How breathing works: diaphragm, nose and nerves

The diaphragm does the heavy lifting

The diaphragm is a thin, dome-shaped muscle that sits under your lungs and heart. When you breathe in, it tightens and flattens, moving down towards your belly. At the same time the muscles between your ribs lift the rib cage up and out. Your chest gets bigger and air flows in.

When you breathe out, the diaphragm relaxes and curves back up into its dome, and the chest gets smaller. At rest this takes no effort at all. Only during exercise, or with some lung diseases, do you need muscles to push air out.

This is why breathing exercises such as the NHS exercise for stress ask you to let the breath flow deep into your belly: the movement you feel there is the diaphragm moving down. How to practise it: belly breathing.

How much and how often

A quiet breath moves about 500 ml of air in an adult man and about 400 ml in a woman. At rest, adults usually breathe 12 to 18 times a minute. A resting rate below 12 or above 25 can be a sign of a health problem.

In research, breathing slower than 10 times a minute is called slow breathing. Every technique in the app is in that range, because each one stretches a single breath over 9 to 19 seconds.

In through the nose

Your nose prepares the air before it reaches the lungs. Nose hairs trap dust and dirt, bony folds called turbinates warm and moisten the air, and the sinuses make the mucus that keeps everything moist. This matters because cold, dry air can irritate the lungs.

The sinuses also make nitric oxide. Early research suggests it may play a role in the airways, but the studies are small and old, so it is not a reason to expect more oxygen from nose breathing. Breathing in through the nose is simply the gentler, more comfortable default, and it is how most of the techniques on this site start.

What makes you breathe

Breathing is mostly automatic. Sensors in the brainstem and near large blood vessels track carbon dioxide and oxygen in the blood, and your brain adjusts each breath to keep carbon dioxide and the acidity of the blood stable. You can take over for a while: hold your breath, sing or slow down on purpose. Breathing exercises use exactly this.

If you breathe faster or deeper than your body needs, you blow off too much carbon dioxide. This is called hyperventilation. Low carbon dioxide narrows blood vessels, including those that supply the brain, and can cause lightheadedness, tingling in the hands or around the mouth, a pounding heart and a feeling that you cannot catch your breath. Anxiety and panic attacks are the most common causes of occasional hyperventilation. More about that in the guide on breathing for anxiety and panic.

An accelerator and a brake

The autonomic nervous system runs your heart rate, digestion and breathing without you having to think about them. It has two branches that work like an accelerator and a brake:

• Sympathetic, “fight or flight”: speeds up the heart and breathing and slows digestion.
• Parasympathetic, “rest and digest”: slows the heart and breathing and helps digestion.

The main nerve of the parasympathetic branch is the vagus nerve, which makes up about three quarters of it. It helps control heart rate, blood pressure, breathing and digestion.

Your heart follows your breath

Your heart rate rises a little as you breathe in and falls as you breathe out. This normal rhythm is called respiratory sinus arrhythmia. Doctors use it as one sign of how much the vagus nerve is influencing the heart, although it is not a perfect measure.

When you breathe slowly, this rise and fall gets bigger and the heart and breath start to move in step. In heart rate variability biofeedback, the effect is usually strongest at about six breaths a minute. Later work put the average closer to 5.5, and the best pace varies a little from person to person. The app’s resonance technique uses 5.5.

A 2018 review of 15 studies linked slow breathing to higher heart rate variability and to people feeling more relaxed and alert and less anxious. The studies were small and none were randomised trials, so this is an early, consistent signal rather than proof.

What this means in practice

• Slow down. Fewer breaths a minute is what the calming techniques have in common.
• Make the breath out unhurried. The heart slows while you breathe out, which is why many techniques make the exhale the longest part.
• Breathe in through your nose when you can.
• Do not force it. If you feel dizzy or tingly, you are probably breathing more than you need. Stop and breathe normally.