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The Respiratory System Study Guide
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Introduction
Cellular respiration is a part of metabolism and is a set of chemical reactions. In this lesson on the
respiratory system, we will examine the physical process of exchanging gases between our bodies and
the atmosphere – in other words, breathing. It is important for organisms to get rid of waste gases and
to absorb helpful gases necessary for the chemical processes of metabolism.
When Is Respiration Not Breathing?
The word respiration is used in two different but related ways in biology. In one sense, respiration
means the act of bringing air into the lungs and expelling waste gases. We call this form of respiration
breathing. However, the chemical reactions of metabolism partly contain a set of reactions that use
enzymes and oxygen to burn food molecules and release the energy of their bonds. This process is
more specifically known as aerobic cellular respiration (or anaerobic when no oxygen is present).
Sometimes, the two types of respiration are differentiated by being internal (cellular respiration) and
external (the act of breathing). Here, we are talking about the process of breathing, which has the
important function of allowing for the exchange of gases.
Who Respires?
All organisms, even single-celled ones, must perform cellular respiration (either aerobically or
anaerobically) during metabolism to obtain necessary energy to support life functions. Because singlecelled animals are small, they have no need of a respiratory system to help them breathe. They can
simply exchange gases across their cell membranes. However, multicellular animals are too large to
rely only upon diffusion for gas exchange. They must also have specialized tissues to perform this
function.
The Respiratory System
Breathing involves inhalation (the taking in of air) and exhalation (the releasing of air). A series of
conducting tubes brings air into the gas exchange portion. Organisms breathe through their nose or
mouth, and air enters the upper part of the throat or the pharynx. The next point of note is the larynx or
what is called the Adam's apple on the inside of the throat. The larynx is also known as the voice box
because the cartilage here changes shape to help form the sounds of speech. Inhaled air next passes
into the trachea, which is a tube in the center of the chest. Within the chest, the trachea branches into
two bronchi. Each bronchus leads to a lung. Once inside the lung, the bronchus branches repeatedly
into small tubes called bronchioles. Bronchiole tubes then lead to a microscopic sac-like structure
called an alveolus (plural is alveoli). The alveoli are the site of the second function of the respiratory
system: gas exchange.
Gas Exchange
The lung provides a moist and warm surface for gas exchange. Each lung has over two million alveoli,
which provide a huge surface area for gas exchange, equal to about 800 square feet. In contrast, the
skin surface area of your body is only ten square feet. All of that is packed within each of your lungs,
which really only have a relatively small volume if it weren't for all of the alveoli. So again we see the
power of branching and folding to increase surface area in relation to volume.
Alveoli are small sacs in the lungs
surrounded by blood vessels. The walls of
the sac and the walls of the surrounding
blood vessels are very thin, allowing for
diffusion of gases in either direction. The
hemoglobin of the red blood cells has a
greater affinity for oxygen than it does for
carbon dioxide. Thus, when the blood stream
interfaces with the outside air in the alveolus,
oxygen diffuses into the blood and carbon
dioxide diffuses into the sac of the alveolus,
to be exhaled with the next breath. Blood
entering the lungs is low in oxygen and high
in carbon dioxide because of cellular
respiration (metabolism), whereas air
entering the lungs from the atmosphere is
high in oxygen and low in carbon dioxide.
Thus, the alveoli contain a high concentration
of oxygen and a low carbon dioxide
concentration. However, the blood in the
blood vessels lining the alveoli has exactly
the opposite condition, so the two gases flow
in opposite directions. This is the gas
exchange of respiration. The blood in the
blood vessels lining the alveoli has a high
oxygen concentration and flows out of the
lungs to go to the rest of the body.
What Happens in Body Cells?
Once the newly oxygenated blood leaves the
lungs, it is distributed throughout the body to
every cell. Each cell engages in cellular
respiration (metabolism), thus using up oxygen and producing carbon dioxide. Inside the cell is thus a
high concentration of carbon dioxide, but in the blood and surrounding fluid outside the cell, the
concentration of carbon dioxide is low. So, again, a gas exchange occurs where oxygen flows from a
high concentration outside the cell to the low-concentration side on the inside of the cell. Carbon
dioxide flows in the opposite direction. The blood is now high in carbon dioxide and makes its way
back to the lungs for more gas exchange with atmospheric air that has been breathed in and conducted
to the lungs.
In Short
The word respiration is used in two different but related ways in biology. In one sense, respiration
means the act of bringing air into the lungs and expelling waste gases. We call this form of respiration
breathing. However, the chemical reactions of metabolism partly contain a set of reactions that use
enzymes and oxygen to burn food molecules and release the energy of their bonds into ATP. This
process is more specifically known as cellular aerobic respiration (or anaerobic when no oxygen is
present). All organisms, even single-celled ones, must perform cellular respiration (either aerobically
or anaerobically) during metabolism to obtain necessary energy in a form used for life functions.