Respiratory Systam
Virtually every book on scuba diving, including the open water
teaching manuals, includes some information on anatomy and
physiology of the respiratory system. Why is this material
important to explain scuba diving? First, because it helps
explain the origin of major problems that can result from
pressure - decompression sickness and air embolism. Second,
because it helps explain the one process vital to every dive:
breathing compressed air under water. If you already feel
comfortable with basic anatomy and physiology of the
respiratory system, please skip to Section D. If not, read on.
(This material is germane to the rest of the book, which is
why
I don't put it in an Appendix.)
WHAT IS THE FUNCTION OF THE RESPIRATORY SYSTEM?
The function of the respiratory system is rather simple in
concept: to bring in oxygen from the atmosphere and get rid
of
carbon dioxide from the blood. Since oxygen (O2) and carbon
dioxide (CO2) are gases, the process of bringing one in and
excreting the other is called gas exchange.
Oxygen is necessary for normal metabolism; lack of it leads to
death in a few minutes. Carbon dioxide is a waste product of
metabolism; if breathing stops, carbon dioxide will quickly
accumulate to a toxic level in the blood. Thus our lungs, the
organs that exchange O2 and CO2 with the atmosphere, are
vital since their total failure is quickly fatal.
We have two lungs, one in the right side of our chest cage and
one in the left (Figure l). Between them is the heart, a midline
organ that tilts slightly to the left within the chest cage. (You
can feel your heart beating by placing your finger tips under
your left breast.) Although gas exchange takes place in the
lungs, the respiratory system also includes two other
components: the part of the central nervous system that
controls our breathing, and the chest bellows.
The part of the nervous system that controls breathing is
located in the mid-brain, also known as the brain stem. It is
an
area more primitive than the area of the brain responsible for
thinking and motor movements, known as the brain cortex.
Brain stem control of breathing is automatic and functions
whether we think about it or not. However, it may be altered
by
drugs or some diseases. A relatively common cause of
respiratory depression is an overdose with narcotics or
sedatives.
Figure 1
Figure 1. Schematic view of the respiratory system, which
consists of: portions of the brain and spinal cord that send
signals to the muscles of breathing; the thoracic cage, which
includes the rib cage (not shown), pleural membranes and
diaphragms; and the lungs and airways.
The chest bellows component of the respiratory system
includes the bony thoracic cage that contains the lungs; the
diaphragms, which are the major muscles of breathing; and
pleural membranes, thin tissues that line both the outside of
the lungs and the inside of the thoracic cage. The thoracic or
chest cage consists of the ribs that protect the lungs from
injury; the muscles and connective tissues that tie the ribs
together; and all the nerves that lead into these muscles.
Approximately 10-12 times a minute, the brain stem sends
nerve impulses that tell the diaphragms and thoracic cage
muscles to contract. Contraction of these muscles expands the
rib cage, leading to the expansion of the lungs contained
within. With each expansion of the lungs we inhale a breath of
fresh air containing 21 percent oxygen and almost no carbon
dioxide. After full expansion the brain command to inhale
ceases and the thoracic cage passively returns to its resting
position, at the same time allowing the lungs to return to
their
resting size. As the lungs return to their resting position we
exhale a breath of stale air, containing about 16 percent
oxygen
and 6 percent carbon dioxide.
In health this breathing cycle is silent, automatic, and
effortless. In the process, oxygen is delivered from the
atmosphere into our blood and carbon dioxide is excreted from
our blood into the atmosphere.
Although "respiratory disease" is often thought of as only a
lung problem, malfunction of any component of the
respiratory system can cause a breathing problem. For example,
if the brain stem center that controls breathing is depressed, as
may occur with a drug overdose, failure of the respiratory
system will occur (and the victim can die because of it) even
though the lungs are normal. Polio, a common disease before
discovery of the vaccine, can cause respiratory system failure
by
damaging the nerves leading to the thoracic cage muscles. In
this situation the brain and lungs can be normal, but the chest
may not be able to expand in order to move air into the lungs;
as a result, the polio patient may have respiratory failure.
Thus all parts of the respiratory system must function properly
for normal breathing to occur. Yet, despite the importance of
all respiratory system components, there is good reason why
the lungs are usually thought of when one hears about
respiratory disease. Lung disease accounts for the vast majority
of respiratory illness. Emphysema, bronchitis, asthma,
pneumonia, lung cancer Ä all originate in the lungs. Our
lungs
are the only internal organ directly in contact with the
atmosphere, making them vulnerable to all pollutants,
including cigarette smoke, as well as airborne viruses and
bacteria. Our lungs are also the only internal organ in direct
contact with the increased ambient pressure of diving, making
them uniquely vulnerable to pressure changes.
Because most serious diving-related injuries occur from
pressure-related problems and/or from drowning, a more
detailed discussion of gas exchange should enhance your
understanding of scuba diving.
HOW DOES GAS EXCHANGE OCCUR?
Oxygen and carbon dioxide, symbolized O2 and CO2
respectively, are colorless, odorless gases. The atmosphere, or
air around us, contains approximately 21 percent oxygen and
78
percent nitrogen. There is almost no CO2 in air (about 0.03
percent); the carbon dioxide humans and animals exhale is a
negligible part of the entire atmosphere. The nitrogen is inert
and does not take part in gas exchange. (Except in
altered-pressure environments nitrogen is of no consequence.
Nitrogen takes on critical importance when breathing
compressed air under water.) The remainder of the air is made
up of some rare inert gases, such as argon, that also play no
role in gas exchange.
To accomplish gas exchange the air we inhale is delivered, via
our mouth and nose (Figure 2), to tiny sacs, called alveoli,
which are the terminal or end units of the airways (Figures 3
and 4). Oxygen from the air diffuses across a thin membrane
into tiny blood capillaries surrounding the alveoli. At the
same
time CO2 diffuses from the blood capillaries into the alveoli
and out of the lungs with each exhalation. The combination of
one alveolus (containing air) and its surrounding capillaries
(containing blood) is called an alveolar-capillary unit. Both
lungs contain an estimate 300,000,000 alveolar-capillary
units; the surface area of the alveolar membranes, if placed end
to end, would cover a tennis court!
This overview can be expanded by dividing gas exchange into
the
processes of alveolar ventilation (bringing air into the lungs
for transfer of oxygen and carbon dioxide) and pulmonary
circulation (bringing blood to the lungs to take up oxygen
and
excrete carbon dioxide).
Alveolar Ventilation. We inhale the air around us with each
breath. The air enters the mouth and nose (Figure 2). In the
nose and upper airway many of the dust particles are filtered
out, purifying the air. Air from the mouth and nose come
together in the throat and begin the journey into the lungs.
First air enters the larynx (voice box) and then the trachea
(just
below the Adam's apple). The trachea divides into two air
tubes, the right and left main bronchi. The trachea and
bronchial tubes that branch from it are lined with cartilage.
Cartilage provides a firm structure so the airways stay open
when we inhale and exhale.
The trachea and air passages above it (i.e., mouth, throat,
larynx) are collectively called the upper airways or upper
respiratory system. Air entering the upper airways is warmed
to
body temperature and humidified (water vapor is added to it).
(The right and left main bronchi and all airways that lead from
them are collectively called the lower airway system, which is
another term for our lungs.)
The right and left main bronchi represent the first of over 20
divisions of airways to come in the lower airway system
(Figure
2). With each division the air passages become narrower, but
the number of airways increases geometrically. By the 20th
division there are a huge number of individual, tiny airways
and
air has been distributed to each of them. Also at the 20th
division, where the diameter of each airway is less than 1 mm,
air sacs (the alveoli) begin to appear; this is where gas
exchange
actually takes place. Eventually, each airway ends in a grape-
like cluster of these alveoli.
At the alveolar-capillary membrane gas exchange takes place.
Oxygen is delivered to, and carbon dioxide removed from, the
capillary blood (Figures 3 and 4). This gas exchange converts
the oxygen-poor blood entering the pulmonary capillary into
oxygen-rich blood. At the same time the air we inhale (21
percent O2, almost no CO2) has been converted into stale air
(16 percent O2, 6 percent CO2) that we exhale.
Air enters through the mouth and nose, then travels down the
larynx (voice box) and trachea (windpipe). Air then enters the
lungs, which consist (in part) of multiple branching airways
called bronchi. These bronchi end in clusters of air sacs Ä the
alveoli. Each alveolus is surrounded by blood capillaries, which
take up the oxygen and give off carbon dioxide.
Each alveolar sac, or alveolus, is surrounded by one or more
pulmonary capillaries. This alveolar-capillary unit is where
oxygen (O2 ) and carbondioxide (CO2) are exchanged with the
atmosphere.
Each minute, under resting conditions, we breathe in about six
liters of fresh air. About 1/3 of this air stays in the mouth,
throat, and large airways where no gas exchange takes place;
this region (the upper airways and part of our lungs) is
referred
to as "dead space," because air in this space doesn't take part in
gas exchange. The remaining four liters of fresh air breathed in
each minute are distributed to the hundreds of millions of
alveoli and it this air that takes part in gas exchange and
constitutes the alveolar ventilation.
Pulmonary Circulation. Each minute our heart pumps
approximately five liters of blood through the lung capillaries,
distributing blood among the hundreds of millions of alveoli
so gas exchange can take place. Because the lungs are three
dimensional, one alveolar sac may be surrounded by several
pulmonary capillaries. Each alveolus and all of its
accompanying capillaries constitute the gas exchange unit
(Figure 3). If there was no blood flow around the alveolus or
there was blood flow without an accompanying alveolus, there
would be no gas exchange. Thus alveolar ventilation is but one
part of respiration; the other necessary part, the delivery of
blood to the capillaries surrounding the alveoli, is the
pulmonary circulation.
The total circulation of the blood in our body is a circular
affair. Blood flowing from one part of the heart to the lungs
and back to another part of the heart constitutes one part of
this circle; the other part is the systemic blood circulation,
which is blood flowing from the heart to the rest of the body
and then back to the heart (Figure 5). Arbitrarily, we can start
this circle with one of the four chambers of the heart, the
right
atrium.
Blood that enters the right atrium has given up much of its
oxygen to the tissues, and is called venous blood
(oxygen-poor
or de-oxygenated blood). After collecting in the right atrium
of the heart, venous blood then goes to the right ventricle
from
where it is pumped to the lungs in order to receive a fresh
supply of oxygen.
Blood leaving the right ventricle is pumped into the lungs via
one large blood vessel, the main pulmonary artery. This large
artery divides into two smaller pulmonary arteries, one to each
of the lungs. Each pulmonary artery gives rise to many
divisions, and in short order the blood supply pumped from
the
heart is divided among millions of tiny pulmonary capillaries,
the smallest unit of circulation. These capillaries are in
contact
with the hundreds of millions of alveoli, the tiny sacs that
receives the fresh air we inhale. The distance between each
alveolus or air sac, and its surrounding capillaries, is very
short, only the diameter of a thin membrane; oxygen easily
diffuses from the air sac into capillary blood while, at the same
time, CO2 diffuses out of the capillary blood and into the air
sac. The CO2-laden stale air is then exhaled, and fresh air is
brought into the lungs with the next breath.
Oxygenated blood leaving the millions of pulmonary
capillaries
enters the other side of the heart, first the left atrium and
then
the left ventricle. From the left ventricle oxygen-rich blood is
pumped, via the body's arterial circulation, to all the muscles,
tissues and organs. In this way our kidneys, brain, liver, heart,
bones, and other tissues receive vital oxygen. (When an
analysis of arterial blood gas is performed for oxygen
measurements, the blood sample is obtained by inserting a
small needle into the radial artery of the wrist, just behind the
thumb. Unlike venous blood, arterial blood reflects the status
of gas exchange in the lungs and is therefore useful to examine
in patients with many types of lung disease.)
The pulmonary and systemic (non-pulmonary) circulation
systems are schematically shown in Figure 5. The heart,
normally between our two lungs (Figure 1), is here separated
to
show its four chambers and the vessels leading to and from
them. Each pulmonary artery branches into millions of tiny
capillaries before picking up oxygen and giving off CO2 (gas
exchange). Then the millions of capillaries merge to become
the pulmonary veins. The pulmonary veins carry oxygenated
blood back to the left heart, from where it is pumped, via the
systemic arterial circulation, to the organs and tissues of the
body. From these organs and tissues it then returns, via the
systemic venous system, back to the right heart.
After entering the tissue, organ, or muscle, each systemic
artery divides into smaller and smaller vessels, the smallest of
which is the systemic capillary. These capillaries are
structurally similar to the pulmonary capillaries and have the
same function: to allow gas exchange to occur by simple
diffusion. In the lung, oxygen diffuses into the capillaries and
CO2 diffuses out. In all other capillaries (non-pulmonary or
systemic capillaries) oxygen diffuses out of the capillary into
the cells of the organ, and CO2 diffuses into the capillary
from
the cells of the organ. In this way oxygen is delivered for
cellular metabolism and CO2, a waste product of metabolism,
is removed.
Gas exchange is a vital process; it occurs not only in the lungs
but in all other tissues as well. Gas exchange requires both
ventilation, provided by breathing adequate amounts of fresh
air, and circulation, provided by the heart pumping blood to
the lungs (pulmonary circulation) and then out to all other
parts of the body (systemic circulation).
Blood entering the systemic (non-pulmonary) capillaries is
oxygenated. When blood leaves these capillaries it has given up
some (not all) of its oxygen, and is venous or de-oxygenated
blood. Venous blood, which appears blue in our veins under
the
skin, is actually dark red in a test tube. Arterial blood is
normally bright red (although if the patient is low on oxygen
arterial blood will also look dark red).
The systemic capillaries, after delivering their oxygenated
blood
to the tissues, merge and form the veins that carry the venous
blood; eventually all the systemic veins in the body come
together to form the two great vena cavae, the superior, that
carries blood leaving the head and neck, and the inferior, that
carries blood leaving the rest of the body. Both vena cavae
enter
the heart at the level of the right atrium. Blood from the right
atrium enters the right ventricle and then is pumped to the
lungs to once again begin the process of oxygenation.
The trachea and main bronchi are also effective in keeping
dusts
and other large particles from reaching the alveoli. Coughing
is
one way we attempt to clear the large airways of noxious
material.
Even when our nose and normal cough response are not
helping
keep out dusts, our bronchi function silently to move out any
unwanted material. This is accomplished by a blanket of mucus
that covers the bronchi, and tiny hairs (cilia) which sweep the
mucus out of the airways.
Each bronchus is lined with cells that are covered with tiny
hairs, called cilia, that project into the airway. Cilia are
covered
with a blanket of mucus, which serves to collect dusts and
other
pollutants. This "mucus blanket," along with its collected
dusts, is normally swept by the cilia up and out of the airways.
Even if particles get past all of these defenses, special alveolar
cells (called macrophages) are mobilized to help digest any
foreign substances such as bacteria or tiny particles of dust.
All
of these normal defense mechanisms may be damaged from
cigarette smoke, making smokers much more vulnerable to
inhaled dusts and other impurities in the air.
Virtually every book on scuba diving, including the open water
teaching manuals, includes some information on anatomy and
physiology of the respiratory system. Why is this material
important to explain scuba diving? First, because it helps
explain the origin of major problems that can result from
pressure - decompression sickness and air embolism. Second,
because it helps explain the one process vital to every dive:
breathing compressed air under water. If you already feel
comfortable with basic anatomy and physiology of the
respiratory system, please skip to Section D. If not, read on.
(This material is germane to the rest of the book, which is
why
I don't put it in an Appendix.)
WHAT IS THE FUNCTION OF THE RESPIRATORY SYSTEM?
The function of the respiratory system is rather simple in
concept: to bring in oxygen from the atmosphere and get rid
of
carbon dioxide from the blood. Since oxygen (O2) and carbon
dioxide (CO2) are gases, the process of bringing one in and
excreting the other is called gas exchange.
Oxygen is necessary for normal metabolism; lack of it leads to
death in a few minutes. Carbon dioxide is a waste product of
metabolism; if breathing stops, carbon dioxide will quickly
accumulate to a toxic level in the blood. Thus our lungs, the
organs that exchange O2 and CO2 with the atmosphere, are
vital since their total failure is quickly fatal.
We have two lungs, one in the right side of our chest cage and
one in the left (Figure l). Between them is the heart, a midline
organ that tilts slightly to the left within the chest cage. (You
can feel your heart beating by placing your finger tips under
your left breast.) Although gas exchange takes place in the
lungs, the respiratory system also includes two other
components: the part of the central nervous system that
controls our breathing, and the chest bellows.
The part of the nervous system that controls breathing is
located in the mid-brain, also known as the brain stem. It is
an
area more primitive than the area of the brain responsible for
thinking and motor movements, known as the brain cortex.
Brain stem control of breathing is automatic and functions
whether we think about it or not. However, it may be altered
by
drugs or some diseases. A relatively common cause of
respiratory depression is an overdose with narcotics or
sedatives.
Figure 1
Figure 1. Schematic view of the respiratory system, which
consists of: portions of the brain and spinal cord that send
signals to the muscles of breathing; the thoracic cage, which
includes the rib cage (not shown), pleural membranes and
diaphragms; and the lungs and airways.
The chest bellows component of the respiratory system
includes the bony thoracic cage that contains the lungs; the
diaphragms, which are the major muscles of breathing; and
pleural membranes, thin tissues that line both the outside of
the lungs and the inside of the thoracic cage. The thoracic or
chest cage consists of the ribs that protect the lungs from
injury; the muscles and connective tissues that tie the ribs
together; and all the nerves that lead into these muscles.
Approximately 10-12 times a minute, the brain stem sends
nerve impulses that tell the diaphragms and thoracic cage
muscles to contract. Contraction of these muscles expands the
rib cage, leading to the expansion of the lungs contained
within. With each expansion of the lungs we inhale a breath of
fresh air containing 21 percent oxygen and almost no carbon
dioxide. After full expansion the brain command to inhale
ceases and the thoracic cage passively returns to its resting
position, at the same time allowing the lungs to return to
their
resting size. As the lungs return to their resting position we
exhale a breath of stale air, containing about 16 percent
oxygen
and 6 percent carbon dioxide.
In health this breathing cycle is silent, automatic, and
effortless. In the process, oxygen is delivered from the
atmosphere into our blood and carbon dioxide is excreted from
our blood into the atmosphere.
Although "respiratory disease" is often thought of as only a
lung problem, malfunction of any component of the
respiratory system can cause a breathing problem. For example,
if the brain stem center that controls breathing is depressed, as
may occur with a drug overdose, failure of the respiratory
system will occur (and the victim can die because of it) even
though the lungs are normal. Polio, a common disease before
discovery of the vaccine, can cause respiratory system failure
by
damaging the nerves leading to the thoracic cage muscles. In
this situation the brain and lungs can be normal, but the chest
may not be able to expand in order to move air into the lungs;
as a result, the polio patient may have respiratory failure.
Thus all parts of the respiratory system must function properly
for normal breathing to occur. Yet, despite the importance of
all respiratory system components, there is good reason why
the lungs are usually thought of when one hears about
respiratory disease. Lung disease accounts for the vast majority
of respiratory illness. Emphysema, bronchitis, asthma,
pneumonia, lung cancer Ä all originate in the lungs. Our
lungs
are the only internal organ directly in contact with the
atmosphere, making them vulnerable to all pollutants,
including cigarette smoke, as well as airborne viruses and
bacteria. Our lungs are also the only internal organ in direct
contact with the increased ambient pressure of diving, making
them uniquely vulnerable to pressure changes.
Because most serious diving-related injuries occur from
pressure-related problems and/or from drowning, a more
detailed discussion of gas exchange should enhance your
understanding of scuba diving.
HOW DOES GAS EXCHANGE OCCUR?
Oxygen and carbon dioxide, symbolized O2 and CO2
respectively, are colorless, odorless gases. The atmosphere, or
air around us, contains approximately 21 percent oxygen and
78
percent nitrogen. There is almost no CO2 in air (about 0.03
percent); the carbon dioxide humans and animals exhale is a
negligible part of the entire atmosphere. The nitrogen is inert
and does not take part in gas exchange. (Except in
altered-pressure environments nitrogen is of no consequence.
Nitrogen takes on critical importance when breathing
compressed air under water.) The remainder of the air is made
up of some rare inert gases, such as argon, that also play no
role in gas exchange.
To accomplish gas exchange the air we inhale is delivered, via
our mouth and nose (Figure 2), to tiny sacs, called alveoli,
which are the terminal or end units of the airways (Figures 3
and 4). Oxygen from the air diffuses across a thin membrane
into tiny blood capillaries surrounding the alveoli. At the
same
time CO2 diffuses from the blood capillaries into the alveoli
and out of the lungs with each exhalation. The combination of
one alveolus (containing air) and its surrounding capillaries
(containing blood) is called an alveolar-capillary unit. Both
lungs contain an estimate 300,000,000 alveolar-capillary
units; the surface area of the alveolar membranes, if placed end
to end, would cover a tennis court!
This overview can be expanded by dividing gas exchange into
the
processes of alveolar ventilation (bringing air into the lungs
for transfer of oxygen and carbon dioxide) and pulmonary
circulation (bringing blood to the lungs to take up oxygen
and
excrete carbon dioxide).
Alveolar Ventilation. We inhale the air around us with each
breath. The air enters the mouth and nose (Figure 2). In the
nose and upper airway many of the dust particles are filtered
out, purifying the air. Air from the mouth and nose come
together in the throat and begin the journey into the lungs.
First air enters the larynx (voice box) and then the trachea
(just
below the Adam's apple). The trachea divides into two air
tubes, the right and left main bronchi. The trachea and
bronchial tubes that branch from it are lined with cartilage.
Cartilage provides a firm structure so the airways stay open
when we inhale and exhale.
The trachea and air passages above it (i.e., mouth, throat,
larynx) are collectively called the upper airways or upper
respiratory system. Air entering the upper airways is warmed
to
body temperature and humidified (water vapor is added to it).
(The right and left main bronchi and all airways that lead from
them are collectively called the lower airway system, which is
another term for our lungs.)
The right and left main bronchi represent the first of over 20
divisions of airways to come in the lower airway system
(Figure
2). With each division the air passages become narrower, but
the number of airways increases geometrically. By the 20th
division there are a huge number of individual, tiny airways
and
air has been distributed to each of them. Also at the 20th
division, where the diameter of each airway is less than 1 mm,
air sacs (the alveoli) begin to appear; this is where gas
exchange
actually takes place. Eventually, each airway ends in a grape-
like cluster of these alveoli.
At the alveolar-capillary membrane gas exchange takes place.
Oxygen is delivered to, and carbon dioxide removed from, the
capillary blood (Figures 3 and 4). This gas exchange converts
the oxygen-poor blood entering the pulmonary capillary into
oxygen-rich blood. At the same time the air we inhale (21
percent O2, almost no CO2) has been converted into stale air
(16 percent O2, 6 percent CO2) that we exhale.
Air enters through the mouth and nose, then travels down the
larynx (voice box) and trachea (windpipe). Air then enters the
lungs, which consist (in part) of multiple branching airways
called bronchi. These bronchi end in clusters of air sacs Ä the
alveoli. Each alveolus is surrounded by blood capillaries, which
take up the oxygen and give off carbon dioxide.
Each alveolar sac, or alveolus, is surrounded by one or more
pulmonary capillaries. This alveolar-capillary unit is where
oxygen (O2 ) and carbondioxide (CO2) are exchanged with the
atmosphere.
Each minute, under resting conditions, we breathe in about six
liters of fresh air. About 1/3 of this air stays in the mouth,
throat, and large airways where no gas exchange takes place;
this region (the upper airways and part of our lungs) is
referred
to as "dead space," because air in this space doesn't take part in
gas exchange. The remaining four liters of fresh air breathed in
each minute are distributed to the hundreds of millions of
alveoli and it this air that takes part in gas exchange and
constitutes the alveolar ventilation.
Pulmonary Circulation. Each minute our heart pumps
approximately five liters of blood through the lung capillaries,
distributing blood among the hundreds of millions of alveoli
so gas exchange can take place. Because the lungs are three
dimensional, one alveolar sac may be surrounded by several
pulmonary capillaries. Each alveolus and all of its
accompanying capillaries constitute the gas exchange unit
(Figure 3). If there was no blood flow around the alveolus or
there was blood flow without an accompanying alveolus, there
would be no gas exchange. Thus alveolar ventilation is but one
part of respiration; the other necessary part, the delivery of
blood to the capillaries surrounding the alveoli, is the
pulmonary circulation.
The total circulation of the blood in our body is a circular
affair. Blood flowing from one part of the heart to the lungs
and back to another part of the heart constitutes one part of
this circle; the other part is the systemic blood circulation,
which is blood flowing from the heart to the rest of the body
and then back to the heart (Figure 5). Arbitrarily, we can start
this circle with one of the four chambers of the heart, the
right
atrium.
Blood that enters the right atrium has given up much of its
oxygen to the tissues, and is called venous blood
(oxygen-poor
or de-oxygenated blood). After collecting in the right atrium
of the heart, venous blood then goes to the right ventricle
from
where it is pumped to the lungs in order to receive a fresh
supply of oxygen.
Blood leaving the right ventricle is pumped into the lungs via
one large blood vessel, the main pulmonary artery. This large
artery divides into two smaller pulmonary arteries, one to each
of the lungs. Each pulmonary artery gives rise to many
divisions, and in short order the blood supply pumped from
the
heart is divided among millions of tiny pulmonary capillaries,
the smallest unit of circulation. These capillaries are in
contact
with the hundreds of millions of alveoli, the tiny sacs that
receives the fresh air we inhale. The distance between each
alveolus or air sac, and its surrounding capillaries, is very
short, only the diameter of a thin membrane; oxygen easily
diffuses from the air sac into capillary blood while, at the same
time, CO2 diffuses out of the capillary blood and into the air
sac. The CO2-laden stale air is then exhaled, and fresh air is
brought into the lungs with the next breath.
Oxygenated blood leaving the millions of pulmonary
capillaries
enters the other side of the heart, first the left atrium and
then
the left ventricle. From the left ventricle oxygen-rich blood is
pumped, via the body's arterial circulation, to all the muscles,
tissues and organs. In this way our kidneys, brain, liver, heart,
bones, and other tissues receive vital oxygen. (When an
analysis of arterial blood gas is performed for oxygen
measurements, the blood sample is obtained by inserting a
small needle into the radial artery of the wrist, just behind the
thumb. Unlike venous blood, arterial blood reflects the status
of gas exchange in the lungs and is therefore useful to examine
in patients with many types of lung disease.)
The pulmonary and systemic (non-pulmonary) circulation
systems are schematically shown in Figure 5. The heart,
normally between our two lungs (Figure 1), is here separated
to
show its four chambers and the vessels leading to and from
them. Each pulmonary artery branches into millions of tiny
capillaries before picking up oxygen and giving off CO2 (gas
exchange). Then the millions of capillaries merge to become
the pulmonary veins. The pulmonary veins carry oxygenated
blood back to the left heart, from where it is pumped, via the
systemic arterial circulation, to the organs and tissues of the
body. From these organs and tissues it then returns, via the
systemic venous system, back to the right heart.
After entering the tissue, organ, or muscle, each systemic
artery divides into smaller and smaller vessels, the smallest of
which is the systemic capillary. These capillaries are
structurally similar to the pulmonary capillaries and have the
same function: to allow gas exchange to occur by simple
diffusion. In the lung, oxygen diffuses into the capillaries and
CO2 diffuses out. In all other capillaries (non-pulmonary or
systemic capillaries) oxygen diffuses out of the capillary into
the cells of the organ, and CO2 diffuses into the capillary
from
the cells of the organ. In this way oxygen is delivered for
cellular metabolism and CO2, a waste product of metabolism,
is removed.
Gas exchange is a vital process; it occurs not only in the lungs
but in all other tissues as well. Gas exchange requires both
ventilation, provided by breathing adequate amounts of fresh
air, and circulation, provided by the heart pumping blood to
the lungs (pulmonary circulation) and then out to all other
parts of the body (systemic circulation).
Blood entering the systemic (non-pulmonary) capillaries is
oxygenated. When blood leaves these capillaries it has given up
some (not all) of its oxygen, and is venous or de-oxygenated
blood. Venous blood, which appears blue in our veins under
the
skin, is actually dark red in a test tube. Arterial blood is
normally bright red (although if the patient is low on oxygen
arterial blood will also look dark red).
The systemic capillaries, after delivering their oxygenated
blood
to the tissues, merge and form the veins that carry the venous
blood; eventually all the systemic veins in the body come
together to form the two great vena cavae, the superior, that
carries blood leaving the head and neck, and the inferior, that
carries blood leaving the rest of the body. Both vena cavae
enter
the heart at the level of the right atrium. Blood from the right
atrium enters the right ventricle and then is pumped to the
lungs to once again begin the process of oxygenation.
The trachea and main bronchi are also effective in keeping
dusts
and other large particles from reaching the alveoli. Coughing
is
one way we attempt to clear the large airways of noxious
material.
Even when our nose and normal cough response are not
helping
keep out dusts, our bronchi function silently to move out any
unwanted material. This is accomplished by a blanket of mucus
that covers the bronchi, and tiny hairs (cilia) which sweep the
mucus out of the airways.
Each bronchus is lined with cells that are covered with tiny
hairs, called cilia, that project into the airway. Cilia are
covered
with a blanket of mucus, which serves to collect dusts and
other
pollutants. This "mucus blanket," along with its collected
dusts, is normally swept by the cilia up and out of the airways.
Even if particles get past all of these defenses, special alveolar
cells (called macrophages) are mobilized to help digest any
foreign substances such as bacteria or tiny particles of dust.
All
of these normal defense mechanisms may be damaged from
cigarette smoke, making smokers much more vulnerable to
inhaled dusts and other impurities in the air.
Figure 1
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