Human Physiology ❮ 179Blood is able to flow so efficiently because it contains primarily water. The liquid por-
tion of the blood, the plasma,contains minerals, hormones, antibodies, and nutritional
materials. Another common component seen in the bloodstream is the platelet,which is
involved in the clotting of blood. You might ask, “What are the white cells flowing
around?” The white blood cells are the protection system for our body. We will be seeing
those up close when we talk about the immune system.
Thelymphatic systemis worth a brief mention here because it is an important part of
the circulatory system. When blood flows through the capillaries of the body, proteins and
fluid leak out during the exchange. The lymphatic system functions as the route by which
these poor lost souls find their way back into the bloodstream. The lymphatic system also
functions as a protector for the body because of the presence of structures known as lymph
nodes,which are full of white blood cells that live to fight infection. If your neck some-
times swells when you are sick with the flu, for instance, it is probably the multiplication
of white blood cells in the lymph nodes of your neck.Diseases of the Cardiovascular System
Two diseases that you should be familiar with for the exam are hypertensionandarterio-
sclerosis. Hypertensionis high blood pressure and is a major cause of strokes and heart
attacks.Arteriosclerosisis a big word that means hardening of the arteries. These hardened
arteries become narrower and are a prime risk factor for death by embolism—the breaking
off of a piece of tissue that lodges in an artery, blocking the flow of blood to vital tissues.Respiratory System
We are going to head down to the lungs now. Please stay close because it will get a little
loud in these windy tunnels. Air comes into the body through the mouth and the nose. We
are currently in the nasal passages, and along with the air that came into the nose, we are
being warmed and moistened in the nasal cavitybefore we head down toward the pharynx
region, where the air and food passages cross. We will come back to this area again later on
in the tour when we take the road that food uses to get from the mouth to the stomach.
During inhalation, the air goes through a structure called the glottisinto the larynx(human
voicebox). From there, the air moves into the trachea,which contains rings of cartilage that
help it maintain its shape. Each trachea is the tunnel that leads the air into the thoracic
cavity.If you look outside your windows, you will notice some tiny arms waving at us as
we go by. They are the cilia,which beat in rhythmical waves to carry foreign particles (like
our tour bus) and mucus away from the respiratory tract.
We are now at a fork in the road. Here the trachea divides into two separate tunnels:
the two bronchi,which are also held open by cartilage rings, one going to the left lung, and
one going to the right lung. Each bronchus divides into smaller branches, which divide into
even smaller branches, which divide into tunnels called bronchioles.These bronchioles
branch repeatedly until they conclude as tiny air pockets containing alveoli.
In Figure 15.2, notice how thin the walls of the alveoli are. They are usually a single
cell in thickness, are covered by a thin film of water, and are surrounded by a dense bed of
capillaries. You might have questioned earlier exactly where the exchange of O 2 and CO 2
actually occurs—this is the place. The alveoli are considered to be the primary functional
unit of the lung. Oxygen enters the alveolus the same way we just did—it dissolves in the
water lining of the wall and diffuses across the cells into the bloodstream. At the same time,
the CO 2 , which is carried by the blood, primarily in the form of bicarbonate (HCO 3 −),
passes out of the blood in a similar manner. The O 2 moves easily into the bloodstreamBIG IDEA 2.A.3
The increased
surface area of
alveoli helps in gas
exchange with the
environment.
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