Editing Anatomy and Physiology (section)

== Respiratory System ==

=== Structure ===

The upper airway (nose, pharynx, larynx) warms, humidifies, and filters air. The lower airway (trachea, bronchi, bronchioles, alveoli) conducts air and performs gas exchange. There are roughly 300 million alveoli, each wrapped in capillaries. '''Surfactant''', produced by type II alveolar cells, lowers surface tension and prevents alveolar collapse. Insufficient surfactant causes respiratory distress syndrome in the preterm newborn.

The right mainstem bronchus is shorter, wider, and more vertical than the left, so aspirated material and a malpositioned endotracheal tube most often go right.

=== Mechanics of Breathing ===

Inspiration is active: the diaphragm contracts and flattens, the external intercostals lift the ribs, thoracic volume increases, intrapleural pressure falls, and air flows in. Expiration at rest is passive elastic recoil. Negative intrapleural pressure keeps the lung expanded against the chest wall; when air enters the pleural space, the lung collapses, which is a pneumothorax.

=== Gas Exchange and Control ===

Oxygen and carbon dioxide move by simple diffusion down partial pressure gradients across the alveolar-capillary membrane. Oxygen is carried mostly bound to hemoglobin; carbon dioxide is carried mostly as bicarbonate.

Breathing is controlled by the medulla and pons. '''Central chemoreceptors''' respond primarily to rising carbon dioxide (sensed as falling cerebrospinal fluid pH), which is the main normal stimulus to breathe. '''Peripheral chemoreceptors''' in the carotid and aortic bodies respond to low oxygen.

Typical adult arterial blood gas reference values: pH 7.35 to 7.45, PaCO2 35 to 45 mmHg, PaO2 80 to 100 mmHg, bicarbonate 22 to 26 mEq/L, and arterial oxygen saturation 95% to 100%.

'''Nursing application:'''
* Ventilation is reflected by PaCO2 and oxygenation by PaO2 and saturation. A patient can have a normal saturation while retaining carbon dioxide, so a normal pulse oximetry reading does not rule out respiratory failure.
* '''Ventilation-perfusion mismatch''' underlies most hypoxemia. Pulmonary embolism creates ventilation without perfusion; pneumonia and atelectasis create perfusion without ventilation.
* Pulse oximetry is unreliable in poor perfusion, hypothermia, motion, and carbon monoxide poisoning, where the reading may be falsely normal.
* Positioning matters: the "good lung down" principle places the healthier lung dependent to improve perfusion where ventilation is best, except in lung abscess or hemorrhage where the affected side is kept down to protect the healthy lung.
* Early hypoxia presents as restlessness, anxiety, tachycardia, and tachypnea. Cyanosis and bradycardia are late. Confusion in an older adult may be the first sign of hypoxia.