Lungs Explained
Two spongy organs with the surface area of a tennis court. How the lungs are structured and how breathing is controlled without a thought.
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What the Lungs Are
The lungs are two large, spongy organs that sit inside the ribcage on either side of the heart. Their job is to bring fresh air into close contact with blood, allowing oxygen to enter the bloodstream and carbon dioxide — a waste gas — to leave it.
Despite the enormous amount of air they process every day, the lungs are surprisingly light. Together they weigh roughly 1 to 1.3 kilograms, and their spongy texture comes from the hundreds of millions of tiny air-filled spaces packed inside them. For a full picture of the breathing process, see the respiratory system guide.
Structure of the Lungs
The two lungs are not identical. The right lung is slightly larger and is divided into three sections called lobes: the upper, middle, and lower lobes. The left lung has only two lobes — the upper and the lower — because it shares space with the heart, which sits slightly to the left of centre.
Each lung is encased in a thin, slippery double membrane called the pleura. The inner layer (visceral pleura) wraps the lung surface; the outer layer (parietal pleura) lines the chest wall. A small film of fluid between the two layers allows the lungs to slide smoothly against the chest wall during breathing without friction.
Each lung has a central entry and exit point called the hilum, where the main bronchus, blood vessels, lymph vessels, and nerves connect. From the hilum, the branching airway system fans out in all directions through the lung tissue.
Alveoli — Where Gas Exchange Happens
Alveoli (singular: alveolus) are microscopic, balloon-like air sacs clustered at the tips of the smallest airways. They have extremely thin walls — just one cell thick — and are surrounded by a dense web of tiny blood vessels called pulmonary capillaries.
This arrangement creates the ideal conditions for gas exchange by diffusion. Oxygen in the inhaled air moves across the thin alveolar membrane into the blood. Carbon dioxide in the blood simultaneously moves in the opposite direction, into the alveolar air, to be exhaled.
The alveoli are coated with a substance called surfactant. Surfactant reduces the surface tension inside these tiny sacs, stopping them from collapsing on themselves when you breathe out. Without surfactant the lungs would be extremely difficult to inflate — a serious problem seen in premature babies, whose lungs have not yet produced enough of it.
The Lungs' Blood Supply
The lungs receive blood through two separate systems. The pulmonary circulation brings oxygen-depleted blood from the right side of the heart to the lungs for gas exchange. This blood arrives via the pulmonary arteries and leaves, now oxygen-rich, via the pulmonary veins heading to the left side of the heart.
The lungs also have their own dedicated blood supply for their own tissue needs, called the bronchial circulation. Bronchial arteries branch from the aorta and deliver oxygenated blood to the airway walls, connective tissue, and pleura. This is the blood supply that keeps the lung tissue itself alive — separate from the blood that flows through for gas exchange.
This dual blood supply connects the lungs intimately with the heart. You can explore how the heart drives these circuits in the guides on heart anatomy and blood circulation.
| Feature | Right lung | Left lung |
|---|---|---|
| Number of lobes | 3 (upper, middle, lower) | 2 (upper, lower) |
| Relative size | Slightly larger | Slightly smaller |
| Why different | More room on right side | Shares space with the heart |
| Main bronchus angle | Steeper, more vertical | More horizontal |
How Breathing Moves Air In and Out
The lungs cannot inflate themselves — they have no muscle. Instead, they are stretched open or allowed to recoil by changes in the size of the chest cavity, driven by the breathing muscles.
The diaphragm is the primary muscle of breathing. It is a dome-shaped sheet of muscle that forms the floor of the chest cavity. When it contracts, it flattens downward, increasing the volume of the chest and drawing air in. When it relaxes, it springs back upward and air flows out.
The intercostal muscles between the ribs assist by lifting the rib cage outward and upward during inhalation, further increasing chest volume. Deeper or more forceful breathing can recruit additional muscles in the neck, shoulders, and abdomen.
Because the lungs are attached to the chest wall by the pleural membrane, they expand and contract with the chest — like a balloon inside a larger balloon. When the chest gets bigger, the lungs stretch; when the chest shrinks, the elastic lung tissue recoils.
How the Lungs Defend Themselves
The lungs are exposed to everything we breathe — dust, pollen, bacteria, viruses, and chemical irritants — so they have several built-in defence systems.
- Mucus and cilia: The airways are lined with cells that secrete mucus, which traps particles. Hair-like cilia beat rhythmically to sweep mucus and trapped material upward toward the throat — a process called the mucociliary escalator.
- Cough reflex: Irritation of the airway triggers a powerful cough, expelling material at high speed.
- Alveolar macrophages: Immune cells that patrol the alveolar surface, engulfing bacteria, dust particles, and debris that reach the gas-exchange zone.
- Immune proteins: The fluid lining the airways contains antibodies and other immune molecules that can neutralise pathogens.
Lung Capacity and Volumes
Not all the air in the lungs is replaced with each breath. Clinicians use specific terms to describe different volumes of air the lungs can hold:
- Tidal volume: The amount of air moved in a single normal breath at rest — roughly 500 millilitres in an average adult.
- Vital capacity: The maximum air exhaled after a maximum inhalation — typically 3 to 5 litres, depending on sex, age, and height.
- Residual volume: The air that remains in the lungs even after the most forceful exhalation — approximately 1.2 litres. This prevents the alveoli from collapsing completely.
- Total lung capacity: The sum of all volumes — roughly 4 to 6 litres in most adults.
A medical test called spirometry measures how much air a patient can exhale and how fast. It is an important tool for diagnosing and monitoring conditions such as asthma and chronic obstructive pulmonary disease (COPD). Spirometry results are always interpreted by a clinician in the context of the individual patient — if you have concerns about your lung function, ask a doctor.
Lung Health — What to Know
The lungs are remarkably resilient but are also vulnerable to a range of conditions, from common infections like pneumonia and bronchitis to chronic diseases such as asthma and COPD, to the long-term effects of smoking and air pollution. Maintaining good lung health is a topic that mainstream medical science takes seriously, and there is strong evidence that not smoking is one of the most impactful things a person can do for their respiratory system.
Regular aerobic exercise strengthens the breathing muscles and improves how efficiently the cardiovascular system uses the oxygen the lungs deliver. Activities like brisk walking, swimming, and cycling can all support respiratory muscle endurance over time. This guide is educational only — if you have concerns about your breathing or lung health, please consult a healthcare professional rather than trying to self-diagnose.
You can explore the body's interconnected systems further with the Body Systems Explorer or look up terms in the anatomy glossary. The lungs and breathing category has additional guides on related topics, including the full respiratory system.