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Systems of Gas Exchange
∗
Robert Bear
David Rintoul
Based on Systems of Gas Exchange† by
OpenStax College
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 4.0‡
Introduction
The inspired and expired air may be sometimes very useful, by condensing and cooling the blood
that passeth through the lungs; I hold that the depuration of the blood in that passage, is not
only one of the ordinary, but one of the principal uses of respiration.
Robert Boyle, in New Experiments ... Touching the Spring of Air, 1660
The primary function of the respiratory system is to deliver oxygen to the cells of the body's tissues and
remove a waste product, carbon dioxide (the process which Boyle called "depuration"). The main structures
of the human respiratory system are the nasal cavity, the trachea, and lungs, and these structures are what
brings oxygen into the human body and removes carbon dioxide from the human body.
As you learned
previously, the circulatory system is responsible for moving oxygen from the lungs to the tissues and for
moving carbon dioxide from the tissues and taking it to the lungs. At the cellular level the oxygen is needed
to make ATP from the energy stored in glucose and other organic molecules; carbon dioxide is a waste
product of harvesting that energy. In other words, the respiratory system gets the oxygen inside the body,
the circulatory system moves the oxygen around the body getting it to the cells, the cells use the oxygen
to produce energy and in the process produce carbon dioxide as a waste, the circulatory system removes
the carbon dioxide from the cell and delivers it to the lungs, and the respiratory system removes the carbon
dioxide from the body. In vertebrates, the respiratory and circulatory work very closely together in order to
allow for gas exchange between the inside and outside of the organism.
All aerobic organisms require oxygen to carry out their metabolic functions.
Over evolutionary time,
dierent organisms have devised dierent means of obtaining oxygen from the surrounding atmosphere. The
environment in which the animal lives greatly determines how an animal respires. The complexity of the
respiratory system is correlated with the size of the organism. As animal size increases, diusion distances
increase and the ratio of surface area to volume drops.
In unicellular organisms, diusion across the cell
membrane is sucient for supplying oxygen to the cell (Figure 1).
Diusion is a slow, passive transport
process. Therefore, dependence on diusion as a means of obtaining oxygen and removing carbon dioxide
remains feasible only for small organisms or those with highly-attened bodies, such as many atworms
∗ Version
1.4: Jul 27, 2014 8:59 pm -0500
† http://legacy.cnx.org/content/m44792/1.7/
‡ http://creativecommons.org/licenses/by/4.0/
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(Platyhelminthes). Larger organisms had to evolve specialized respiratory tissues, such as gills, lungs, and
respiratory passages accompanied by a complex circulatory systems, to transport oxygen throughout their
entire body.
Figure 1: The cell of the unicellular algae Ventricaria ventricosa is one of the largest known, reaching
one to ve centimeters in diameter. Like all single-celled organisms, V. ventricosa exchanges gases across
the cell membrane.
1 Direct Diusion
For small multicellular organisms, diusion across the outer membrane is sucient to meet their oxygen
needs. Gas exchange by direct diusion across surface membranes is ecient for organisms less than 1 mm
in diameter. In simple organisms, such as cnidarians and atworms, every cell in the body is close to the
external environment. Their cells are kept moist and gases diuse quickly via direct diusion. Flatworms
are small, literally at worms, which `breathe' through diusion across the outer surface (Figure 2). The at
shape of these organisms increases the surface area for diusion, ensuring that each cell within the body is
close to the outer surface and has access to oxygen. If the atworm had a cylindrical body, then the cells in
the center would not be able to get oxygen.
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Figure 2: This atworm's process of respiration works by diusion across the outer membrane. (credit:
Stephen Childs)
2 Skin and Gills
Earthworms and amphibians use their skin (integument) as a respiratory organ. A dense network of capillaries
lies just below the skin and facilitates gas exchange between the external environment and the circulatory
system. The respiratory surface must be kept moist in order for the gases to dissolve and diuse across cell
membranes.
Organisms that live in water need to obtain oxygen from the water. Oxygen dissolves in water but at a
lower concentration than in the atmosphere. The atmosphere has roughly 21 percent oxygen. In water, the
oxygen concentration is much less than that. Fish and many other aquatic organisms have evolved gills to
take up the dissolved oxygen from water (Figure 3). Gills are thin tissue laments that are highly branched
and folded. When water passes over the gills, the dissolved oxygen in water rapidly diuses across the gills
into the bloodstream. The circulatory system can then carry the oxygenated blood to the other parts of the
body. In animals that contain coelomic uid instead of blood, oxygen diuses across the gill surfaces into
the coelomic uid. Gills are found in mollusks, annelids, and crustaceans.
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Figure 3: This common carp, like many other aquatic organisms, has gills that allow it to obtain oxygen
from water. (credit: "Guitardude012"/Wikimedia Commons)
The folded surfaces of the gills provide a large surface area to ensure that the sh gets sucient oxygen.
Diusion is a process in which material travels from regions of high concentration to low concentration
until equilibrium is reached.
In this case, blood with a low concentration of oxygen molecules circulates
through the gills. The concentration of oxygen molecules in water is higher than the concentration of oxygen
molecules in gills.
As a result, oxygen molecules diuse from water (high concentration) to blood (low
concentration), as shown in Figure 4. Similarly, carbon dioxide molecules in the blood diuse from the blood
(high concentration) to water (low concentration).
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Figure 4: As water ows over the gills, oxygen is transferred to blood via the veins. (credit "sh":
modication of work by Duane Raver, NOAA)
3 Tracheal Systems
Insect respiration is independent of its circulatory system; therefore, the blood does not play a direct role
in oxygen transport. Insects have a highly specialized type of respiratory system called the tracheal system,
which consists of a network of small tubes that carries oxygen to the entire body. The tracheal system is the
most direct and ecient respiratory system in active animals.
Insect bodies have openings, called spiracles, along the thorax and abdomen. These openings connect to
the tubular network, allowing oxygen to pass into the body (Figure 5) and regulating the diusion of CO
2
and water vapor. Air enters and leaves the tracheal system through the spiracles. Some insects can ventilate
the tracheal system with body movements.
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Figure 5: Insects perform respiration via a tracheal system.
4 Mammalian Systems
In mammals, pulmonary ventilation occurs via inhalation (breathing).
During inhalation, air enters the
human body through the nasal cavity located just inside the nose (Figure 6). As air passes through the
nasal cavity, the air is warmed to body temperature and humidied. The respiratory tract is coated with
mucus to seal the tissues from direct contact with air. Mucus is high in water. As air crosses these surfaces
of the mucous membranes, it picks up water. These processes help equilibrate the air to the body conditions,
reducing any damage that cold, dry air can cause. Particulate matter that is oating in the air is removed
in the nasal passages via mucus and cilia. The processes of warming, humidifying, and removing particles
are important protective mechanisms that prevent damage to the trachea and lungs. Thus, inhalation serves
several purposes in addition to bringing oxygen into the respiratory system.
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Figure 6: Air enters the respiratory system through the nasal cavity and pharynx, and then passes
through the trachea and into the bronchi, which bring air into the lungs. (credit: modication of work
by NCI)
From the nasal cavity, air passes through the pharynx (throat) and the larynx (voice box), as it makes
its way to the trachea (Figure 6).
The main function of the trachea is to funnel the inhaled air to the
lungs and the exhaled air back out of the body. The human trachea is a cylinder about 10 to 12 cm long
and 2 cm in diameter that sits in front of the esophagus and extends from the larynx into the chest cavity
where it divides into the two primary bronchi at the midthorax (Figure 6). The trachea is lined with mucusproducing goblet cells and ciliated epithelia. The cilia propel foreign particles trapped in the mucus toward
the pharynx. The cartilage provides strength and support to the trachea to keep the passage open. The
smooth muscle can contract, decreasing the trachea's diameter, which causes expired air to rush upwards
from the lungs at a great force. The forced exhalation helps expel mucus when we cough. Smooth muscle
can contract or relax, depending on stimuli from the external environment or the body's nervous system.
4.1 Lungs: Bronchi and Alveoli
The end of the trachea bifurcates (divides) to the right and left lungs. The lungs are not identical. The
right lung is larger and contains three lobes, whereas the smaller left lung contains two lobes (Figure 7). The
muscular diaphragm, which facilitates breathing, is inferior (below) to the lungs and marks the end of the
thoracic cavity.
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Figure 7: The trachea bifurcates into the right and left bronchi in the lungs. The right lung is made of
three lobes and is larger. To accommodate the heart, the left lung is smaller and has only two lobes.
In the lungs, air is diverted into smaller and smaller passages, or bronchi. Air enters the lungs through
the two primary (main) bronchi (singular: bronchus). Each bronchus divides into secondary bronchi, then
into tertiary bronchi, which in turn divide, creating smaller and smaller diameter bronchioles as they split
and spread through the lung.
The terminal bronchioles subdivide into microscopic branches called respiratory bronchioles. The respiratory bronchioles subdivide into several alveolar ducts. Numerous alveoli and alveolar sacs surround the
alveolar ducts. The alveolar sacs resemble bunches of grapes tethered to the end of the bronchioles (Figure 8). Alveoli are made of thin-walled parenchymal cells, typically one-cell thick, that look like tiny bubbles
within the sacs. Alveoli are in direct contact with capillaries (one-cell thick) of the circulatory system. Such
intimate contact ensures that oxygen will diuse from alveoli into the blood and be distributed to the cells
of the body. In addition, the carbon dioxide that was produced by cells as a waste product will diuse from
the blood into alveoli to be exhaled.
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Figure 8: Terminal bronchioles are connected by respiratory bronchioles to alveolar ducts and alveolar
sacs. Each alveolar sac contains 20 to 30 spherical alveoli and has the appearance of a bunch of grapes.
Air ows into the atrium of the alveolar sac, then circulates into alveoli where gas exchange occurs with
the capillaries. Mucous glands secrete mucous into the airways, keeping them moist and exible. (credit:
modication of work by Mariana Ruiz Villareal)
5 Transport of Gases in Blood
Once the oxygen diuses across the alveoli, it enters the bloodstream and is transported to the tissues where
it is unloaded, and carbon dioxide diuses out of the blood and into the alveoli to be expelled from the body.
Although gas exchange is a continuous process, the oxygen and carbon dioxide are transported by dierent
mechanisms.
5.1 Transport of Oxygen in the Blood
Although oxygen dissolves in blood, only a small amount of oxygen is transported this way. Only 1.5 percent
of oxygen in the blood is dissolved directly into the blood itself. Most oxygen98.5 percentis bound to a
protein called hemoglobin and carried to the tissues.
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5.1.1 Hemoglobin
Hemoglobin, or Hb, is a protein molecule found in red blood cells (erythrocytes) made of four subunits:
two alpha subunits and two beta subunits (Figure 9).
Each subunit surrounds a central heme group
that contains iron and binds one oxygen molecule, allowing each hemoglobin molecule to bind four oxygen
molecules. Molecules with more oxygen bound to the heme groups are brighter red. As a result, oxygenated
arterial blood where the Hb is carrying four oxygen molecules is bright red, while venous blood that is
deoxygenated is darker red.
Figure 9: The protein inside (a) red blood cells that carries oxygen to cells and carbon dioxide to the
lungs is (b) hemoglobin. Hemoglobin is made up of four symmetrical subunits and four heme groups.
Iron associated with the heme binds oxygen. It is the iron in hemoglobin that gives blood its red color.
5.2 Transport of Carbon Dioxide in the Blood
Carbon dioxide molecules are transported in the blood from body tissues to the lungs by one of three methods:
dissolution directly into the blood, binding to hemoglobin, or carried as a bicarbonate ion. Several properties
of carbon dioxide in the blood aect its transport. First, carbon dioxide is more soluble in blood than oxygen.
About 5 to 7 percent of all carbon dioxide is dissolved in the plasma. Second, carbon dioxide can bind to
plasma proteins or can enter red blood cells and bind to hemoglobin. This form transports about 10 percent
of the carbon dioxide. When carbon dioxide binds to hemoglobin, a molecule called carbaminohemoglobin
is formed. Binding of carbon dioxide to hemoglobin is reversible. Therefore, when it reaches the lungs, the
carbon dioxide can freely dissociate from the hemoglobin and be expelled from the body.
Third, the majority of carbon dioxide molecules (85 percent) are carried as part of the bicarbonate buer
system. In this system, carbon dioxide diuses into the red blood cells. Carbonic anhydrase (CA) within the
2
3
red blood cells quickly converts the carbon dioxide into carbonic acid (H CO ). Carbonic acid is an unstable
+
3 ) and hydrogen (H ) ions.
Since carbon dioxide is quickly converted into bicarbonate ions, this reaction allows for the continued uptake
+
of carbon dioxide into the blood down its concentration gradient. It also results in the production of H ions.
+
+
If too much H is produced, it can alter blood pH. However, hemoglobin binds to the free H ions and thus
−
intermediate molecule that immediately dissociates into bicarbonate ions (HCO
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limits shifts in pH. The newly synthesized bicarbonate ion is transported out of the red blood cell into the
-
liquid component of the blood in exchange for a chloride ion (Cl ); this is called the chloride shift. When the
blood reaches the lungs, the bicarbonate ion is transported back into the red blood cell in exchange for the
+
chloride ion. The H
ion dissociates from the hemoglobin and binds to the bicarbonate ion. This produces
the carbonic acid intermediate, which is converted back into carbon dioxide through the enzymatic action
of CA. The carbon dioxide produced is expelled through the lungs during exhalation.
CO2 + H2 O
↔
H2 CO3
(carbonic acid)
↔
HCO3 + H
+
(1)
(bicarbonate)
The benet of the bicarbonate buer system is that carbon dioxide is soaked up into the blood with little
change to the pH of the system. This is important because it takes only a small change in the overall pH of
the body for severe injury or death to result. The presence of this bicarbonate buer system also allows for
people to travel and live at high altitudes: When the partial pressure of oxygen and carbon dioxide change
at high altitudes, the bicarbonate buer system adjusts to regulate carbon dioxide while maintaining the
correct pH in the body.
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