_Demonstration._--Comparison of lungs of frog with those of
bird or mammal.
_Experiment._--The changes of blood within the lungs.
_Experiment._--Changes taking place in air in the lungs.
_Experiment._--The use of the ribs in respiration.
_Demonstration experiment._--What causes the filling of air
sacs of the lungs?
_Demonstration experiment._--What are the best methods of
ventilating a room?
_Demonstration._--Best methods of dusting and cleaning.
_Demonstration._--Beef or sheep's kidney to show areas.
Necessity for Respiration.--We have seen that plants and animals need
oxygen in order that the life processes may go on. Food is oxidized to
release energy, just as coal is burned to give heat to run an engine. As a
draft of air is required to make fire under the boiler, so, in the human
body, oxygen must be given so that food in tissues may be oxidized to
release energy used in work. This oxidation takes place in the cells of the
body, be they part of a muscle, a gland, or the brain. _Blood, in its
circulation to all parts of the body, is the medium which conveys the
oxygen to that place in the body where it will be used._
[Illustration: Air passages in the human lungs. _a_, larynx; _b_, trachea
(or windpipe); _c_, _d_, bronchi; _e_, bronchial tubes; _f_, cluster of air
cells.]
The Organs of Respiration in Man.--We have alluded to the fact that the
lungs are the organs which give oxygen to the blood and take from it carbon
dioxide. The course of the air passing to the lungs in man is much the same
as in the frog. Air passes through the nose, and into the windpipe. This
cartilaginous tube, the top of which may easily be felt as the Adam's apple
of the throat, divides into two _bronchi_. The bronchi within the lungs
break up into a great number of smaller tubes, the _bronchial tubes_, which
divide somewhat like the small branches of a tree. The bronchial tubes,
indeed all the air passages, are lined with ciliated cells. The cilia of
these cells are constantly in motion, beating with a quick stroke toward
the outer end of the tube, that is, toward the mouth. Hence any foreign
material will be raised from the throat first by the action of the cilia
and then by coughing or "clearing the throat." The bronchi end in very
minute air sacs, little pouches having elastic walls, into which air is
taken when we inspire, or take a deep breath. In the walls of these pouches
are numerous capillaries, the ends of arteries which pass from the heart
into the lung. _It is through the very thin walls of the air sacs that an
interchange of gases takes place which results in the blood giving up part
of its load of carbon dioxide, and taking up oxygen in its place._ This
exchange appears to be aided by the presence of an enzyme in the lung
tissues. This is another example of the various kinds of work done by the
enzymes of the body.
[Illustration: Diagram to show what the blood loses and gains in one of the
air sacs of the lungs.]
Changes in the Blood within the Lungs.--Blood, after leaving the lungs, is
much brighter red than just before entering them. The change in color is
due to a taking up of oxygen by the _haemoglobin_ of the red corpuscles.
Changes taking place in blood are obviously the reverse of those which take
place in air in the lungs. Every hundred cubic centimeters of blood going
into the lungs contains 8 to 12 c.c. of oxygen, 45 to 50 c.c. of carbon
dioxide, and 1 to 2 c.c. of nitrogen. The same amount of blood passing out
of the lungs contains 20 c.c. of oxygen, 38 c.c. of carbon dioxide, and 1
to 2 c.c. of nitrogen. The water, of which about half a pint is given off
daily, is mostly lost from the blood.
Changes in Air in the Lungs.--Air is much warmer after leaving the lungs
than before it enters them. Breathe on the bulb of a thermometer to prove
this. Expired air contains a considerable amount of moisture, as may be
proved by breathing on a cold polished surface. This it has taken up in the
air sacs of the lungs. The presence of carbon dioxide in expired air may
easily be detected by the limewater test. Air such as we breathe out of
doors contains, by volume:--
Nitrogen 76.95
Oxygen 20.61
Carbon dioxide .03
Argon 1.00
Water vapor (average) 1.40