The respiratory system is formed by organs involved in breathing, including air passages and paired respiratory organs - the lungs. 

Breathing is a process that includes:

• Ventilation - exchanging air between the outer environment and the lungs;
• Respiration - exchanging gases (oxygen and carbon dioxide) between the outer environment and tissue and cells of the body. 

Before reaching the lungs, the air passes through the air passages, which include the nasal cavity, nasopharynx, larynx, trachea, and bronchi. Air in the air passages undergoes several transformations, including cleaning, warming, and humidifying.

The lungs are the most prominent organs of the respiratory system, and they are essential for respiration.

The respiratory system may also be called the respiratory tract, and it can be divided into two major parts depending on their position in the body. The two parts are called the upper and lower respiratory tracts.

The upper respiratory tract includes the following structures:

• Nose and nasal cavity
• Paranasal sinuses
• Oral cavity
• Pharynx
• Larynx (above the vocal cords)

The lower respiratory tract consists of the following organs:

• Larynx (below the vocal cords)
• Trachea
• Bronchi
• Lungs

The lower respiratory tract terminates with the lungs, which are covered by pleura - a thin and serous double-layered membrane that lines the thoracic cavity and envelops each lung.

The primary respiratory muscle in the human body is the diaphragm, which is situated below the lungs, separating the thoracic cavity from the abdominal cavity.