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BRONCHIOLES AND ALVEOLI

The narrowest bronchioles end in 300 million air-filled bags called alveoli that fill most of the lungs and are surrounded by blood capillaries. Oxygen passes through the wall of each alveolus into the bloodstream in exchange for carbon dioxide, which moves in the opposite direction. The alveoli provide a large surface across which this exchange can take place efficiently.

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BRONCHIAL TREE

Once inside a lung, each bronchus divides into smaller bronchi that then spilt even further. These, in turn, divide repeatedly to form smaller branches called bronchioles. This arrangement is often called “the bronchial tree” because its structure looks like an upside-down tree with the trachea as the trunk, bronchi as branches, and bronchioles as twigs.

Branches of^71

the smallest bronchioles

reach the

deepest parts

of the lung

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TRACHEA

Also called the windpipe, this flexible tube carries air between the larynx (voice box)

at the base of the throat and the two

bronchi that arise at its lower end. Up

to 20 C-shaped rings of cartilage that

encircle the trachea hold it open when

you breathe in. Mucus lining the

trachea cleans the incoming air

by trapping dirt and germs, a process that began in the nasal cavity.

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DIAPHRAGM

Situated just below the lungs, the diaphragm plays a key role in breathing. When breathing in, the diaphragm contracts and flattens as muscles pull the ribs upwards and outwards. This increases the space in the chest so that air is sucked into the lungs. When breathing out the relaxed diaphragm is pushed upwards, and the ribs move downwards and inwards, squeezing air from the lungs.

The diaphragm is a domed sheet of muscle that separates chest from abdomen

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