How does the respiratory system work?

Your respiratory system has one job: bring oxygen in and take carbon dioxide out, about 20,000 times a day. It is a chain of parts, and every breath passes along the whole chain.

Air enters the nose, where it is warmed, wetted and cleaned. Next to the nose are the sinuses, hollow spaces in your skull bones. The air goes down the pharynx (your throat) and through the larynx, the voice box. A flap on top, the epiglottis, keeps food out.

Then comes the trachea, or windpipe. It splits into two bronchi, and these split again and again, about 23 times, into thin bronchioles. They end in about 480 million alveoli, tiny air sacs where oxygen crosses into the blood. LungsClear zooms in on them.

The lungs cannot move by themselves. The diaphragm, a dome of muscle under them, and the intercostal muscles between your ribs do the pulling. The brainstem tells them when, as you will see in chapter 5.

Your lungs are not pumps. They are stretchy sponges stuck to the inside of your chest wall by a thin film of fluid. So when the chest gets bigger, the lungs have to get bigger too.

To breathe in, the diaphragm contracts and its dome flattens and moves down. The external intercostal muscles lift your ribs up and out, like a bucket handle. The air inside the lungs now has more room, and Boyle’s law says a gas with more room has a lower pressure (see GasLawClear). The pressure in your air sacs drops about 1 cmH₂O, only 0.1% below the air outside, and air flows in until they match.

Breathing out at rest is free. The muscles relax, the stretchy lungs spring back, and the air is squeezed out.

A quiet breath, the tidal volume, is about 0.5 L. Adults at rest take 12 to 16 breaths a minute, so about 6 to 8 litres flow in and out every minute. A spirometer draws it as a wavy line.

Your biggest breath in, then all the way out, is your vital capacity: about 4.6 L for a young adult. Even then about 1.2 L stays behind, the residual volume, so your lungs never fully collapse.

Your nose is an air conditioner. Inside it, three curled shelves called turbinates stick out from each side wall. They are covered in wet, warm lining full of blood vessels. As air swirls past them it is warmed almost to body heat and wetted to nearly 100% humidity before it reaches your lungs. Nose hairs and sticky mucus catch dust and germs.

The airways are lined with cilia, tiny hairs that beat 10 to 15 times a second in a wave. They push a carpet of mucus, with the dirt stuck in it, up to your throat, where you swallow it. This is the mucociliary escalator.

Air and food share your throat. When you swallow, your soft palate lifts to seal the nose, your larynx jumps up about 2 cm, and the epiglottis tips back like a lid. For about a second you stop breathing, and the food slides safely into the oesophagus, the food pipe (see StomachClear).

Speaking is breathing out through a gap. Two vocal folds in the larynx close and flap open hundreds of times a second as your breath pushes past them. Tighter folds vibrate faster, so the pitch goes up.

Breathing is only half the story. The oxygen still has to reach every cell, and that is the job of your blood, pumped by your heart (see HeartClear and CirculationClear).

In the lungs, oxygen slips from the air sacs into red blood cells and sticks to haemoglobin, a protein with iron at its heart. Each haemoglobin carries up to four oxygen molecules. Blood leaving the lungs is about 97% full.

The S-shaped oxygen–haemoglobin curve on the board shows the trick. At the high oxygen pressure in the lungs, haemoglobin fills up. At the low pressure in a busy muscle, it lets go. Half-full happens at about 27 mmHg, the P50. A working muscle is also warmer, more acidic and full of CO₂, and all three make haemoglobin let go even more easily. This is the Bohr effect.

Inside each cell, tiny power stations called mitochondria use the oxygen to burn glucose: glucose + oxygen → carbon dioxide + water + ATP. This is cellular respiration, and ATP is the energy every cell spends.

The carbon dioxide goes back the other way. Most of it, about 70%, travels dissolved in the blood as bicarbonate. In the lungs it turns back into CO₂ and you breathe it out.

You can hold your breath or take a deep one on purpose, but most of the time an autopilot in your brainstem runs your breathing. A group of nerve cells in the medulla and pons fires a burst of signals for every breath. They travel down the phrenic nerves to the diaphragm and the intercostal nerves to the rib muscles (see NervousClear and BrainClear).

What does the autopilot watch? Mostly carbon dioxide, not oxygen. Central chemoreceptors on the medulla sense CO₂ (through the acid it makes), and carotid bodies in your neck and aortic bodies near the heart sense low oxygen. Keeping CO₂ near 40 mmHg is the main goal.

When you exercise, your muscles make more CO₂ and you breathe deeper and faster, from about 6 to over 80 litres a minute. At high altitude, like Leh at 3,500 m, the air is thinner: each breath holds about 35% less oxygen. Your oxygen sensors kick in and over a few days your body resets, which is why climbers acclimatise slowly.

When you hold your breath, CO₂ builds up until the urge to breathe wins. That is also why you cannot hold your breath until you pass out on purpose. Yawns and hiccups are the autopilot’s odd moments: a hiccup is a sudden twitch of the diaphragm, and the “hic” is your vocal folds snapping shut.

Most of the time the system looks after itself. But because every breath passes through the same tubes, anything that narrows them or fills the air sacs is felt at once.

In asthma, the muscle around small airways squeezes and the lining swells. Airflow resistance grows with 1/r⁴, so an airway at half its width is 16 times harder to breathe through. A reliever inhaler sends medicine straight to the airways to relax that muscle. COPD is long-lasting narrowing and damaged air sacs, mostly from tobacco smoke and smoky cooking fires.

Pneumonia is an infection that fills air sacs with fluid, and it is a leading killer of young children worldwide. Tuberculosis (TB) is a germ spread by coughing. India has about a quarter of the world’s TB cases, but TB is curable, and testing and treatment are free under India’s national TB programme.

Air pollution matters too. The finest specks, PM2.5, reach deep into the lungs. The WHO guideline is a yearly average of 5 µg/m³; Delhi’s is around 100. Smoking paralyses the cilia that keep your airways clean.

These are general facts, not medical advice. A cough lasting more than two weeks, breathlessness, wheezing, chest pain or coughing blood are reasons to see a doctor. If someone collapses and is not breathing normally, call 112 or 108 and push hard and fast in the middle of their chest, 100 to 120 times a minute, until help comes: learn CPR properly on a first-aid course.