CONCEPTUAL LIFE SCIENCE
Respiration
Breathing is generally called respiration. We use respiration to distinguish
breathing from cellular respiration, which is the oxidation of glucose for energy
production. All higher organisms require O2. Unless the O2 diffuses into the body, they
must have a mechanism and a system for bring in O2 and eliminating the CO2 that results
from cellular respiration.
Plants and the lower members of the animal kingdom can obtain their O2 and
eliminate their CO2 by diffusion. They do not require lungs or gills. Higher animals,
including humans, require a breathing apparatus in order to exchange these gases.
RESPIRATON IN SOME REPRESENTATIVE ORGANISMS
Unicellular organisms
In unicellular organisms, gases exchange with the surrounding medium by
diffusion across the moist cell membrane of the cell. The membrane must be moist. It is
more difficult for gases to diffuse across a dry membrane, Besides, a dried out organism
is usually dead. No special breathing apparatus is required by these organisms.
Green plants
Gases enter and exit the air spaces of leaves via the stomates. The openings of the
stomates are controlled by the guard cells.
Older stems are usually impermeable to gases. Some trees have lenticels that let
air into the cambium. In many large trees, there are enough cracks in the bark so that air
is available for the cambium and living phloem cells.
Roots can exchange gases with the soil. This is done via the moist cell
membranes of the cells in the lower parts of the roots.
Coelenterates
Hydra does not have any specialized breathing apparatus. It has only two cell
layers and therefore does not need one. Each cell layer is in contact directly with the
exterior aqueous environment. Gas exchange is across the moist cell membranes of the
individual cells.
Higher marine animals
Gills are used by marine worms, molluscs (clams), crustacea (lobsters),
echinoderms (starfishes), and fishes. The gill lamellae (layers) have a large area of
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exchange of gases. The gases are transported to the rest of the body by the circulatory
system.
Gill function
Water enters the mouth of the fish, passes over the gills, and exits under the
operculum. The gill rakers prevent food and other substances from clogging the gills.
The gill lamellae provide a large surface area for exchange of gases.
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The afferent vessel carries blood toward the gill (afferent and approach both begin
with “a”). This blood is high in CO2 and low in O2.
The blood in the gill exchanges gases with the water. The CO2 leaves the gill and
enters the water while the O2 leaves the water and enters the gill.
Then, the efferent vessel carries blood away from the gill (efferent and exit both begin
with “e”). The blood is high in O2 and low in CO2.
Insects and spiders
Each cell in the body of an insect or spider is in contact or near a breathing tube
known as a trachiole. A group of trachioles will be combined to form a larger breathing
tube called a trachea. These animals have numerous tracheas that exit through holes on
the side of the body called spiracles. These spiracles are used for gas exchange, the
“spir” part is also found is the word “respiration.”
Frog
In the frog and other amphibians, about half of their gas exchange takes place
across the skin. The other half of the respiration is done using lungs. Frogs do not have a
diaphragm so they must force air into the lungs. This is called positive pressure
breathing. Humans have a diaphragm so human breathing is by negative pressure.
Birds
Birds have a complicated respiratory system consisting of lungs and air sacs in the
bones. Air is cycled through the air sacs in the hollow spaces in the bones.
Human respiration—the big picture
The human respiratory system consists of the lungs, bronchi and trachea. The
lungs are located in the thoracic (chest) cavity, the area in the chest above the diaphragm.
The heart is also located in the chest cavity.
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In humans, the diaphragm contracts an makes the chest cavity larger. This creates
a partial vacuum in the chest cavity. Air rushes in because the pressure is lower in the
chest cavity when the diaphragm contracts. The air that enters the lungs from the outside
is high in O2 and low in CO2. Inspiration is the term used for inhaling air. When the
diaphragm relaxes, the pressure in the chest increases and forces the air out. The air that
leaves the lungs is low in O2 and high in CO2. Expiration is the term used for exhaling
air.
Control of breathing is done by the nervous system. The brain stem has a control
center that detects the concentration of CO2 in the blood. When CO2 increases as a result
of exercise, for example, the breathing rate and heart rate increase. When there is little
CO2 in the blood, the nervous system slows the breathing rate.
Gas exchange in the lungs—the little picture
Gases are exchanged in the air sacs (alveoli) of the lungs. Air makes its way
down to the air sacs where O2 passes through the capillary wall by diffusion, and into the
blood stream where it enters the red blood cells. Simultaneously, CO2 and some water
leave the blood stream and enter the alveoli of the lungs. These respiratory gases pass up
through the respiratory system to the outside. All air exchange in the lungs in external
respiration and occurs through the moist surfaces of the cell membranes of the air sacs of
the lungs and the capillaries of the blood stream. Cellular respiration is internal
respiration.