Respiratory System

Core Concepts

Ventilation: moving air

Ventilation is the physical movement of air into and out of the lungs. Air enters through the upper airway (nose, pharynx, larynx), travels down the trachea and bronchial tree, and reaches small airways ending in clusters of air sacs. Inspiration is active: the diaphragm contracts and flattens, the chest expands, pressure inside drops, and air flows in. Expiration at rest is mostly passive: muscles relax, the chest recoils, and air flows out. The conducting airways warm, filter, and humidify the air; a mucus blanket and tiny cilia trap particles and sweep them upward, and a cough expels what accumulates. The work of breathing depends on airway resistance and the stiffness of the lungs and chest wall — narrowed airways and stiff lungs force harder work, visible as increased effort, accessory muscle use, and changes in rate or depth.

Diffusion: gas exchange at the membrane

At the end of the airway tree sit the alveoli — millions of tiny, thin-walled air sacs wrapped in a dense capillary network. The wall between the air and the blood is only a couple of cell layers thick, and oxygen and carbon dioxide cross it by diffusion, each moving from where it is more concentrated to where it is less concentrated. A thin, moist surface coated with surfactant keeps the alveoli open: surfactant reduces surface tension so the sacs do not collapse on expiration. Anything that thickens the membrane (inflammation), fills the alveoli with fluid or debris (pneumonia, pulmonary edema), or removes surfactant interferes with diffusion, and gas exchange becomes less efficient.

Perfusion: blood flow to the exchange surface

Gas exchange also requires blood to arrive at the alveoli. The right side of the heart pumps blood through the pulmonary circulation to the alveolar capillaries, where it picks up oxygen and drops off carbon dioxide before returning to the left heart for distribution. For efficient exchange, air and blood should meet in the same regions at the same time — ventilation-perfusion (V/Q) matching. When an area is ventilated but not perfused, or perfused but not ventilated, gas exchange is wasted; the body compensates by shifting blood flow toward better-ventilated areas, but significant mismatching (mucus plugs, collapsed alveoli, fluid, blood clots in the lung) lowers overall oxygen levels.

Oxygen transport and delivery

Once oxygen crosses into the blood, almost all of it is carried by hemoglobin inside red blood cells as oxyhemoglobin; only a tiny fraction dissolves in plasma. A pulse oximeter estimates the percentage of hemoglobin saturated with oxygen (SpO₂) by shining light through a fingertip or other tissue. It is fast, continuous, and noninvasive — but not a complete oxygenation assessment: it says nothing about how much oxygen is delivered to tissues (which depends on hemoglobin amount, cardiac output, and perfusion), and readings can be distorted by poor circulation, motion, nail polish, and ambient light. SpO₂ targets are individualized and order-driven — there is no universal normal — so nurses titrate and interpret against the plan of care and the clinical picture. Verify current targets and protocols against facility policy and provider orders. Skin color is a poor guide: cyanosis (bluish discoloration) is a late, unreliable sign of inadequate oxygenation — especially in people with darker skin — and should never be the basis for deciding whether a person is adequately oxygenated.

What the nurse assesses

Respiratory assessment combines what you see, hear, and measure: rate, rhythm, depth, and effort; whether the person can speak in full sentences; use of accessory muscles or nasal flaring; posture (some people lean forward — "tripod" — to ease breathing); and breath sounds. Normal breath sounds are quiet and clear. Adventitious sounds include wheezes (whistling, often from narrowed airways), crackles (popping, often from fluid or collapsed airway reopening), rhonchi (lower-pitched sounds often associated with secretions), and stridor (a harsh inspiratory sound that can signal upper-airway narrowing — an urgent finding). Sound descriptions vary somewhat by source and require supervised practice; auscultation findings are always interpreted in the full clinical context.

When oxygenation fails

Three terms describe the main failure modes, and they are not interchangeable: hypoxemia is low oxygen in the blood; hypoxia is low oxygen at the tissue level (tissues can be hypoxic even when blood oxygen looks acceptable if perfusion or hemoglobin is inadequate); hypercapnia is elevated carbon dioxide in the blood, typically from ventilation that cannot keep pace — slow or shallow breathing. Slow, shallow breathing (hypoventilation) raises carbon dioxide; rapid, deep breathing (hyperventilation) blows it off. The early warning signs of inadequate oxygenation — restlessness, confusion, headache, fatigue — are subtle and nonspecific, which is why trended assessment matters more than any single vital sign.

Common Confusions