Respiration refers to two processes - breathing and cellular respiration. Cellular respiration occurs within cells and converts food into energy through a series of chemical reactions that require oxygen. It is the reverse of photosynthesis in plants. Respiration can be aerobic, using oxygen, or anaerobic, not requiring oxygen. Different organisms use various mechanisms for respiration, ranging from direct diffusion of gases in simple organisms to more complex systems using trachea, gills, or lungs to facilitate gas exchange with the environment.

1. Respiration in Organism

2. RESPIRATION
CONCEPT
Respiration is much more than just breathing; in
fact, the term refers to two separate processes, only
one of which is the intake and outflow of breath. At
least cellular respiration, the process by which
organisms convert food into chemical energy,
requires oxygen; on the other hand, some forms of
respiration are anaerobic, meaning that they require
no oxygen. Such is the case, for instance, with
some bacteria, such as those that convert ethyl
alcohol to vinegar. Likewise, an anaerobic process
can take place in human muscle tissue, producing
lactic acid—something so painful that it feels as
though vinegar itself were being poured on an open
sore.

3. HOW IT WORKS
Forms of Respiration
Respiration can be defined as the process by which an
organism takes in oxygen and releases carbon dioxide, one
in which the circulating medium of the organism (e.g., the
blood) comes into contact with air or dissolved gases. Either
way, this means more or less the same thing as breathing. In
some cases, this meaning of the term is extended to the
transfer of oxygen from the lungs to the bloodstream and,
eventually, into cells or the release of carbon dioxide from
cells into the bloodstream and thence to the lungs, from
whence it is expelled to the environment. Sometimes a
distinction is made between external respiration, or an
exchange of gases with the external environment, and
internal respiration, an exchange of gases between the
body's cells and the blood, in which the blood itself "bathes"
the cells with oxygen and receives carbon dioxide to transfer

4. This is just one meaning—albeit a more familiar
one—of the word respiration. Respiration also
can mean cellular respiration, a series of
chemical reactions within cells whereby food is
"burned" in the presence of oxygen and
converted into carbon dioxide and water. This
type of respiration is the reverse of
photosynthesis, the process by which plants
convert dioxide and water, with the aid of solar
energy, into complex organic compounds known
as carbohydrates. (For more about carbohydrates
and photosynthesis, see Carbohydrates.)

5. How Gases Move Through the Body
Later in this essay, we discuss some of the ways in
which various life-forms breathe, but suffice it to say
for the moment—hardly a surprising revelation!—that
the human lungs and respiratory system are among
the more complex mechanisms for breathing in the
animal world. In humans and other animals with
relatively complex breathing mechanisms (i.e., lungs
or gills), oxygen passes through the breathing
apparatus, is absorbed by the bloodstream, and then
is converted into an unstable chemical compound
(i.e., one that is broken down easily) and carried to
cells. When the compound reaches a cell, it is
broken down and releases its oxygen, which passes
into the cell.

6. On the "return trip"—that is, the reverse process,
which we experience as exhalation—cells release
carbon dioxide into the bloodstream, where it is
used to form another unstable chemical
compound. This compound is carried by the
bloodstream back to the gills or lungs, and, at the
end of the journey, it breaks down and releases
the carbon dioxide to the surrounding
environment. Clearly, the one process is a mirror
image of the other, with the principal difference
being the fact that oxygen is the key chemical
component in the intake process, while carbon
dioxide plays the same role in the process of
outflow.

8. HEMOGLOBIN AND OTHER COMPOUNDS.
In humans the compound used to transport oxygen is
known by the name hemoglobin. Hemoglobin is an ironcontaining protein in red blood cells that is responsible for
transporting oxygen to the tissues and removing carbon
dioxide from them. In the lungs, hemoglobin, known for its
deep red color, reacts with oxygen to form oxyhemoglobin.
Oxyhemoglobin travels through the bloodstream to cells,
where it breaks down to form hemoglobin and oxygen, and
the oxygen then passes into cells. On the return trip,
hemoglobin combines with carbon dioxide to form
carbaminohemoglobin, an unstable compound that, once
again, breaks down—only this time it is carbon dioxide that
it releases, in this case to the surrounding environment
rather than to the cells.

9. In other species, compounds other than
hemoglobin perform a similar function. For
example, some types of annelids, or segmented
worms, carry a green blood protein called
chlorocruorin that functions in the same way as
hemoglobin does in humans. And whereas
hemoglobin is a molecule with an iron atom at
the center, the blood of lobsters and other large
crustaceans contains hemocyanin, in which
copper occupies the central position. Whatever
the substance, the compound it forms with
oxygen and carbon dioxide must be unstable, so
that it can break down easily to release oxygen
to the cells or carbon dioxide to the environment.

11. Cellular Respiration
Both forms of respiration involve oxygen, but cellular respiration also
involves a type of nutrient—materials that supply energy, or the
materials for forming new tissue. Among the key nutrients are
carbohydrates, naturally occurring compounds that consist of carbon,
hydrogen, and oxygen. Included in the carbohydrate group are
sugars, starches, cellulose, and various other substances.
Glucose is a simple sugar produced in cells by the breakdown of
more complex carbohydrates, including starch, cellulose, and such
complex sugars as sucrose (cane or beet sugar) and fructose (fruit
sugar). In cellular respiration, an organism oxidizes glucose (i.e.,
combines it with oxygen) so as to form the energy-rich compound
known as adenosine triphosphate (ATP). ATP, critical to metabolism
(the breakdown of nutrients to provide energy or form new material),
is the compound used by cells to carry out most of their ordinary
functions. Among those functions are the production of new cell parts
and chemicals, the movement of compounds through cells and the
body as a whole, and growth.

12. In cellular respiration, six molecules of
glucose (C6H12O6) react with six molecules of
oxygen (O2) to form six molecules of carbon
dioxide (CO2), six molecules of water (H2O),
and 36 molecules of ATP. This can be
represented by the following chemical
equation:
6C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + 36 ATP
The process is much more complicated than
this equation makes it appear: some two
dozen separate chemical reactions are
involved in the overall conversion of glucose
to carbon dioxide, water, and ATP.

14. The Mechanics of Breathing
All animals have some mechanism for removing oxygen from
the air and transmitting it into the bloodstream, and this same
mechanism typically is used to expel carbon dioxide from the
bloodstream into the surrounding environment. Types of animal
respiration, in order of complexity, include direct diffusion,
diffusion into blood, tracheal respiration, respiration with gills,
and finally, respiration through lungs. Microbes, fungi, and
plants all obtain the oxygen they use for cellular respiration
directly from the environment, meaning that there are no
intermediate organs or bodily chemicals, such as lungs or
blood. More complex organisms, such as sponges, jellyfish,
and terrestrial (land) flatworms, all of which have blood, also
breathe through direct diffusion. The latter term describes an
exchange of oxygen and carbon dioxide directly between an
organism, or its bloodstream, and the surrounding environment.

15. More complex is the method of diffusion into blood whereby
oxygen passes through a moist layer of cells on the body surface
and then through capillary walls (capillaries are small blood
vessels that form a network throughout the body) and into the
bloodstream. Once oxygen is in the blood, it moves throughout
the body to different tissues and cells. Among the organisms that
rely on diffusion into blood are annelids, a group that includes
earthworms, various marine worms, and leeches.
In tracheal respiration air moves through openings in the body
surface called spiracles. It then passes into special breathing
tubes called tracheae that extend into the body. The tracheae
divide into many small branches that are in contact with muscles
and organs. In small insects, air simply moves into the tracheae,
while in large insects, body movements assist tracheal air
movement. Insects and terrestrial arthropods (land-based
organisms with external skeletons) use this method of respiration.

16. Much more complicated than tracheae, gills are
specialized tissues with many infoldings. Each gill
is covered by a thin layer of cells and filled with
blood capillaries. These capillaries take up
oxygen dissolved in water and expel carbon
dioxide dissolved in blood. Fish and other aquatic
animals use gills, as did the early ancestors of
humans and other higher animals. A remnant of
this chapter from humans' evolutionary history
can be seen in the way that an embryo breathes
in its mother's womb, not by drawing in oxygen
through its lungs but through gill-like mechanisms
that disappear as the embryo develops.

18. LUNGS.
Lungs are composed of many small chambers or air sacs
surrounded by blood capillaries. Thus, they work with the
circulatory system, which transports oxygen from inhaled air to
all tissues of the body and also transports carbon dioxide from
body cells to the lungs to be exhaled. After air enters the lungs,
oxygen moves into the bloodstream through the walls of these
capillaries. It then passes from the lung capillaries to the
different muscles and organs of the body.
Although they are common to amphibians, reptiles, birds, and
mammals, lungs differ enormously throughout the animal
kingdom. Frogs, for instance, have balloon-like lungs that do not
have a very large surface area. By contrast, if the entire surface
of an adult male human's lungs were spread flat, it would cover
about 750 sq. ft. (70 m2), approximately the size of a handball
court. The reason is that humans have about 300 million gasfilled alveoli, tiny protrusions inside the lungs that greatly
expand the surface area for gas exchange.

19. Birds have specialized lungs that use a mechanism called crosscurrent
exchange, which allows air to flow in one direction only, making for more
efficient oxygen exchange. They have some eight thin-walled air sacs
attached to their lungs, and when they inhale, air passes through a tube
called the bronchus and enters posterior air sacs—that is, sacs located
toward the rear. At the same time, air in the lungs moves forward to
anterior air sacs, or ones located near the bird's front. When the bird
exhales, air from the rear air sacs moves to the outside environment,
while air from the front moves into the lungs. This efficient system
moves air forward through the lungs when the bird inhales and exhales
and makes it possible for birds to fly at high altitudes, where the air has
a low oxygen content.
Humans and other mammals have lungs in which air moves in and out
through the same pathway. This is true even of dolphins and whales,
though they differ from humans in that they do not take in nutrition
through the same opening. In fact, terrestrial mammals, such as the
human, horse, or dog, are some of the only creatures that possess two
large respiratory openings: one purely for breathing and smelling and
the other for the intake of nutrients as well as air (i.e., oxygen in and
carbon dioxide out).

21. REAL-LIFE APPLICATIONS
Anaerobic Respiration
Activity that involves oxygen is called aerobic; hence the term
aerobic exercise, which refers to running, calisthenics, biking, or any
other form of activity that increases the heart rate and breathing.
Activity that does not involve oxygen intake is called anaerobic.
Weightlifting, for instance, will increase the heart rate and rate of
breathing if it is done intensely, but that is not its purpose and it does
not depend on the intake and outflow of breath. For that reason, it is
called an anaerobic exercise—though, obviously, a person has to
keep breathing while doing it.
In fact, a person cannot consciously stop breathing for a prolonged
period, and for this reason, people cannot kill themselves simply by
holding their breath. A buildup of carbon dioxide and hydrogen ions
(electrically charged atoms) in the bloodstream stimulates the
breathing centers to become active, no matter what we try to do. On
the other hand, if a person were underwater, the lungs would draw in
water instead of air, and though water contains air, the drowning
person would suffocate.

23. ANAEROBIC BACTERIA.
Some creatures, however, do not need to breathe air but instead survive by
anaerobic respiration. This is true primarily of some forms of bacteria, and
indeed scientists believe that the first organisms to appear on Earth's
surface were anaerobic. Those organisms arose when Earth's atmosphere
contained very little oxygen, and as the composition of the atmosphere
began to incorporate more oxygen over the course of many millions of
years, new organisms evolved that were adapted to that condition.
The essay on paleontology discusses Earth's early history, including the
existence of anaerobic life before the formation of oxygen in the
atmosphere. The appearance of oxygen is a result of plant life, which
produces it as a byproduct of the conversion of carbon dioxide that takes
place in photosynthesis. Plants, therefore, are technically anaerobic lifeforms, though that term usually refers to types of bacteria that neither inhale
nor exhale oxygen. Anaerobic bacteria still exist on Earth and serve humans
in many ways. Some play a part in the production of foods, as in the
process of fermentation. Other anaerobic bacteria have a role in the
treatment of sewage. Living in an environment that would kill most
creatures—and not just because of the lack of oxygen—they consume
waste materials, breaking them down chemically into simpler compounds.

24. HUMANS AND ANAEROBIC RESPIRATION.
Even in creatures, such as humans, that depend on aerobic
respiration, anaerobic respiration can take place. Most cells are
able to switch from aerobic to anaerobic respiration when
necessary, but they generally are not able to continue producing
energy by this process for very long. For example, a person who
exercises vigorously may be burning up glucose faster than
oxygen is being pumped to the cells, meaning that cellular
respiration cannot take place quickly enough to supply all the
energy the body needs. In that case, cells switch over to
anaerobic respiration, which results in the production of lactic
acid, or C3H6O3. One advantage of anaerobic respiration is that it
can take place very quickly and in short bursts, as opposed to
aerobic respiration, which is designed for slower and steadier use
of muscles. The disadvantage is that anaerobic respiration
produces lactic acid, which, when it builds up in muscles that are
overworked, causes soreness and may even lead to cramps.

25. RESPIRATION:
A term that can refer either to cellular
respiration (see definition) or, more
commonly, to the process by which an
organism takes in oxygen and releases
carbon dioxide. Sometimes a distinction
is made between external respiration, or
an exchange of gases with the external
environment, and internal respiration, an
exchange of gases between the body's
cells and the blood.