The respiratory system is the network of organs and tissues that brings oxygen into your body and removes carbon dioxide. It stretches from your nose and mouth all the way down to tiny air sacs deep inside your lungs, where oxygen passes into your bloodstream and carbon dioxide passes out. A healthy adult at rest takes 12 to 18 breaths per minute, and each breath moves through a surprisingly complex series of structures designed to warm, filter, and transport air.
Upper and Lower Airways
Your respiratory system is split into two sections. The upper airways include your nose, mouth, throat (pharynx), and voice box (larynx). These structures warm and humidify incoming air before it travels deeper. Short hairs lining the inside of your nostrils are coated in mucus, which traps larger particles like dust and pollen before they reach your lungs.
Below the larynx, the lower airways begin with the trachea, or windpipe. The trachea branches into two large tubes called bronchi, one leading to each lung. Those bronchi divide again and again into progressively smaller tubes called bronchioles, forming a branching structure sometimes called the bronchial tree. At the very tips of the smallest bronchioles sit clusters of tiny air sacs called alveoli. This is where the real work of breathing happens.
How Gas Exchange Works
Your lungs contain millions of alveoli, and each one is wrapped in a web of extremely small blood vessels called capillaries. The walls of the alveoli and the capillaries share a membrane so thin that oxygen and carbon dioxide can move freely across it. When you inhale, oxygen fills the alveoli, crosses that shared membrane, and attaches to red blood cells. Those red blood cells carry the oxygen back to your heart, which pumps it out to the rest of your body.
At the same time, carbon dioxide (a waste product your cells generate as they use oxygen and nutrients) travels in the opposite direction. It moves from the blood into the alveoli and leaves your body the next time you exhale. This two-way exchange happens with every single breath.
The Mechanics of Breathing
Breathing depends on pressure changes inside your chest, driven primarily by two muscle groups. When you inhale, your diaphragm, a dome-shaped muscle below your lungs, contracts and moves downward. This increases the space inside your chest cavity, and your lungs expand to fill it. The muscles between your ribs help by contracting to pull your rib cage upward and outward at the same time.
Exhaling is mostly passive. Your diaphragm and rib muscles relax, the chest cavity shrinks, and your lungs deflate, pushing air out the way air releases from a balloon. During exercise or heavy breathing, your abdominal muscles can contract to force air out more quickly, but at rest, exhalation requires almost no effort.
Healthy adult lungs can hold about 6 liters of air at maximum capacity. During a normal, relaxed breath you only use a fraction of that. Your brain constantly adjusts how fast and deep you breathe based on how much oxygen your body needs and how much carbon dioxide it needs to get rid of.
Built-In Defense System
Every breath pulls in more than just oxygen. Bacteria, viruses, fungal spores, and tiny particles ride along with the air. Your respiratory system has a layered defense system to deal with them.
The most important layer is the mucociliary escalator. The airways from the trachea down to the bronchioles are lined with tiny, finger-like projections called cilia, all coated in a layer of mucus. Mucus is a gel made of sticky proteins, defense proteins, salt, and water. It traps inhaled particles and pathogens. The cilia then beat in coordinated waves, pushing that mucus upward and out of the lungs like a slow-moving escalator. Once the mucus reaches the throat, you either cough it up or swallow it without noticing.
This system has a blind spot: the alveoli don’t have cilia. Very small particles that make it all the way to the air sacs have to be dealt with by immune cells stationed there instead.
Regulating Blood pH
Beyond delivering oxygen, the respiratory system plays a critical role in keeping your blood at the right pH level. Carbon dioxide is mildly acidic. As it accumulates in the blood, acidity rises. By controlling how much carbon dioxide you exhale, your lungs act as a minute-by-minute pH regulator.
If your blood becomes too acidic, your brain signals you to breathe faster and deeper, expelling more carbon dioxide and bringing pH back toward normal. If your blood becomes too alkaline, your breathing rate slows, allowing carbon dioxide to build up slightly and lower pH. This feedback loop between the brain and lungs adjusts constantly, often without you noticing any change in your breathing.
Oxygen Levels and How They’re Measured
A pulse oximeter, the small clip-on device often placed on your fingertip, measures how much of the oxygen-carrying protein in your red blood cells is saturated with oxygen. For most people, a normal reading falls between 95% and 100%. Readings consistently below 95% can signal that the lungs aren’t transferring oxygen effectively, which may point to conditions like pneumonia, COPD, or other lung diseases.
Common Respiratory Diseases
Lung diseases generally fall into two broad categories based on how they affect airflow.
Obstructive Lung Diseases
These conditions narrow or block the airways, making it hard to exhale fully. Air gets trapped inside the lungs, leading to shortness of breath and reduced oxygen exchange. The most familiar examples include asthma, where irritants or allergens cause the airways to swell and tighten, and emphysema, where the air sacs lose their elasticity and become permanently overinflated. Chronic bronchitis causes recurring airway inflammation and excess mucus production. Cystic fibrosis, an inherited condition, fills the lungs with thick, sticky mucus that’s difficult to clear. These conditions are collectively referred to as chronic obstructive pulmonary disease, or COPD.
Restrictive Lung Diseases
Instead of blocking airflow, restrictive diseases prevent the lungs from expanding fully. Scarring, inflammation, or thickening of lung tissue makes the lungs stiff, so they can’t take in a full breath. Some autoimmune conditions, like scleroderma, can cause this kind of scarring. Physical factors outside the lungs can also be restrictive: obesity and scoliosis, for instance, can compress the chest cavity enough to limit how far the lungs can stretch.
Both categories reduce the lungs’ ability to move oxygen into the blood and carbon dioxide out. The symptoms often overlap (shortness of breath, fatigue, reduced exercise tolerance), but the underlying mechanics differ, which determines how each type is managed.