Respiratory System

Breathing has two linked tasks: ventilation moves air into and out of the alveoli, and perfusion brings blood past them. Oxygen crosses the alveolar-capillary membrane into pulmonary blood; carbon dioxide crosses in the opposite direction to be exhaled. Gas exchange therefore fails when air cannot reach the alveoli, blood cannot reach them, the membrane is impaired, or the patient cannot sustain the work of breathing. This distinction helps interpret findings. A low oxygen saturation suggests impaired oxygen transfer, but pulse oximetry does not show how effectively the patient is eliminating carbon dioxide. A patient receiving oxygen can have an acceptable saturation while developing carbon dioxide retention, increasing fatigue, or declining consciousness. Respiratory rate, depth, effort, speech, posture, and mental status must be read alongside the saturation. Airflow obstruction makes exhalation especially difficult. Narrowed airways and loss of elastic recoil leave air trapped in the lungs, increasing the work required for the next breath. Pursed-lip breathing creates slight back-pressure during exhalation, helping splint smaller airways open long enough for trapped air to escape.