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Did you know..

.. if a person sneezed with their eyes open, their eyes would pop out?

Lungs bring oxygen into our bodies from the air we breathe, but they are also vital in the excretion of the carbon dioxide produced by the cells of the body.

Trachea: Simply the "windpipe", the trachea is the primary passage for air. It is a tube that measures about 23cm, and is lined with a mucous membrane. Within this mucous membrane are tiny hair-like structures called flagella. Should any foreign objects enter the trachea, such as dust particles, they become trapped in the mucous membrane, and the flagella push them up towards the throat. A little like at a rock concert when a band member dives into the crowd and is carried along by dozens of hands, flagella work in the same way. The foreign objects are moved up to the throat where they get swallowed, and any germs are killed by the digestive juices in the stomach. 

Air passes down the trachea, the bottom of which divides in two, forming primary bronchi. The bronchi continue to divide and subdivide into secondary and tertiary bronchi. These divisions end in tiny, thin walled, air-filled sacs called alveoli. In these alveoli is where the exchange of gases occur.

Alveoli: In the alveoli, oxygen from the air we breathe diffuses* into the capillary vessels on their outer wall where it is taken up and transported by red blood cells to all tissues.

*Diffusion: the difference in pressure of a particular gas on either side of a porous membrane will result in diffusion. That is to say, a higher level of a particular gas on one side of the membrane will, if able to, pass through to the other side where there is a lower concentration of that gas. This is why oxygen is easily absorbed from the alveoli into the capillaries, because there is a lower concentration of oxygen in the capillaries. This is also why carbon dioxide, the waste product of cells, diffuses into the alveoli from the capillaries. There is a lower concentration of carbon dioxide in the alveoli than in the capillaries.

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So, how does oxygen get into our bloodstream?

We breathe air in through our mouth or nose, where it flows down the trachea. This then divides into two hollow branches called bronchial tubes (primary bronchi), or bronchi. These further divide and subdivide into smaller tubes called bronchioles (secondary and tertiary bronchi). These eventually end in small air sacs called alveoli, which is where oxygen is diffused into the bloodstream, and where wastes, such as carbon dioxide, are passed from the bloodstream into the alveoli. What we exhale is air, carbon dioxide and water, and is breathed out and into the air. The new oxygen in the blood is then carried to the rest of the body and organs via red blood cells.

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