The Respiratory System

What the Respiratory System Does

Every cell in your body needs a continuous supply of oxygen to produce energy, and every cell produces carbon dioxide as a waste product. The respiratory system is the set of organs and structures that manages this ongoing exchange: drawing fresh air in, extracting its oxygen, and expelling the carbon dioxide that cells have produced.

Breathing feels automatic because it largely is — your brainstem monitors blood chemistry and adjusts your breathing rate without you having to think about it. Yet you can also override this automatic control for a short time, choosing to breathe deeply, hold your breath, or sing a long note.

The respiratory system works in close partnership with the circulatory system. Once oxygen enters the blood at the lungs, the heart pumps that oxygenated blood to tissues throughout the body. To understand that side of the process, see the guides on heart anatomy and blood circulation. For a detailed look at the lungs themselves, see lungs explained.

The Upper Airway — Nose to Throat

Air typically enters the respiratory system through the nose, which is the preferred route for several reasons. The nasal passages are lined with tiny hairs called cilia and coated with mucus that trap dust, pollen, and microbes before they can reach the lungs. The rich blood supply in the nose also warms and humidifies incoming air, preventing the delicate lung tissue from being chilled or dried out.

The nose connects to the nasal cavity, a large air space divided by the nasal septum. Projecting from its walls are shelf-like bony ridges called turbinates (conchae), which create turbulent airflow to maximise contact between air and the warm, moist lining.

Air then passes through the pharynx (throat), a shared passage for both food and air. At the bottom of the pharynx the paths diverge: the oesophagus leads to the stomach, and the larynx leads into the airway. A small flap of tissue called the epiglottis automatically covers the larynx when you swallow, directing food away from the airway. When the system malfunctions and food or liquid "goes down the wrong way", coughing is triggered as a reflex to expel it.

The Larynx and Trachea

The larynx — commonly called the voice box — serves two purposes. It is a gateway to the lower airway, and it contains the vocal cords: two bands of elastic tissue that vibrate as air passes over them to produce sound. The pitch and volume of speech depend on the tension of the vocal cords and the rate of airflow. The prominent bulge in the neck sometimes called the "Adam's apple" is actually part of the thyroid cartilage, one of the rings of cartilage that form the larynx's framework.

Below the larynx lies the trachea, or windpipe — a tube about 10–12 cm long and roughly 2 cm in diameter. Its walls are reinforced by C-shaped rings of cartilage that keep it open even when the neck is bent or the muscles around it contract. The opening at the back of each ring (where the C is incomplete) faces the oesophagus, allowing it to bulge slightly when a large mouthful of food is swallowed.

The trachea is also lined with cilia and mucus-producing cells. Mucus traps particles, and the cilia beat rhythmically upward — a process called the mucociliary escalator — moving trapped debris and microorganisms up toward the throat, where it is swallowed or cleared. This is one of the respiratory system's key defence mechanisms.

Bronchi and Bronchioles — The Branching Airway Tree

At its lower end, the trachea splits into two tubes: the right primary bronchus and the left primary bronchus, one heading into each lung. These are the largest branches of what anatomists often call the bronchial tree, because the airways branch repeatedly like a tree, getting progressively narrower with each division.

Inside each lung, the primary bronchus divides into secondary (lobar) bronchi — one for each lobe of the lung. The right lung has three lobes; the left has two (the heart occupies some of the space on the left side). The secondary bronchi divide further into tertiary (segmental) bronchi, then into smaller bronchioles, and finally into terminal bronchioles.

The bronchi have cartilage in their walls to keep them open. As the airways get smaller, cartilage gives way to smooth muscle, which can contract or relax to widen or narrow the airways. During an asthma attack, for example, this smooth muscle goes into spasm and the lining becomes inflamed, narrowing the airways and making it harder to move air in and out.

Beyond the terminal bronchioles, the airways enter the gas-exchange zone. Here, respiratory bronchioles lead into clusters of tiny air sacs called alveoli — the end destination of every breath.

Alveoli and Gas Exchange

The alveoli (singular: alveolus) are microscopic, thin-walled air sacs clustered at the ends of the bronchiole tree. An adult has somewhere between 300 and 500 million of them. When all those tiny surfaces are unfolded and spread flat, the total surface area of the alveoli is roughly 70 square metres — about the area of one side of a tennis court — packed into just two fist-sized organs.

This enormous surface area is necessary because gas exchange must happen fast enough to keep pace with the body's demands. Wrapped around each alveolus is a dense network of pulmonary capillaries — the tiny blood vessels of the pulmonary (lung) circulation. The alveolar wall and the capillary wall together form a barrier that is less than half a micrometre thick. Gases can cross this paper-thin barrier in a fraction of a second by diffusion — the movement of molecules from an area of higher concentration to lower concentration.

Here is how the exchange works: incoming air in the alveoli has a high concentration of oxygen and a low concentration of carbon dioxide (because you have just inhaled fresh air). The blood arriving in the pulmonary capillaries has delivered its oxygen to body tissues and picked up carbon dioxide — so it has a low oxygen concentration and a high carbon dioxide concentration.

Oxygen therefore diffuses from the alveolar air into the blood. Carbon dioxide simultaneously diffuses in the opposite direction — from the blood into the alveolar air. When you breathe out, that carbon dioxide-rich air is expelled. Meanwhile the newly oxygenated blood travels back to the heart and is pumped to the body.

The alveoli are also coated with a substance called surfactant, produced by specialised cells in the alveolar lining. Surfactant reduces surface tension in the tiny air sacs, preventing them from collapsing on themselves during exhalation. Premature babies are born before their lungs produce enough surfactant, which is one reason why respiratory difficulties are a major concern in premature infants and why surfactant therapy is an important part of neonatal care.

The Mechanics of Breathing — Inhalation and Exhalation

Moving air in and out of the lungs is a mechanical process driven by changes in chest volume and pressure. The lungs themselves have no muscle — they cannot expand on their own. Instead, breathing is driven by muscles that change the size of the chest cavity, causing the lungs (which are attached to the chest wall by a thin membrane) to stretch or recoil.

Inhalation (breathing in) is an active process that requires muscular effort:

• The diaphragm — a dome-shaped muscle below the lungs — contracts and flattens downward.
• The external intercostal muscles between the ribs contract, pulling the rib cage upward and outward.
• These movements increase the volume of the chest cavity. As volume increases, pressure inside the lungs falls below atmospheric pressure.
• Air flows in through the airway to equalise the pressure — filling the lungs.

Exhalation (breathing out) is normally passive at rest:

• The diaphragm and intercostal muscles relax.
• The elastic lungs and chest wall recoil to their resting position, reducing chest volume.
• As volume decreases, pressure inside the lungs rises above atmospheric pressure, and air flows out.

During vigorous exercise or when breathing is forceful (as when playing a wind instrument), additional muscles help. The internal intercostal muscles, abdominal muscles, and other accessory muscles of breathing can all be recruited to move air more rapidly and completely.

How Breathing is Controlled

Breathing is regulated by the respiratory centre — a network of neurons in the brainstem, primarily in the medulla oblongata and pons. This centre generates a rhythmic signal that drives the breathing muscles, adjusting rate and depth automatically to match the body's needs.

The most important trigger for increasing the breathing rate is a rise in blood carbon dioxide levels (which also causes a slight drop in blood pH, making it more acidic). This is detected by chemoreceptors — sensors sensitive to chemical changes — located in the brainstem and in the walls of the aorta and carotid arteries.

Interestingly, the immediate drive to breathe comes more from carbon dioxide accumulation than from falling oxygen. You can feel this if you try to hold your breath: the growing urge to inhale is caused by rising CO₂, not by oxygen running out. In most situations, oxygen levels in the blood stay relatively high even when CO₂ is rising.

Other factors also influence breathing: physical activity increases CO₂ production and triggers deeper, faster breathing. Emotional states (anxiety can cause rapid, shallow breathing), temperature, pain, and many medical conditions can all affect the respiratory pattern. This is also why understanding the control of breathing is important in conditions such as sleep apnoea, where breathing becomes disrupted during sleep.

Lung Volumes and Capacities

Clinicians and physiologists measure different aspects of lung function to assess respiratory health. Key terms include:

• Tidal volume — the amount of air inhaled or exhaled in one normal, restful breath: roughly 500 millilitres (0.5 litres) in an average adult.
• Vital capacity — the maximum amount of air you can forcefully exhale after a maximum inhalation: typically 3 to 5 litres, varying with age, sex, height and fitness.
• Residual volume — the air that stays in the lungs even after the most complete exhalation, keeping them from collapsing completely: roughly 1.2 litres.
• Total lung capacity — the sum of all volumes; roughly 4 to 6 litres in adults.

A test called spirometry measures how much air a person can exhale and how quickly, providing important information for diagnosing and monitoring conditions such as asthma and chronic obstructive pulmonary disease (COPD). If you have any concerns about your breathing or lung health, a healthcare professional is the right person to advise you.

Respiratory Defences

The respiratory system doubles as a formidable defence against the environment. Every day the lungs are exposed to thousands of litres of air containing dust, smoke particles, pollen, bacteria, viruses, and a wide range of chemical irritants.

Multiple defence layers operate throughout the airway. The mucus-and-cilia system (the mucociliary escalator) traps and removes particles in the conducting zone. The cough and sneeze reflexes expel larger irritants rapidly. Deeper in the lung, specialised immune cells called alveolar macrophages patrol the alveolar surface, engulfing and destroying particles and microorganisms that make it past the upper defences. The lungs also produce antibodies and other immune proteins in their lining fluid.

When these defences are overwhelmed — by heavy smoking, severe infection, or certain environmental exposures — the delicate alveolar tissue can be damaged. This is why respiratory diseases are such a significant cause of illness worldwide, and why protecting lung health is a topic that respiratory medicine takes very seriously.

To build on what you have learned here, explore the lungs explained guide for a detailed look at lung structure, use the Body Systems Explorer to see how the respiratory system connects to others, or take the anatomy quiz. The lungs and breathing category has more guides on this topic.