Your respiratory system is how your body gets oxygen, clears out carbon dioxide, helps keep your blood pH balanced, filters the air you breathe, and even helps cool you down.

Gas Exchange

Inside your lungs are tiny air sacs called alveoli, where oxygen moves into the blood and carbon dioxide moves out. This works because gases naturally spread from where they’re more concentrated to where they’re less concentrated. There’s more oxygen in the air inside the alveoli than in the blood, so it diffuses into the capillaries. At the same time, CO₂ (which your cells produce) is more concentrated in the blood, so it moves into the alveoli to be exhaled.

Oxygen binds to hemoglobin inside red blood cells. Meanwhile, most of the CO₂ in your blood is carried as bicarbonate. When it reaches the lungs, it reacts with hydrogen ions to form CO₂ again, which is why you also breathe out water vapour.

The structure of the lungs helps this exchange happen efficiently. Each alveolus has thin walls and sits right next to a capillary, which creates a short distance for gas to travel. Because there are millions of alveoli, there’s also a huge surface area. This is exactly what Fick’s Law says you need for fast diffusion: a big surface area and a short distance.

Breathing and Blood pH

Your body uses breathing to help control the acidity of your blood. CO₂ dissolves in blood to form carbonic acid, so if you don’t breathe it out fast enough, your blood gets more acidic. That can happen if you’re breathing too slowly or shallowly (hypoventilation), like in COPD.

On the other hand, if you’re breathing too fast (hyperventilating), you get rid of CO₂ too quickly, and your blood becomes too alkaline. This can happen during anxiety or panic attacks and may lead to symptoms like dizziness or tingling.

Built-In Defences 

The air we breathe is full of dust, microbes, and other particles. Your respiratory tract is lined with mucus and tiny hairs called cilia that work together to keep unwanted stuff out of your lungs.

Goblet cells produce the mucus, which traps particles. The cilia beat rhythmically to move that mucus upward toward your throat, where you either swallow it (and destroy the contents in your stomach) or cough it out. This system is called the mucociliary escalator.

If something goes wrong, like in cystic fibrosis, where mucus is too thick to move properly, or in smokers, where cilia get damaged, it’s harder to clear out invaders, and the risk of infections goes up.

Temperature Regulation

Every time you breathe out, you lose water, and with it, heat. That’s part of why animals like dogs pant when they’re hot. And during exercise, you breathe faster not just to get more oxygen, but also to help get rid of excess heat.

Anatomy of the Respiratory System

Upper Tract

When you breathe in through your nose, the air gets warmed and moistened. The nasal cavity has mucus and cilia to trap dust, and blood vessels to warm the air.

From there, air travels through the pharynx, which is a shared passageway for food and air. The epiglottis acts like a flap to make sure food goes down the right pipe.

Next is the larynx, or voice box, where your vocal cords live. They vibrate to create sound. The pitch of your voice depends on how long and tight these cords are.

Lower Tract

The trachea, or windpipe, is a sturdy tube held open by cartilage rings. Like the upper tract, it’s lined with mucus and cilia.

It branches into two bronchi (one per lung), which split into smaller bronchioles. These bronchioles can widen or narrow, depending on your body’s needs. For example, they dilate during exercise. In asthma, they constrict too much, which makes breathing difficult.

At the end of the line are alveoli, tiny, balloon-like sacs where gas exchange happens. They’re lined with a surfactant, made by Type II alveolar cells. The surfactant prevents the alveoli from collapsing by reducing their surface tension.

How Breathing Works

Inhaling

When you breathe in, your diaphragm contracts and flattens, and the external intercostal muscles lift your ribs. This expands the chest cavity, and lowers the pressure inside your lungs so air flows in.

The lungs themselves aren’t muscles! They move because they’re stuck to the chest wall via two thin membranes (the pleura) with a small amount of fluid between them. If that seal breaks (like in a pneumothorax), the lung can collapse.

Exhaling

At rest, exhaling is passive. The diaphragm and rib muscles relax, and the lungs recoil and push air out. When you’re breathing hard, like during exercise, you recruit internal intercostal and abdominal muscles to push air out more forcefully.

How Breathing Is Controlled

Your breathing is controlled automatically by your brainstem, so you don’t have to think about it. The medulla oblongata sets the basic rhythm, like a built-in pacemaker for your lungs, while the pons helps coordinate the timing so that each breath flows smoothly into the next.

The main signal that tells your body to breathe isn’t a lack of oxygen, but the buildup of carbon dioxide. As CO₂ accumulates in your blood, it makes the blood more acidic. Your body senses this change through chemoreceptors. Central chemoreceptors in the medulla are especially sensitive to CO₂ levels in the fluid around your brain, while peripheral chemoreceptors in the carotid and aortic bodies can also detect low oxygen and shifts in blood pH.

Because CO₂ has such a strong influence on breathing, giving someone a mix of oxygen and CO₂, rather than just pure oxygen, can sometimes be more effective in triggering the urge to breathe. 

Stretch receptors in your lungs also prevent over-inflation by sending feedback to the brain when your lungs are too full.

Lung Diseases

Restrictive lung diseases make it difficult to fully expand the lungs when you breathe in. This can happen when the lung tissue itself becomes stiff or scarred, as in pulmonary fibrosis, or when something outside the lungs physically limits their expansion. For example, a curved spine in scoliosis can compress the lungs, and in cases of severe obesity, excess weight can restrict how far the diaphragm can move. In all these situations, the total lung capacity is reduced because the lungs simply can’t stretch as much as they need to.

Obstructive lung diseases, on the other hand, make it harder to get air out of the lungs. The airways may be narrowed, inflamed, or clogged, which traps air inside and makes exhalation hard to do. In asthma, the airways constrict in response to triggers like allergens or exercise. Chronic bronchitis causes long-term inflammation and mucus buildup, which narrows the breathing passages. Cystic fibrosis also leads to thick, sticky mucus that blocks the airways and makes it harder to clear out infections.

Pulmonary Function Testing

To see how well your lungs are working, doctors use pulmonary function tests, which give measurements of how much air you can move in and out of your lungs, and how efficiently you’re doing it.

One basic measure is tidal volume, which is the amount of air you breathe in or out during a normal, relaxed breath. When you take a deep breath in and then exhale as much as you can, the total amount of air you push out is called your vital capacity. Even after that full exhale, though, there’s still some air left in your lungs. This remaining air is known as residual volume, and it keeps your alveoli from collapsing completely. If you add up the vital capacity and residual volume, you get your total lung capacity, the maximum volume of air your lungs can hold.

In restrictive diseases, since lungs can’t expand fully, both vital capacity and total lung capacity are reduced. In obstructive diseases, since the lungs can’t push air out easily, they lead to a higher residual volume and even an increased total lung capacity, but the extra air isn’t helpful, since gas exchange becomes less efficient.