Only two gases are important, oxygen and carbon dioxide. Do you remember from Cells Are Us why this is so? If not, review the unit on Energy.
Why do we not care about nitrogen, which makes up 78%
of the air? Nitrogen does not normally exchange in the sense that
we discuss oxygen and carbon dioxide. Nitrogen only becomes
important in scuba diving, where it bubbles out of the tissues if you
have been deep in the water.
|Blood carbon dioxide||decreases||increases|
Oxygen is consumed by cells in the process of making energy. It thus has to be replenished and air is the only source.
Carbon dioxide is a waste product (from the Krebs cycle) and needs to be continually removed from the tissues that generate it. If left to accumulate, carbon dioxide forms an acid (carbonic acid), which would alter the normal function of body enzymes.
For these exchanges to occur, blood must circulate between lungs and tissue, a function which is carried out by the heart and blood vessels.
|Cow heart, with part of
the wall cut away to show interior of a pump chamber (ventricle).
Note the valve at the top of the chamber. When blood fills the artery
(part of its wall is also cut away) during contraction of the ventricle,
the backpressure closes the valve.
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