How do we know what happens to the
air we breathe?
A simple way of following air
through are body is to look at the color of blood. When blood is
bright red it means that oxygen is bound to the
blood. When less oxygen is in the blood, the blood becomes a
darker shade of red, very close in color to maroon.
Arteries are red,
veins are blue
Most arteries carry
oxygenated blood from the heart to the rest of the body. As the oxygen
is released into the tissues, the blood slowly turns a darker shade of
red and returns to the heart in veins.
arteries will be a pink to reddish color, while veins are a blue or
Arteries are thick and muscular.
Veins are thin and flabby.
From dissection of an animal body,
you can see where oxygen is
going from the heart by following the arteries. You can see where carbon
dioxide is leaving by following the veins back to the heart (See
Story Time about how the circulation paths were discovered).
What you can NOT see (without a
microscope) is how arteries are connected to veins.
Carbon dioxide moves from high to low areas of
concentration. The same is true for oxygen.
The heart has
an artery that carries blood to the lungs to pick up oxygen, and
release carbon dioxide; this is the only artery in the body that does
not have oxygen-rich blood.
When blood from the heart reaches the lungs, the carbon dioxide leaves the blood much like the
carbon dioxide that leaves a coke bottle when it is
|A device that doctors use to measure oxygen
in the blood is an oxymeter.
An oxymeter is clipped on a finger or ear lobe and uses light beams
to determine how much oxygen is attached to the hemoglobin in the blood
cells. This reflects the amount of oxygen getting into the blood stream.
Some oxymeters also provide sensors for carbon dioxide levels to help
make sure its levels are not toxic. A more accurate measure of
testing the body's oxygen levels is to test the blood directly by taking
a small blood sample from an artery.
A pulse oxymeter
How do we know about gas exchange in body
Scientists measure oxygen and carbon dioxide in
various heart chambers and blood vessels. They notice that the big
changes in both oxygen and carbon dioxide occur in the small
capillaries, because the gas composition is very different between blood
entering a capillary bed and blood leaving it. How do the gases get from the blood to the cells in the
Why do gases
move in and out of the blood?
- As the blood gets closer to its destination, blood vessels get
smaller and thinner, until they get so small and thin that gases and other
molecules can freely move in and out of the blood stream.
- The idea is similar to
diffusion, which we learned about in the
Are Us lesson. What this means is that if there is
more oxygen in the
blood, some will move into the cells that need oxygen.
- The cell is always using up oxygen and needs more from the blood. We know this because
we can observe the color change from red to maroon of blood as oxygen is released
from the blood to the tissues.
As the cell uses
oxygen, it produces carbon dioxide. The carbon dioxide diffuses from the
cell into the blood. How do we know this?
Scientists have come up with clever ways of measuring the
concentrations of gases at these locations.
- For example, to measure the concentration of
scientists measure the acidity of the blood (carbon dioxide in blood
converts to an acid compound.)
- To measure acidity, scientists measure the acidity
of the solution with a pH meter.
- The higher the concentration of carbon dioxide,
the more acidic the blood is, and the more toxic it is for the cells.
- The enzymes, that make cells work, function best
when the cells are not acidic. The further a cell gets from its "optimal"
acidity level, the less effective its enzymes become and the more
likely the cell is to die.
Meter for measuring acidity
| You can observe this effect of acid
you exercise very hard. You may feel a bit of a "burn"
in your muscles. This "burn" is an acid build up in the
muscles due to a build up of carbon-dioxide and acid (see
"Cells Are Us" lesson on energy production).
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