Sometimes breathing is calledrespiration. But, there is much more to respiration than just
breathing. There are actually two parts to respiration. The movement of oxygen into the
body and carbon dioxide out of the body is calledexternal respiration. The exchange of
gases between the blood and the cells of the body is celledinternal respiration.
The Journey of a Breath of Air
Breathing is only part of the process of delivering oxygen to where it is needed in the body.
Gas exchangeoccurs in the alveoli by passive diffusion of gases between the alveoli and
the blood in the capillaries of the lungs. The passive diffusion of oxygen and carbon dioxide
is shown inFigure19.5.
Recall that diffusion is the movement of substances from an area of higher concentration
to an area of lower concentration. The difference between the high concentration of oxygen
(O 2 ) in the alveoli and the low O 2 concentration of the blood in the capillaries is enough to
cause O 2 molecules to diffuse across the thin walls of the alveoli and capillaries and into the
blood. Carbon dioxide (CO 2 ) moves out of the blood and into the alveoli in a similar way.
After leaving the lungs, the oxygenated blood returns to the heart to be pumped through the
aorta and around the body. The oxygenated blood travels through the aorta, to the smaller
arteries and finally to the capillaries where gas exchange occurs. The oxygen molecules move
out of the capillaries and into the body cells. While oxygen diffuses from the capillaries and
into body cells, carbon dioxide diffuses from the cells into the capillaries.
Figure 19.5: Gas exchange is the movement of oxygen into the blood and carbon dioxide out
of the blood. ( 17 )