Respiratory System

Core Concepts

Anatomy: The Airway

• Upper airway: the nose and nasal passages, pharynx, and larynx. Its jobs are to warm, filter, and humidify air and to protect the airway via the cough and gag reflexes.
• Lower airway: the trachea, bronchi, bronchioles, and finally the alveoli. The branching tubes are the conducting zone (they move air only); the respiratory zone — the alveoli — is where exchange happens.
• Alveoli: tiny, thin-walled sacs at the ends of the airways. Their design is perfect for exchange: enormous combined surface area, walls one cell layer thick, and a dense capillary web. Surfactant coats the inner surface, lowering surface tension so the sacs do not collapse on expiration — like a soap film stopping a balloon from sticking to itself.

Ventilation: The Mechanics of Breathing

• Inspiration is active: the diaphragm contracts and flattens and the intercostal muscles lift the ribs. Chest volume increases, pressure inside falls below atmospheric pressure, and air flows in — negative-pressure breathing, the same principle as a straw.
• Expiration is mostly passive at rest: muscles relax and the lungs' elastic recoil pushes air out.
• Work of breathing rises when airflow is obstructed (narrowed airways, secretions) or when the lungs become stiff (fluid, inflammation, fibrosis).
• Ventilation fails when the pressure difference is lost (a pneumothorax collapses the lung) or when air cannot move through the tubes (swelling, mucus, foreign objects) — both are nursing-emergency situations.

Gas Exchange and Transport

• Exchange is simple diffusion across the alveolar-capillary membrane: oxygen moves into the blood and carbon dioxide out, each driven by its own pressure (concentration) difference. Anything that thickens the membrane (fluid, inflammation) or shrinks its surface area slows exchange.
• Oxygen transport: most oxygen travels bound to hemoglobin inside red blood cells; only a small fraction dissolves in plasma — which is why hemoglobin level matters and why pulse oximetry measures hemoglobin saturation, not dissolved oxygen.
• Carbon dioxide transport: most CO2 travels as bicarbonate dissolved in plasma, with the rest bound to hemoglobin or dissolved.
• Ventilation-perfusion (V/Q) matching: exchange works best where air and blood meet — ventilated and perfused alveoli. When fluid fills an alveolus, air cannot reach it but blood still flows past (a V/Q mismatch), lowering oxygen levels.
• Breathing is driven largely by the body's sensing of carbon dioxide, which is why breathing speeds up when CO2 rises. A traditional teaching point holds that some people with long-standing CO2 retention rely on a low-oxygen stimulus; this concept is debated, so assess each person individually.

Lung Volumes and Capacities

• Tidal volume: the air moved in a normal, quiet breath — roughly half a liter in an average adult (approximate; values vary with body size).
• Vital capacity: the maximum air you can exhale after a maximum inhale.
• Residual volume: the air that stays in the lungs even after a forced exhale — it keeps alveoli from collapsing.
• Spirometry measures these volumes and flows; reduced capacity or flow points to restrictive or obstructive problems, covered later in this chapter.

Assessment Basics

• Rate, depth, rhythm: count respirations, note shallow versus deep breaths, and watch for irregular patterns.
• Effort: watch for accessory muscle use (neck and shoulders), nasal flaring, retractions (skin pulling between ribs or above the collarbones), and tripod positioning — leaning forward to breathe.
• Breath sounds: normal sounds are soft and clear; wheezes suggest narrowed airways, crackles suggest fluid, and diminished sounds mean less air movement. Sounds are one piece of the picture, not a diagnosis.
• Pulse oximetry (SpO2): a noninvasive estimate of hemoglobin oxygen saturation. It is quick and continuous but does not measure ventilation directly — a person can have an acceptable SpO2 while CO2 climbs — and readings can be affected by poor perfusion and movement. Target values vary by condition and institutional protocol.
• Mental status and color: confusion, restlessness, or irritability can signal inadequate oxygenation and deserve immediate attention; cyanosis is a late and unreliable sign, especially in people with darker skin — do not wait for it.

Protective Mechanisms

The respiratory system defends itself: the cough reflex clears large airways, the mucociliary escalator (tiny hairs and mucus) sweeps particles upward, and alveolar macrophages engulf invaders. Smoking, dehydration, and immobility weaken these defenses — which is why respiratory infection risk rises in hospitalized patients.

Common Confusions