The Respiratory System: Oxygen In, Carbon Dioxide Out

1) Respiratory Tract:

Passages through which oxygen enters the lungs and carbon dioxide is expelled.

• Nasal 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, which is part of both systems.
• Larynx: Made of cartilage. The vocal cords vibrate and enable humans 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 from the trachea into each lung. They divide into small tubes called bronchioles, rounded sacs with very thin walls called pulmonary alveoli.

2) Lungs:

Spongy organs protected by the ribs. The left lung is smaller. Surrounded by pleura, which 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 end in pulmonary alveoli, which are grouped in alveolar sacs that give the lungs their spongy appearance. The pulmonary arteries carry deoxygenated blood to the lungs. The pulmonary veins carry oxygenated blood.