Functional Organisation of the Respiratory System
The respiratory system supplies the body with oxygen and removes carbon dioxide. To do this, air must pass through the airways to the lungs, oxygen must move into the blood, and carbon dioxide must move from the blood back into the air to be breathed out.
Main Parts of the Respiratory System
The major components are the air passages, lungs, respiratory muscles and pulmonary blood vessels. Inspired air travels through the nose or mouth → pharynx → larynx → trachea → main bronchi → smaller bronchi → bronchioles → alveoli. The alveoli are the tiny air sacs where gas exchange occurs.
The Conducting Zone
The conducting zone extends from the nose to the terminal bronchioles. It does not normally take part in gas exchange. Instead, it conducts air and prepares it for the delicate alveoli by warming, humidifying and filtering it. Mucus traps particles, while cilia move the mucus towards the throat. This protective cleaning system is called the mucociliary escalator.
The Respiratory Zone
The respiratory zone includes the respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli. Its walls are very thin and lie closely beside pulmonary capillaries. This allows oxygen to diffuse from the alveoli into the blood and carbon dioxide to diffuse from the blood into the alveoli.
Type I alveolar cells
These thin cells form most of the gas-exchange surface.
Type II alveolar cells
These cells produce surfactant, which reduces surface tension and helps prevent alveolar collapse.
Pulmonary Circulation
Deoxygenated blood leaves the right ventricle through the pulmonary artery and reaches the capillaries around the alveoli. Here, carbon dioxide is removed and oxygen is taken up. Oxygenated blood then returns through the pulmonary veins to the left atrium, before the left ventricle pumps it to the body. Effective gas exchange depends on matching ventilation (airflow) with perfusion (blood flow).
Other Important Functions
Asthma and chronic bronchitis narrow or obstruct the conducting airways.
Pneumonia, pulmonary oedema and emphysema damage the respiratory zone or interfere with diffusion, which may cause hypoxaemia.
Organise your answer as: Main parts → Conducting zone → Respiratory zone → Alveoli → Pulmonary circulation → Functions.
The conducting zone conditions the air; the respiratory zone exchanges gases.