The respiratory system is situated in the thorax, and is responsible for gaseous exchange between the circulatory system and the outside world. Air is taken in via the upper airways (the nasal cavity, pharynx and larynx) through the lower airways (trachea, primary bronchi and bronchial tree) and into the small bronchioles and alveoli within the lung tissue.
Move the pointer over the coloured regions of the diagram; the names will appear at the bottom of the screen)

The lungs are divided into lobes; The left lung is composed of the upper lobe, the lower lobe and the lingula (a small remnant next to the apex of the heart), the right lung is composed of the upper, the middle and the lower lobes.

To take a breath in, the external intercostal muscles contract, moving the ribcage up and out. The diaphragm moves down at the same time, creating negative pressure within the thorax. The lungs are held to the thoracic wall by the pleural membranes, and so expand outwards as well. This creates negative pressure within the lungs, and so air rushes in through the upper and lower airways.

Expiration is mainly due to the natural elasticity of the lungs, which tend to collapse if they are not held against the thoracic wall. This is the mechanism behind lung collapse if there is air in the pleural space (pneumothorax).

Each alveolus is very closely associated with a network of capillaries containing deoxygenated blood from the pulmonary artery. The capillary and alveolar walls are very thin, allowing rapid exchange of gases by passive diffusion along concentration gradients.
CO2 moves into the alveolus as the concentration is much lower in the alveolus than in the blood, and O2 moves out of the alveolus as the continuous flow of blood through the capillaries prevents saturation of the blood with O2 and allows maximal transfer across the membrane.