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The Importance of the Respiratory System
You live in a sea of air. Nitrogen, oxygen, carbon dioxide, and trace gases are taken into and
expelled from your body with every breath. Earth’s atmosphere is made up of approximately 78%
nitrogen and 21% oxygen; the remaining gases, argon, carbon dioxide, and others, make up about 1%.
The second most common component, oxygen, is vital to life. Cells obtain energy through a chemical
reaction called oxidation, in which organic compounds are broken down using oxygen. Although a small
amount of energy can be obtained in anaerobic conditions (in the absence of oxygen), life processes in
humans cannot be maintained without an adequate supply of oxygen.
Oxygen is so essential to the survival of humans that just a few minutes without oxygen will
result in death. By comparison, individuals can live for a number of days without water and several
weeks without food. It has been estimated that an average adult utilizes 250 mL of oxygen every minute
while resting. Oxygen consumption may increase up to 20 times with strenuous exercise.
Respiration and Breathing
The term respiration can be used to describe all processes that supply oxygen to the cells of the
body for the breakdown of glucose and to describe the process by which wastes are transported to the
lungs for exhalation. Animals use oxygen for cellular respiration. We know that Small organelles called
mitochondria are the centres of cellular respiration. During the process of cellular respiration, oxygen
and sugar molecules react, resulting in the production of carbon dioxide and water. The energy
released is used to maintain cell processes, such as growth, movement, and the creation of new
The concentration of oxygen in cells is much lower than in their environment because cells continuously
use it for cellular respiration. Oxygen must be constantly replenished if a cell is to survive.
Breathing, or ventilation, involves the movement of gases between the external environment
and the location where they can enter and leave the body. In aquatic organisms, such as fish, oxygen is
extracted from the surrounding water. Land animals have approximately 20 times more oxygen
available to them through the atmosphere. The intake of oxygen and the release of carbon dioxide by
cells take place across a respiratory membrane.
The Challenge of Getting Oxygen
As animals increase in size, more oxygen is required to meet their energy needs. In order to
deliver greater amounts of oxygen to cells, the respiratory membranes of the more complex animals
must have an increased surface area. Some animals, such as earthworms, use their skin as a respiratory
membrane. The skin must be kept moist at all times to allow the proper diffusion of gases.
Fish, some salamanders, clams, starfish, and crayfish exchange gases through their gills. Gills
are, essentially, extensions of the outer surface of the body. The extensive folding and branching of the
gills provide increased surface area for the diffusion of gases, improving the efficiency of the respiratory
organ. Fish also use a countercurrent flow—the water moves over the gills in one direction while the
blood, contained within the capillaries inside the gill, moves in the opposite direction. Countercurrent
flow increases the efficiency of oxygen intake and ensures that the oxygen diffuses into the blood over
the entire length of the blood vessel inside of the gill. Because the blood and water move in opposite
directions, oxygen-poor blood is in contact with oxygen-rich water, while blood that is relatively rich in
oxygen is in contact with oxygen-poor water. This ensures that oxygen always diffuses from the water
into blood that has a low concentration of oxygen. If the blood and water flowed in the same direction,
the amount of oxygen diffusion would be reduced because there would be only a small difference
between the oxygen concentrations in the water and in the blood. Although gills are ideal for aquatic
environments, they are poorly adapted for land. Exposing the large surface area of the respiratory
membrane to air causes too much evaporation. If the gills become dry, the membrane becomes
impermeable to the diffusion of gases.
Insects do not have the problem of losing too much water by evaporation through their
respiratory membrane because it is located inside their body. A tracheal system, consisting of branching
respiratory tubes, connects cells directly to the atmosphere by openings in the exoskeleton called
spiracles. Oxygen enters the body through the spiracles and is then delivered to the cells by the
tracheae and blood. For land animals, the tracheal system is a vast improvement over gills, but it does
not provide enough oxygen for larger animals. The limited oxygen delivery by a tracheal system is one
factor that keeps insects small.
Although the atmosphere contains 20 times more oxygen than water, the size and activity of
land animals are still limited by their ability to secure oxygen. Larger animals require an even more
efficient system for the delivery of gases. The respiratory system consists of a windpipe or trachea that
branch of into the lungs. Frog lungs are balloon like structures that allow the exchange of gases
between the air and the blood. Although frogs, like earthworms, can absorb oxygen through their skin,
the lungs provide substantially greater surface area. The internal location helps solve the problem of
evaporation. Internal folds inside the lung increase the surface area for diffusion. The greater the
surface area, the greater the amount of oxygen, that can be absorbed.