Anatomy of the respiratory system
Every cell needs oxygen and must get rid of carbon dioxide. The respiratory system brings air close enough to blood for gases to swap.
Functions
The respiratory system supplies oxygen, removes carbon dioxide, helps regulate blood pH, produces sound for speech, and protects against inhaled particles and microbes. It works closely with the cardiovascular system, which carries the gases to and from tissues.
The upper respiratory tract
Air enters through the nose, where hairs and mucus trap particles and the nasal cavity warms and humidifies it. It passes through the pharynx (throat), shared with the digestive tract, and the larynx (voice box), which holds the vocal cords. The epiglottis flaps over the larynx during swallowing so food goes down the oesophagus instead.
The lower respiratory tract
Below the larynx, the trachea (windpipe) is held open by C-shaped rings of cartilage. It splits into left and right primary bronchi, which branch about 23 times into smaller bronchi and bronchioles, forming the 'respiratory tree'. The right bronchus is wider and more vertical, so inhaled objects more often lodge there.
The mucociliary escalator
The airways are lined with ciliated epithelium and mucus-secreting goblet cells. Mucus traps dust and microbes; cilia beat to sweep it up toward the throat, where it is swallowed or coughed out. Smoking paralyses and destroys cilia, which is one reason smokers develop a chronic cough.
Alveoli
Bronchioles end in clusters of alveoli, tiny air sacs. The two lungs contain roughly 300-500 million alveoli with a total surface area of about 70 square metres, the size of a badminton court. Alveolar walls are a single layer of thin cells wrapped in capillaries, so the barrier between air and blood is less than a micrometre thick.
Surfactant
Some alveolar cells secrete surfactant, a detergent-like fluid that lowers surface tension so alveoli do not collapse when you breathe out. Babies born very prematurely may lack enough surfactant and struggle to breathe; treatment with artificial surfactant has saved many lives.
The lungs and pleura
The right lung has three lobes; the left has two, leaving room for the heart. Each lung is wrapped in a double pleural membrane with a thin fluid film between the layers. This fluid makes the lungs cling to the chest wall, so they expand when the chest does. Air entering this space (pneumothorax) can collapse a lung.
Worked example
A child inhales a peanut. Because the right primary bronchus is wider and more vertical than the left, the peanut is more likely to lodge on the right side, blocking airflow to part of the right lung. Doctors remove it with a bronchoscope passed down the airway.
Key takeaways
Air passes through nose, pharynx, larynx, trachea, bronchi, and bronchioles to the alveoli. The epiglottis protects the airway during swallowing. Mucus and cilia clean the airways. Alveoli provide a huge, thin surface for gas exchange, kept open by surfactant. The right lung has three lobes, the left two, and pleural fluid links lungs to chest wall.
Flashcard
Throat
Check your understanding
Why do premature babies sometimes struggle to breathe?