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Animals need to breathe to supply the cells with
oxygen
and remove the waste product
carbon dioxide
.
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The lungs are situated in the
pleural cavities
of the
thorax
.
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Gas exchange
occurs in the
alveoli
of the lungs that provide a large surface area. Here oxygen diffuses from the alveoli into the red blood cells in the capillaries that surround the alveoli. Carbon dioxide, at high concentration in the blood, diffuses into the alveoli to be breathed out.
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Inspiration
occurs when muscle contraction causes the ribs to move up and out and the diaphragm to flatten. These movements increase the volume of the pleural cavity and draw air down the respiratory system into the lungs.
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The air enters the nasal cavity and passes to the
pharynx
and
larynx
where the
epiglottis
closes the opening to the lungs during swallowing. the air passes down the trachea kept open by rings of cartilage to the
bronchi
and
bronchioles
and then to the alveoli.
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Expiration
is a passive process requiring no energy as it relies on the relaxation of the muscles and recoil of the elastic tissue of the lungs.
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The rate of breathing is determined by the concentration of carbon dioxide in the blood. As carbon dioxide makes blood acidic, the rate of breathing helps control the
acid/base balance
of the blood.
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The cells lining the respiratory passages produce mucus which traps dust particles, which are wafted into the nose by cilia.