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-Ventilation requires pressure changes to occur in the lungs, so that air can be forced in an out of the alveoli.
-The pressure changes are brought by the movement of 2 sets of muscles.

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https://image.baidu.com

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Step 1: When we breathe out the air, the internal intercostal muscles contract and the external intercostal muscles relax.
Step 2: The ribs move downwards and inwards.
Step 3: The diaphragm relaxes.
Step 4: The volume which inside the thorax will decreases, and the pressure will decreases too.
Step 5: Elastic recoil of alveoli helps to push the air out of the lungs.

# data[].attributes.body
Step 1: When we breathe in the air, the external intercostal muscles contract, the internal intercostal muscles relax, they push the ribs upwards and outwards.
Step 2: The diaphragm contracts too.
Step 3: The volume of the thorax increases and the pressure in the thorax increases too.
Step 4: The pressure of atmosphere becomes bigger, so air moves into the lungs and they inflate.

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When the inhaled air reaches the alveolus, it contains a lot of oxygen. Each alveoli contains capillary networks, the oxygen molecules will diffuse into red blood cell, then the red blood cells will transport the oxygen to rest of the body. There are a lot of carbon dioxide in capillaries, and  it will diffuses through the capillary wall across to  the alveolar wall into the alveolus, then we will breathe out the exhaled air with carbon dioxide.

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Step 3: At the end of trachea, it separates to two bronchi.
Step 4: The air will pass through the bronchi then divides to form many bronchioles.
Step 5: In the end of bronchioles, there are many air sacs called alveoli where the gas exchange takes place.

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Step 1: We inhale the air through our nose and mouth.
Step 2: The air will goes to trachea, the epiglottis prevents the food goes in to your trachea, when people swallow the food, the epiglottis will close the opening side of trachea.