Respiratory organs and the human respiratory system

Breathing brings oxygen from the air into the body and takes out the carbon dioxide made by the cells. Different animals do it differently:

• Sponges, coelenterates and flatworms: simple diffusion over the whole body surface.
• Earthworms: through their moist skin (cuticle).
• Insects: a network of air tubes, the tracheae.
• Most aquatic arthropods, molluscs and fishes: gills (branchial respiration).
• Land vertebrates (reptiles, birds, mammals): lungs (pulmonary respiration).
• Frogs: lungs, and also their moist skin (cutaneous respiration).

The path of air in humans. Air enters through the two external nostrils you can see, into the nasal chamber, then the pharynx (a passage shared by food and air), then the larynx, the sound box. When you swallow, a flap called the epiglottis covers the opening of the larynx (the glottis) so food does not go the wrong way. Air then goes down the windpipe, the trachea.

At about the level of the 5th thoracic vertebra, the trachea splits into the right and left primary bronchi. These branch again and again (secondary and tertiary bronchi, bronchioles), ending in very fine terminal bronchioles. Rings of cartilage (incomplete, C-shaped) keep these tubes from collapsing. Each terminal bronchiole ends in clusters of tiny, thin-walled, blood-rich air sacs: the alveoli.

The two lungs are wrapped in a double layer, the pleura, with fluid between the layers that reduces friction as the lungs move. The airways from the nostrils to the terminal bronchioles are the conducting part: they carry air, clean it, moisten it and warm it to body temperature. The alveoli and their ducts are the exchange part, where gases actually pass into and out of the blood.

The lungs sit in the chest (thoracic) chamber, which is air-tight: bounded by the backbone, breastbone (sternum), ribs, and the diaphragm below. So any change in the chest's volume changes the lungs' volume too. Respiration has five steps: breathing air in and out (ventilation), gases crossing the alveolar walls, transport in the blood, gases crossing into and out of the tissues, and cells using O₂ to release energy.