Ever wondered why you can sprint up stairs and then feel out of breath a few minutes later? That quick switch of air is the drama of breathing and gas exchange at work.
Breathing is just the body’s way of swapping stale air for fresh air. In simple terms, we pull in oxygen (O₂) and push out carbon dioxide (CO₂) – the waste gas our cells make.
What is Breathing?
Breathing, also called ventilation, is the mechanical movement of air in and out of the lungs. Think of it like a bellows that a blacksmith uses to blow air into a fire – the lungs expand (inhalation) and contract (exhalation) to move air.
Inhalation – pulling fresh air
When you inhale, the diaphragm (a dome‑shaped muscle under the lungs) contracts and moves down, while the rib cage lifts up. This creates a larger space inside the chest cavity, lowering the air pressure inside the lungs compared to the outside world. Air rushes in to equalise the pressure, just like water flows into a bigger bucket when the level drops.
Exhalation – pushing out waste
During exhalation the diaphragm relaxes and moves up, the ribs drop, and the chest cavity gets smaller. Pressure inside the lungs becomes higher than outside, so air is forced out, carrying carbon dioxide (CO₂) with it.
How Does Gas Exchange Happen?
The real magic happens in tiny balloon‑like structures called alveoli (singular: alveolus). Each alveolus is wrapped in a thin network of capillaries – the smallest blood vessels. The walls of both alveoli and capillaries are only one cell thick, making it easy for gases to slip through.
Diffusion – the key word here – means molecules move from an area where they are crowded (high concentration) to where they are sparse (low concentration). It’s like dropping a sugar cube in a cup of tea; the sugar spreads out until the sweetness is the same everywhere.
Oxygen’s journey
Air in the alveoli contains about 21% O₂. Blood arriving in the lung capillaries has already given up most of its O₂ and is rich in CO₂, so its O₂ level is low. Because O₂ concentration is higher in the alveoli than in the blood, O₂ diffuses across the thin walls into the blood. Once inside, O₂ binds to a protein called haemoglobin inside red blood cells, ready to travel to tissues.
Carbon dioxide’s exit
In the opposite direction, CO₂ concentration is higher in the blood (it’s a waste product of cellular metabolism) than in the alveolar air. So CO₂ diffuses out of the blood, into the alveoli, and is later expelled when you exhale.
Why partial pressure matters
Scientists often talk about partial pressure – the pressure contributed by a single gas in a mixture. The greater the partial pressure difference, the faster the diffusion. In the lungs, the partial pressure of O₂ is about 100 mmHg, while in the blood it’s about 40 mmHg, creating a strong pull for O₂ into the blood.
Step‑by‑step Flow of Breathing and Gas Exchange
Quick Comparison: O₂ vs CO₂
| Gas | Direction of movement | Why it moves |
|---|---|---|
| Oxygen (O₂) | Alveoli → Blood | Higher partial pressure in alveoli |
| Carbon dioxide (CO₂) | Blood → Alveoli | Higher partial pressure in blood |
Key Points to Remember
- The diaphragm and intercostal muscles drive the mechanical part of breathing.
- Alveoli provide a huge surface area (about 70 m²) for gas exchange.
- Diffusion is driven by concentration (or partial pressure) gradients.
- Haemoglobin carries most of the O₂ in the blood; about 98% of CO₂ is transported as bicarbonate ions.
- Any blockage of alveoli (like pneumonia) or damage to the diaphragm (like in spinal injuries) can seriously impair gas exchange.
📝 Likely Exam Questions
- Explain how the diaphragm helps in inhalation. The diaphragm contracts and moves downward, increasing thoracic volume and lowering intra‑lung pressure, causing air to flow in.
- Why does oxygen diffuse from alveoli into blood but carbon dioxide moves the opposite way? Because O₂ has a higher partial pressure in alveolar air than in blood, while CO₂ has a higher partial pressure in blood than in alveolar air, creating opposite concentration gradients.
- Describe the role of haemoglobin in gas exchange. Haemoglobin binds to O₂ in the lungs, forming oxyhaemoglobin, and releases O₂ in tissues; it also helps transport CO₂ back to the lungs as carbamino‑haemoglobin.
- List two factors that can reduce the efficiency of gas exchange. (i) Decreased alveolar surface area (e.g., emphysema) and (ii) Thickened alveolar‑capillary membrane (e.g., pulmonary fibrosis).
- Draw and label a simple diagram showing the pathway of air from the nose to the alveoli. (Answer expects a sketch of nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli.)