of you.

Gas Exchange and Transport

At the alveolus, oxygen moves from air into blood and carbon dioxide moves from blood into air, each traveling from where it is more concentrated to where it is less. Nothing pumps them. The gradient does the work, which is why the process is called diffusion.

Oxygen then travels bound to hemoglobin inside red blood cells. Roughly 97 percent of oxygen in the blood is carried this way, with a small remainder dissolved in plasma. A pulse oximeter reads the percentage of hemoglobin that is carrying something, which is worth remembering, because carbon monoxide binds hemoglobin far more tightly than oxygen does and a pulse oximeter cannot tell the difference.

Carbon dioxide travels back mostly as bicarbonate dissolved in plasma, with smaller amounts bound to hemoglobin and dissolved as gas.

Gas exchange fails when any part of that chain breaks: fluid between the alveolus and the capillary, alveoli that are collapsed or full, a blocked pulmonary artery, or not enough hemoglobin to carry anything.

Reading the Breathing

Rate. Adult 12 to 20. Child 15 to 30. Infant 25 to 50. Rate alone tells you very little without depth.

Effort. Accessory muscle use, nasal flaring, retractions between and below the ribs, tripod positioning, and the inability to speak a full sentence all say the same thing: the ordinary work of breathing is no longer enough.

Sounds. Where the sound comes from tells you where the problem is.

Crackles and rhonchi are not interchangeable. Secretions sit in the larger airways and rattle, which is rhonchi. Fluid pushed into the alveoli is what air bubbles through, which is crackles. Crackles point toward the left ventricle. Rhonchi point toward secretions.

Snoring is the tongue against the back of the pharynx and it is corrected by position, not by a drug. Gurgling is fluid in the airway and it is corrected by suction.

What to Take Away

Ventilation, respiration and oxygenation are three different things. The upper airway ends at the vocal cords and the lower airway ends at the alveolus. Inhalation is active, exhalation is passive, and a patient working to exhale has an obstructive problem. Carbon dioxide drives breathing and oxygen is the backup. Dead space is paid on every breath, which is why fast and shallow moves air without moving gas. Never withhold oxygen from a hypoxic patient.