How Breathing Works: Lungs, Diaphragm and Gas Exchange

Why start with the body

Every breathing practice works with a system that already runs by itself. Knowing the basics helps you read claims about breathing more critically. This page describes standard anatomy and physiology as explained by the U.S. National Heart, Lung, and Blood Institute (NHLBI) and MedlinePlus. It is general information, not medical advice. If you have sudden or severe trouble breathing or chest pain, get emergency help (911 in the United States).

The lungs and airways

Your lungs sit on each side of the heart inside the chest cavity. The right lung has three lobes and the left has two. [1] Air enters through the nose or mouth, where it is warmed and moistened, then passes the voice box and windpipe (trachea), and then the bronchial tubes, which branch into thousands of smaller tubes called bronchioles. [1] The bronchioles end in clusters of tiny air sacs called alveoli. The lungs have about 150 million of them. [1]

Gas exchange

In the alveoli, oxygen from the air you breathe in passes through thin walls into tiny blood vessels, where red blood cells pick it up. At the same time, carbon dioxide, a waste gas carried from the body's cells, moves from the blood into the alveoli and is breathed out. [2] The cells need the oxygen to make energy, and making energy creates carbon dioxide. [2] Carbon dioxide is therefore not just waste to be removed: the brain also uses it as a signal, which is why the amount you breathe matters (see below and the guide to CO2, oxygen and blood pH).

The muscles of breathing

The lungs cannot expand on their own. Muscles in the chest and abdomen contract to create a slight vacuum around the lungs, and air flows in. [3] The main muscle is the diaphragm, a dome-shaped muscle below the lungs that separates the chest from the abdomen. [3] When you breathe in, the diaphragm contracts and moves downward, increasing the space in the chest, while the muscles between the ribs pull the rib cage up and outward. [2]

Breathing out is different. When the diaphragm and rib muscles relax, the chest space gets smaller and the lungs deflate like a balloon. This takes no effort from the body unless you have a lung disease or are being physically active. [2] During activity, the abdominal muscles help push air out, and neck and collarbone muscles help with breathing in. [3]

You will often hear "belly breathing" described as using the diaphragm. In fact the diaphragm works in every breath. What changes in "diaphragmatic breathing" is how much you notice and allow the belly to move; see diaphragmatic breathing and chest versus belly breathing myths.

How the body controls breathing

Your breathing usually does not require thought, because the autonomic (involuntary) nervous system controls it. [3] Your brain adjusts your breathing rate by sensing the body's need for oxygen and for getting rid of carbon dioxide. [4] Sensors in the brain and near blood vessels detect carbon dioxide and oxygen levels; sensors in the airways detect irritants and can trigger sneezing or coughing; and sensors in joints and muscles may raise breathing when you move. [3]

Breathing is unusual because you can also take over. NHLBI notes that at times you can control your breathing pattern, such as when you hold your breath or sing. [3] That voluntary control is what breathing practices use. But it has limits. Breathing much faster and deeper than the body needs (overbreathing, or hyperventilation) lowers carbon dioxide in the blood, and this causes many of its symptoms: feeling lightheaded, dizzy or weak, chest pain or a pounding heartbeat, and numbness and tingling in the arms or around the mouth. [5] This is the main reason gentle, comfortable breathing is the safe approach and forceful breathing is not.

The two branches of the autonomic nervous system act on breathing too: the parasympathetic system slows breathing rate and narrows the bronchial tubes, while the sympathetic system increases breathing rate and widens them. [3] What that does and does not mean for slow-breathing practices is covered in the autonomic nervous system and breathing.

When breathing is affected by illness

Damage to the nerves in the upper spinal cord, from a spinal cord injury, a stroke, or a muscle-weakening disease such as muscular dystrophy, can interfere with the diaphragm and other breathing muscles and can cause respiratory failure. [3] Problems in the muscles of the mouth and throat can narrow the airway and contribute to sleep apnea. [3] Damage, infection or inflammation of the lungs or airways can lead to conditions such as asthma, bronchitis, COPD, pneumonia and sleep apnea. [2] If you have a lung or breathing-muscle condition, talk with your clinician before starting any breathing practice; see breathing practices with lung conditions.

What this means for breathing practices

Putting it together: breathing is automatic but adjustable; the amount of carbon dioxide and oxygen in the blood is tightly monitored; and slow, comfortable breathing stays within what the system handles easily. We found nothing in the sources we read that supports breathing "harder" to bring in more oxygen in a healthy person at rest, and forceful methods carry the documented overbreathing symptoms above. For the library overview see breathing and relaxation basics.