Why Do We Need to Breathe?
Every cell in your body needs a continuous supply of oxygen to release energy from glucose in a process called cellular respiration. At the same time, cells produce carbon dioxide as a waste product that must be removed. The respiratory system is the organ system that brings oxygen into the blood and expels carbon dioxide.
Breathing (ventilation) is the mechanical process of moving air in and out of the lungs. Gas exchange is the diffusion of oxygen into the blood and carbon dioxide out.
Structure of the Respiratory System
Nasal cavity / mouth — Air enters the body, is warmed, moistened, and filtered by mucus and tiny hairs (cilia).
Trachea (windpipe) — A tube reinforced with C-shaped rings of cartilage that keep it open. Lined with ciliated cells that sweep mucus and trapped particles upward away from the lungs.
Bronchi — The trachea splits into two bronchi (singular: bronchus), one entering each lung.
Bronchioles — Bronchi branch repeatedly into smaller and smaller tubes called bronchioles, spreading throughout the lungs.
Alveoli — Tiny air sacs at the ends of the bronchioles, about 0.2 mm across. There are roughly 300–500 million alveoli in adult lungs, providing a total surface area of about 70 square metres — roughly the size of a tennis court.
The alveoli are the actual site of gas exchange. Each alveolus is surrounded by a dense network of capillaries. Oxygen diffuses from the alveolar air into the blood; carbon dioxide diffuses from the blood into the alveolar air.
How Breathing Works
Breathing uses two main muscles:
Intercostal muscles (between the ribs) and the diaphragm (a dome-shaped muscle below the lungs).
Inhaling (inspiration):
- Diaphragm contracts and flattens.
- Intercostal muscles contract, lifting the ribcage up and out.
- Chest volume increases → air pressure inside drops → air rushes in.
Exhaling (expiration):
- Diaphragm and intercostal muscles relax.
- Chest volume decreases → air pressure inside rises → air is pushed out.
At rest, a healthy adult breathes about 12–20 times per minute. During vigorous exercise this rises to 40 or more breaths per minute, to supply enough oxygen to working muscles.
Adaptations of the Alveoli
Alveoli are specially adapted for efficient gas exchange:
- Large surface area (millions of alveoli)
- Very thin walls (one cell thick) — short diffusion distance
- Moist lining — gases dissolve in moisture before diffusing
- Dense capillary network — blood is always nearby
- Good blood supply — constantly refreshes blood with low O₂ and high CO₂
Smoking and the Respiratory System
Smoking damages the respiratory system in multiple ways: tar destroys cilia (so mucus and particles build up), toxins cause cancer, and chemicals destroy alveolar walls (reducing surface area), leading to emphysema — a condition where breathing becomes extremely difficult.
Real world: Breathing rate during exercise
During vigorous exercise you breathe faster and deeper — your breathing rate can rise from 12 to over 40 breaths per minute. This is because muscles are using oxygen rapidly and producing more carbon dioxide. Receptors in the brain detect rising CO₂ levels in the blood and signal the respiratory system to speed up. This is the respiratory system in visible, real-time action.
Real world: Asthma and narrowed airways
Asthma affects over 300 million people worldwide. During an asthma attack, the smooth muscle in the walls of bronchioles contracts and the airways become inflamed and narrow, making it extremely difficult to exhale. Reliever inhalers contain bronchodilators that relax the smooth muscle, widening the airways. Understanding the bronchiole structure explains exactly how asthma inhalers work.
Real world: Smoking and emphysema
Long-term smoking destroys the walls of alveoli, merging many small sacs into fewer, larger ones. This dramatically reduces the total surface area for gas exchange. Patients with emphysema are permanently breathless even at rest because the damaged alveoli cannot deliver enough oxygen. Anti-smoking campaigns frequently show images of damaged lungs — the biology behind those images directly relates to alveolar surface area and gas exchange.
Where do you see this in real life? Watch your chest and feel your ribs as you take a deep breath in. You'll feel the ribcage expand outward and upward as the intercostal muscles contract, and the diaphragm pulling the base of your lungs down. This is the mechanics of inspiration — visible and feelable on your own body.
Practice Problems
answer="Any three of: very large surface area (300–500 million alveoli), very thin walls (one cell thick), moist inner lining, dense network of surrounding capillaries." />
Describe what happens to the diaphragm and ribcage during inhalation.