Enables oxygen in the air to enter the body expelling carbon dioxide 

1) Respiratory tract: Passages through which oxygen enters lungs and carbon dioxide is expelled.

- Nassal passages: air enters the nose and continues through these passages, where it is moistened and warmed. Dust and microorganisms in the air are retained here.

- Pharynx: air from the nasal passages reaches the pharynx that is part of both systems.

- Larynx: made of cartilage. The vocal cords vibrate and enable human to make sounds.

- Trachea: C-shaped cartilage rings through which air travels from the larynx to the bronchi.

- Bronchi and Bronchioles: bronchi are two tubes that branch out the trachea into each lung. They divide into small tubes called bronchioles, rounded sacs with ver thin walls called pulmonary alveoli.

2) Lungs: spongy organs protected by the ribs. Left lung is smaller. Surrounded by pleura. Pleura protect the surface of the lungs from damage as they move. Inside the pleura, pleural fluid enables the lungs to move easily during breathing. The bronchi and bronchioles ends in pulmonary alveoli that are graped in alveolar sacs that gives the lungs their spongy appearance. The pulmonary arteries carry deoxygenated blood to the lungs. The pulmonary veins carry oxygenated blood.