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The Human Respiratory System Explained

Every cell in your body needs oxygen to carry out aerobic respiration and must dispose of the carbon dioxide that respiration produces. The respiratory system is the machinery that achieves this exchange: a branching network of airways that draws air deep into the lungs, where oxygen crosses into the blood and carbon dioxide crosses out, breath by breath, 15 times per minute throughout your life.

The Airways: From Nose to Alveoli

Air enters through the nose (or mouth) and passes through the following structures in sequence:

• Nasal cavity: Lined with mucus-secreting cells and fine hairs (cilia), the nasal cavity warms, humidifies, and filters inhaled air, trapping dust and pathogens in mucus. The sticky mucus is swept by cilia toward the throat, where it is swallowed.
• Pharynx and larynx: The pharynx (throat) is a shared passage for air and food. The larynx (voice box) sits below the pharynx; the epiglottis, a flap of cartilage, covers the larynx during swallowing to prevent food from entering the airway.
• Trachea: The windpipe is a tube about 12 cm long, kept open by C-shaped rings of cartilage. Its lining of ciliated mucus-secreting (goblet) cells traps particles and moves them upward in the “mucociliary escalator.”
• Bronchi (singular: bronchus): At the carina, the trachea divides into the left and right primary bronchi, one entering each lung. These divide further into secondary bronchi (one per lobe) and tertiary bronchi. Cartilage rings are present throughout.
• Bronchioles: Smaller branches (less than 1 mm in diameter) that lack cartilage but contain smooth muscle. Contraction of this muscle narrows the bronchioles (bronchoconstriction), which occurs during an asthma attack and reduces airflow.
• Alveoli (singular: alveolus): Tiny air sacs at the ends of the bronchioles where gas exchange actually occurs. An adult lung contains approximately 300–500 million alveoli, giving a total gas-exchange surface area of about 70 m² — roughly the size of a badminton court.

The Lungs and Pleural Membranes

The two lungs sit in the thoracic cavity, separated by the mediastinum (which contains the heart). The right lung has three lobes; the left has two (to accommodate the heart). Each lung is enclosed by two layers of pleural membrane: the inner (visceral) pleura adheres to the lung surface and the outer (parietal) pleura lines the rib cage. Between them is the pleural cavity, filled with a thin layer of pleural fluid that reduces friction during breathing and, crucially, maintains the lungs in an inflated state by surface tension. If air enters the pleural cavity (pneumothorax), this tension is lost and the lung collapses.

The Mechanics of Breathing

Breathing relies on pressure differences created by changing the volume of the thoracic cavity. The diaphragm (a dome-shaped sheet of muscle below the lungs) and the intercostal muscles (between the ribs) do this work.

Inhalation (inspiration): The diaphragm contracts and flattens; the external intercostal muscles contract and pull the rib cage upward and outward. Thoracic volume increases, pressure inside the lungs drops below atmospheric pressure, and air flows in.

Exhalation (expiration): At rest, exhalation is passive. The diaphragm and external intercostals relax; the elastic recoil of the lung tissue reduces thoracic volume, raises pressure above atmospheric, and air flows out. During exercise, the internal intercostal muscles and abdominal muscles contract to force faster, deeper exhalation.

Gas Exchange in the Alveoli

Alveoli are exquisitely adapted for rapid, efficient gas exchange. Key adaptations include:

• Large surface area: Hundreds of millions of alveoli produce the vast surface area noted above.
• Thin walls: The alveolar epithelium is one cell thick (type I pneumocytes), and the capillary endothelium is also one cell thick, giving a combined diffusion distance of less than 0.5 µm.
• Rich blood supply: Each alveolus is surrounded by a dense network of pulmonary capillaries, maintaining steep concentration gradients.
• Moist lining: Gases diffuse across a thin film of water coating the alveolar surface. Surfactant (secreted by type II pneumocytes) reduces surface tension and prevents alveolar collapse.

Oxygen diffuses from the alveolar air (where its partial pressure is high, about 13.3 kPa) into the blood in the capillaries (where it arrives from the body with a much lower partial pressure of about 5.3 kPa). Carbon dioxide diffuses in the opposite direction, from capillary blood (high CO₂) into alveolar air (lower CO₂). Expired air is then removed during exhalation.

Oxygen Transport in the Blood

Only a tiny fraction of oxygen dissolves directly in plasma. The vast majority (about 98%) binds to haemoglobin inside red blood cells to form oxyhaemoglobin. Haemoglobin is a protein with four subunits, each containing an iron-containing haem group that can bind one oxygen molecule. The relationship between oxygen partial pressure and haemoglobin saturation is described by the oxygen-haemoglobin dissociation curve, an S-shaped (sigmoid) curve. In the lungs, where oxygen is plentiful, haemoglobin loads up; in respiring tissues, where oxygen is scarce and CO₂ is high, haemoglobin unloads its oxygen. This automatic response to local conditions is one of the elegant features of biological oxygen transport.

Summary

The respiratory system channels air through progressively narrower airways — trachea, bronchi, bronchioles — to the alveoli, where oxygen and carbon dioxide exchange across a thin, moist, richly vascularised surface. Breathing is driven by muscle contraction that changes thoracic volume and air pressure. Oxygen is transported in the blood mainly bound to haemoglobin in red blood cells and delivered to tissues for use in aerobic respiration. Together, the respiratory and circulatory systems maintain the continuous supply of oxygen every cell in the body needs to survive.