The respiratory system involves the upper and lower respiratory tract, intercostal muscles and the diaphragm, a muscle at the bottom of the rib cage separating the thoracic and abdominal cavities. Breathing is controlled by a portion of the brain called the medulla oblongata.

The respiratory system is responsible for the process of respiration, which is a gas exchange of oxygen for carbon dioxide. We must breathe to stay alive. We breathe in oxygen from the air. It gets into the lungs and from the lungs into the bloodstream. The blood then takes it to the tissue cells that need it for metabolism.

When we breathe in, the thorax enlarges, the ribs move up and out and the diaphragm pushes down. Negative pressure is created and the air is sucked into the lungs. This is called inhalation or inspiration. When we breathe out, the diaphragm relaxes, ribs move back and the air is pushed out of the lungs. This process is called exhalation or expiration.

We divide the respiratory tract into the uper and lower one. The upper respiratory tract is made up of the nose, nasal cavities, sinuses, pharynx and larynx. Air gets into the body through the nose and mouth. In the nasal cavities it is warmed and filtered from the dust by cilia. Sinuses contribute to the quality of our voice, as they serve as resonating chambers. The air passes from the nasal cavities into the pharynx.

The pharynx is a common passageway for food and air. When we swallow, a structure called the epiglottis closes off the larynx. It prevents any food from getting into the larynx. The larynx is also called the voice box. There are vocal cords producing sounds. The larynx is made up of cartilage rings. The biggest one is called the thyroid cartilage, also known as the Adam´s apple.

The lower respiratory tract comprises the trachea, bronchi and lungs. The trachea, also called the windpipe, is a tube connecting the larynx to the lungs. It is about 10 to 16 centimetres long and consists of cartilage rings. This tube divides into two bronchi. Each bronchus enters a lung and divides many times within the lung into smaller structures called bronchioles. Each bronchiole ends up with a microscopic air sac called alveolus. Clusters of alveoli look like bunches of grapes. The lungs are a pair organ. They are situated on either side of the heart. They look similar, but are not identical. The right lung is larger than the left one and it has three lobes, while the left one has only two lobes. The lungs are covered with a membrane called visceral pleura.