The Respiratory System
Breathing is so automatic that most of us never think about it — yet we breathe roughly 20,000 times a day. The respiratory system is the set of organs and structures that allows us to take in oxygen and get rid of carbon dioxide, the waste product of cellular respiration.
The main organs of the respiratory system are the lungs, but they are supported by many other structures. Air enters through the nose and mouth, passes through the trachea (windpipe), and then divides into two bronchi — one for each lung. Inside the lung, bronchi branch into smaller and smaller tubes called bronchioles, which end in tiny air sacs called alveoli.
The Alveoli — Where Gas Exchange Happens
The real work of breathing happens in the alveoli. There are about 700 million alveoli in your lungs, giving them a total surface area of around 70 m² — about the size of a singles tennis court! Each alveolus is surrounded by a dense network of capillaries.
Because the alveolus wall and capillary wall are both just one cell thick, gases can pass through very easily. Oxygen moves from the air inside the alveolus into the blood in the capillary. At the same time, carbon dioxide moves from the blood into the alveolus and is breathed out. This exchange is called gas exchange and happens by a process called diffusion — gases move from areas of high concentration to areas of low concentration.
Your lungs are not equal in size. The right lung has three lobes and is slightly larger than the left lung, which has two lobes. The left lung is smaller to make room for the heart, which sits slightly on the left side of the chest.
Inhaling and Exhaling
Breathing in (inhalation) and breathing out (exhalation) are controlled by muscles. The main breathing muscle is the diaphragm, a dome-shaped sheet of muscle below the lungs. When you inhale, the diaphragm contracts and flattens, and your rib muscles pull your ribcage up and outwards. This increases the volume of your chest, reducing the air pressure inside, so air rushes in.
When you exhale, the diaphragm relaxes and rises back to its dome shape, the ribcage drops, the chest volume decreases, pressure increases, and air is pushed out. No energy is needed to breathe out normally — it happens by elastic recoil of the lungs.
Think of your lungs as balloons inside a container. When you expand the container (by contracting the diaphragm and lifting the ribcage), the pressure drops and air rushes into the "balloons." Relax the container, and air is pushed out.
Keeping the Airways Healthy
The respiratory system has several defences against germs and dust. The nose contains hairs that trap large particles. The lining of the airways produces mucus that traps smaller particles and pathogens. Tiny hair-like structures called cilia beat constantly to move the mucus — and everything trapped in it — up towards the throat, where it is swallowed.
Smoking damages cilia and produces excess mucus, leading to respiratory diseases like bronchitis and increasing the risk of lung cancer. It also reduces the surface area of the alveoli, making gas exchange less efficient.
Remember: breathing (ventilation) and cellular respiration are not the same process! Breathing moves air in and out of the lungs. Cellular respiration is a chemical reaction inside cells that releases energy from glucose. Breathing supports respiration by supplying oxygen and removing CO₂, but they are different processes.
Real world: asthma inhalers
In asthma, the airways (bronchioles) narrow due to inflammation and muscle contraction, reducing airflow to the alveoli. A reliever inhaler contains a bronchodilator drug that relaxes the smooth muscle in bronchiole walls, widening the airways and restoring normal airflow. Understanding the structure of the respiratory system — from bronchi to alveoli — is the basis of all asthma treatment.
Real world: altitude sickness
At high altitude (e.g. 5000 m), atmospheric pressure is lower, so each breath contains fewer oxygen molecules. Alveolar diffusion is driven by concentration gradient — with less O₂ in the air, the gradient is reduced and less O₂ enters the blood. Climbers experience breathlessness and headaches as a result. Acclimatisation involves the body producing more red blood cells to compensate.
Real world: swimming and breath control
Competitive swimmers must time their breaths precisely to maintain speed. Exhaling underwater removes CO₂, but the urge to breathe is actually triggered by rising CO₂ levels, not falling O₂. Swimmers train to increase their tolerance to elevated CO₂, which allows them to take fewer strokes between breaths. This is a direct application of respiratory physiology to athletic performance.
Practice Problems
What is the role of the alveoli in breathing?
Show answer
How does the diaphragm help you breathe in (inhale)?