Below the trachea the airways divide again and again, like the branches of an upside-down tree. The branches get smaller until they end in millions of tiny air sacs called alveoli.
The bronchial tree
The right main bronchus is shorter, wider and more vertical than the left, so inhaled objects and deep tubes tend to go right. Each main bronchus divides into lobar and segmental bronchi, then into small bronchioles that have no cartilage.
Smooth muscle and mucus
Bronchioles are wrapped in smooth muscle that can tighten. A lining of mucus and tiny hairs called cilia traps particles and sweeps them up toward the throat.
Alveoli
Each lung has hundreds of millions of alveoli, giving a huge surface for gas exchange. Their walls are very thin and wrapped in capillaries, so oxygen and CO2 cross easily.
Surfactant
Type II cells in the alveoli make surfactant, which lowers surface tension and keeps small alveoli from collapsing at the end of each breath. Premature babies may not have enough, which causes respiratory distress.
At the bedside
In asthma, smooth muscle spasm, swelling and mucus narrow the bronchioles and cause wheezing, especially when breathing out. Bronchodilators given by inhaler or nebulizer relax that muscle.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).