Bronchi are the tubes that carry air to your lungs. They are made of smooth muscle tissue and lined with mucus membranes. The bronchi branch off from the trachea (windpipe) and get progressively smaller as they travel to the lungs.
The smallest bronchi are called bronchioles. The bronchioles end in tiny air sacs called alveoli. The alveoli are where gas exchange (oxygen and carbon dioxide) occurs.
When you breathe in, the bronchi and bronchioles expand to allow more air to flow into the lungs. When you breathe out, they get smaller so that less air flows out.
The smooth muscle tissue in the walls of the bronchi and bronchioles can contract (get smaller) or relax (get larger). The amount of air that flows into and out of your lungs depends on how much the bronchi and bronchioles are contracted or relaxed.
The bronchi and bronchioles are lined with cilia (tiny hairs). The cilia help to move mucus and debris out of the lungs. A mucus is a sticky substance that traps dust and germs.
The bronchi and bronchioles are also lined with tiny blood vessels called capillaries. The capillaries pick up oxygen from the air in the lungs and deliver it to the body. They also remove carbon dioxide from the body and take it back to the lungs to be exhaled.
The bronchi and bronchioles are part of the respiratory system, which also includes the nose, mouth, sinuses, pharynx (throat), larynx (voice box), and lungs. The respiratory system helps you to breathe. It brings oxygen into your body and gets rid of carbon dioxide.