Human respiratory system explained: airway, lungs and gas exchange

The path of air: upper airway

Air normally enters through the nose, where folded tissues called turbinates warm and moisten it while mucus and tiny hair-like cilia trap dust and microbes. It then passes through the pharynx, the shared passage at the back of the mouth and nose, and the larynx, or voice box, whose vocal cords produce sound.

A flap of cartilage called the epiglottis guards the entrance to the windpipe: when you swallow, it closes over the airway so food and drink go down the oesophagus instead of into the lungs. Below the larynx, the trachea, a tube stiffened by cartilage rings so it stays open, carries air down into the chest, where it splits at a point called the carina into the left and right bronchi.

The lower airway and the lungs

Inside the lungs, the bronchi keep branching into smaller tubes, the bronchioles, which end in clusters of microscopic air sacs called alveoli. The alveoli are wrapped in the body's smallest blood vessels, the capillaries, and the membrane between air sac and blood is so thin that gases cross it easily.

The two lungs are not identical. The left lung has two lobes and a notch where the heart sits, while the right lung has three lobes. Healthy lungs are soft and elastic, stretching with each breath and springing back afterwards. Special cells in the alveoli make surfactant, a substance that stops the tiny sacs collapsing, while other alveolar cells act as cleaners, removing inhaled particles and waste.

How breathing works

Breathing in, or inspiration, is active work. The diaphragm, the large muscle under the lungs, and the muscles between the ribs contract, enlarging the chest cavity. That lowers the pressure inside the air sacs, so air flows in from outside. Breathing out at rest is mostly passive: the muscles relax, the elastic lungs recoil, pressure inside rises above the outside air, and air flows out.

The system also helps keep the blood's acidity in a narrow safe range, roughly pH 7.35 to 7.45. Breathing faster and deeper blows off more carbon dioxide and raises pH, while slower, shallower breathing lets carbon dioxide build up and lowers it. This is one reason breathing rate changes automatically during exercise, fever or illness.

Common disorders to know

When any part of the system is damaged, gas exchange suffers. Asthma and chronic obstructive pulmonary disease narrow or inflame the airways, pneumonia fills air sacs with fluid, and infections such as influenza and tuberculosis attack lung tissue. Lung cancer and other breathing problems can also reduce the lungs' working capacity, and severe failure of oxygen intake or carbon dioxide removal is called respiratory failure.

A healthy adult breathes nearly 25,000 times a day, and the lungs also help regulate blood pressure through enzymes in their capillaries. Keeping the airways clear of smoke and dust, and treating infections early, protects a system the body cannot do without for more than a few minutes. This article gives general educational information only, not personal medical advice.

Editorial disclosure

This article was researched and published with AI assistance. No human prepublication review is claimed. Report factual errors through our contact page.