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Chapter 5
The principal organs of the respiratory system
are the two lungs, which are in the right and left sides of the
chest (thoracic cavity) and are separated from each other by the
heart. Air passes into and out of the lungs through a series of passages and
tubes called the upper airways. The flow of air depends on other
organs, including the muscular diaphragm and the muscles and
bones that make up the wall of the thoracic cavity (Fig.
5.1).
Part of each lung consists of tubes called the lower
airways; they end in the microscopic saclike alveoli,
which make up most of the lungs. The lower airways transport air to and from
the alveoli. Many pulmonary vessels transport blood throughout
the lungs.
Main Functions for Homeostasis
Working in a coordinated fashion controlled mostly by the
nervous system, these structures perform the two functions of the respiratory system;
gas exchange and sound production. Gas exchange
involves two processes; obtaining oxygen and eliminating carbon dioxide.
The respiratory system obtains oxygen by providing
conditions that allow the oxygen contained in air to pass into the blood
flowing through the lungs. The circulatory system then transports the oxygen
throughout the body. Oxygen (O2) must be supplied to body cells
because it is a raw material used by mitochondria to obtain energy from
nutrients. This energy provides the power needed to perform all essential
bodily activities.
The respiratory system eliminates carbon dioxide (CO2)
by providing conditions that allow it to move out of the blood in pulmonary
vessels and into the atmosphere. Carbon dioxide is transported from body cells
to the lungs by the circulatory system.
Carbon dioxide must be eliminated because it is a waste
product from the series of chemical reactions in mitochondria that release
energy from nutrients. When CO2 accumulates within the body, it can
interfere with body functions because CO2 combines with water to
produce carbonic acid. The excess carbonic acid upsets the acid/base balance of
the body. This disturbance can alter body proteins (see animation).
Body structures and functions can be adversely affected, and serious illness or
death can follow. Still, some acidic materials must be present for the body to
achieve a normal acid/base balance, and a deficiency in acids can be as
disastrous as an excess. Furthermore, some CO2 in the blood is used
to make a buffer. Therefore, the respiratory system must eliminate some but not
too much CO2.
Many other acidic materials in the body contribute to the
acidic side of the acid/base balance. If there is an increase in acidic
substances other than CO2, the respiratory system can help maintain
acid/base balance by eliminating more CO2. This occurs in
individuals whose kidneys do not eliminate acids adequately.
The rate of bodily activities changes from time to time.
These changes cause fluctuations in the rates of O2 use, CO2
production and the amount of other acids. To maintain homeostasis (i.e., continuing good health),
negative feedback systems employing the nervous system normally ensure that the
rate of gas exchange by the respiratory system increases or decreases to meet
these fluctuations. This adaptive mechanism occurs when a person begins to
breathe more heavily soon after beginning vigorous physical activity.
The maximum amount that gas exchange can be increased to
compensate for increases in bodily activity constitutes the reserve capacity of
the respiratory system. The limited nature of respiratory capacity seems to
contribute to setting a maximum limit on how vigorously a person can exercise.
This limit is experienced as the sensation of feeling completely out of breath
while exercising. Limitations in the maximum functional capacities of the circulatory,
nervous, and muscle systems may also play a role in establishing the maximum
rate of physical activity attainable.
Three operations are involved in carrying out gas exchange. Ventilation
(breathing) involves moving air through the airways into and out of the lungs.
Perfusion of the lungs involves the movement of blood through the
pulmonary vessels. Diffusion causes the O2 in inhaled
air to move into the blood while CO2 exits into the air in the
lungs.
Sound production, which is the second main function of the
respiratory system, is important because it helps people communicate. A short
section on sound production, including the effects caused by aging, is
presented at the end of this chapter.
Ventilation involves two phases: inhaling (inspiration)
and exhaling (expiration). Inspiration moves air into the
nostrils and mouth, and down the airways to the deepest parts of the lungs,
where the O2 it contains can diffuse into the blood. Expiration
moves air containing CO2 from the innermost parts of the lungs up
and out of the body.
To understand how ventilation occurs, one must realize that
air around the body is under atmospheric pressure. Materials
normally move from areas of higher concentration or pressure to areas of lower
concentration or pressure. For example, air moves into a balloon and inflates
it when more air pressure is applied to its opening than is already in the
balloon. Conversely, releasing the opening of an inflated balloon results in
air rushing out because the pressure within the balloon is higher than
atmospheric pressure.
Inspiration occurs for the same reason that a balloon
becomes inflated. Air moves into the body when the air pressure outside the
body is greater than that inside the respiratory system. A person creates this
difference in pressure by contracting muscles to move the floor or walls of the
thoracic cavity.
The floor of the thoracic cavity consists of the dome‑shaped
diaphragm, a thick sheet whose edge is muscle and whose center is
fibrous material. The muscular edge slants downward sharply and is attached
around its perimeter to the body wall. When the muscle contracts, the rounded
central region is pulled downward within the body wall, moving like a piston
downward in a cylinder. When the central region is pulled downward and the
diaphragm flattens somewhat, the pressure within the thoracic cavity decreases.
Because moisture on the outer surface of the lungs causes them, in effect, to
adhere to the diaphragm, a similar decrease in pressure occurs within the
lungs. Since the pressure in the lungs is then lower than atmospheric pressure,
air flows through the airways and into the lungs, resulting in inspiration (Fig.
5.2).
The walls of the thoracic cavity contain many bones,
including the ribs, the sternum, and some vertebrae. The cartilage and joints
that connect these bones allow them to move somewhat when the muscles attached
to the bones contract. When muscles move the ribs and sternum upward and
outward, pressure in the thoracic cavity decreases. The lungs, whose surfaces
are stuck to the thoracic walls by fluid, also have a decrease in internal
pressure, and inspiration occurs.
Inspiration usually involves simultaneous movement of both
the diaphragm and the bones of the thorax. Some individuals rely mostly on
movement of the diaphragm (diaphragmatic breathing), while in
others movement of the ribs (costal breathing) makes the major
contribution. Inspiration ends when parts of the body stop moving and enough
air has come in to raise the pressure in the lungs to atmospheric pressure. If
the muscles are held in this position, no further air movement occurs and the
lungs remain inflated. People in this state are truly holding their breath.
Inspiration is called an active process because
it requires the use of energy. Obtaining this energy uses some of the oxygen
that inspiration helps obtain. The energy expended and the oxygen used are
called the work of breathing. Usually not more than 5 percent of
the oxygen brought in by inspiration is consumed in this process; the rest is
available for use by other body cells. Since diaphragmatic breathing is more
efficient and requires less energy than does costal breathing, it consumes less
oxygen. These differences leave more of the oxygen obtained from inspiration
for use by other body cells.
Expiration for a person who is resting and breathing
quietly, normally requires no muscle contraction because the movements of
inspiration set up conditions that allow it to occur automatically. For
example, when the diaphragm moves downward, it pushes on the organs below it in
the abdominal cavity, and this increases the pressure in the abdominal cavity.
Also, the movements of the ribs and sternum stretch and bend elastic and
springy structures in the thoracic wall such as ligaments, cartilage, and the
ribs themselves. Finally, the lungs, which are elastic, are stretched outward.
As a result, as soon as the muscles of inspiration relax,
the abdominal organs, structures in the thoracic wall, and the lungs start to
spring back to their original positions. This elastic recoil
increases the pressure in the lungs. The pressure quickly rises above atmospheric
pressure, and expiration occurs. The process is similar to what happens when
the opening of an inflated balloon is released. Each expiration is followed
shortly by the next inspiration (Fig.
5.3).
Since normal quiet expiration requires no muscle contraction
or energy, it is called a passive process. However, when a person
becomes very active, passive expiration occurs too slowly to meet the needs of
the body. Then respiratory muscles and energy can be used to perform active forced
expiration. For example, abdominal muscles can squeeze on the abdominal
organs, causing them to push upward on the diaphragm more forcefully, and chest
muscles can pull the ribs downward. The resulting increase in pressure in the
lungs pushes air out of the respiratory system quickly (Fig.
5.4).
Rate of
Ventilation (Minute Volume)
Ventilation usually occurs continuously to provide ongoing
replacement of the O2 being consumed and elimination of the CO2
being produced. The rate of ventilation must be high enough to maintain
homeostatic levels of these gases in the body. The rate of ventilation is
called the respiratory minute volume, the volume of air inspired
per breath times the number of breaths per minute. The number of breaths per
minute is called the respiratory rate. Minute volume can be
expressed mathematically:
Minute volume = volume per breath x breaths per minute
Lung Volumes The volume of air inspired equals the amount of air expired.
When a person is at rest and breathing quietly, this volume is called the tidal
volume (TV).
When a person is active and has to exchange gases more
quickly, inspiratory and expiratory volumes can be increased considerably by
increasing the distance the respiratory muscles contract. The extra amount a person
can inspire is called the inspiratory reserve volume (IRV);
the extra amount a person can expire is called the expiratory reserve
volume (ERV). The combination of tidal volume,
inspiratory reserve volume, and expiratory reserve volume is called the vital
capacity (VC). Vital capacity is the most air a person
can expire after taking the deepest possible inspiration. This can be expressed
mathematically:
TV + IRV + ERV = VC
Besides increasing the volume of air respired with each
breath, a person can increase the speed at which the air flows. This is
accomplished by increasing the speed and force of respiratory muscle
contractions, which can magnify pressure changes in the lungs more than 25‑fold.
A person who expires as much as possible still has some air
left in the lungs. This volume is called the residual volume (RV).
Thus, the total amount of air the lungs can hold equals TV + IRV + ERV + RV and
is called total lung capacity (TLC). A small amount
of the TLC does not reach the alveoli but remains in the lower airways. This
volume of air – the dead space – cannot be used for gas exchange
because only the alveoli are thin enough for this process to occur.
Respiratory Rate A normal person may have a respiratory rate of 15 to 20
breaths per minute, but this rate can change as needed. If the volume per
breath remains high, an increase in the rate of respiration increases the
minute volume and therefore the rate of gas exchange. Decreases in the
respiratory rate have the opposite effect on the minute volume and the rate of
gas exchange. Such adjustments in the respiratory rate occur as changes in
bodily activity alter the need for gas exchange.
When the rate of respiration increases, there is less time
for each inspiration and expiration. If a person does not increase the rate of
airflow, breathing becomes rapid but shallow. Such breathing delivers little
fresh air to the lungs for gas exchange.
The major features necessary for proper ventilation include
(1) open airways for easy air movement, (2) defense
mechanisms that assure that only clean, moist, warm air reaches the
lungs, (3) proper pressure changes in the thoracic cavity and
lungs to make the air move, (4) compliance in thoracic and lung
components so that pressure changes cause them to expand easily to accept
incoming air, and (5) control systems that ensure that the
process occurs successfully and at the correct rate.
The contributions made by muscles and skeletal components to
the first four of these requirements were described previously. We will now
consider the ways by which the airways contribute to these requirements.
Nasal Cavities Inspired air entering the nostrils passes
through the nasal cavities above the hard palate. These cavities
are held open by the bones of the skull. As air passes through the nasal
cavities, it is cleaned, moistened, warmed, and monitored so that it does not
harm the delicate structures deep within the lungs (Fig.
5.5).
The air is cleaned because dust and other particles are
trapped by hairs inside the nostrils and by the sticky mucus that coats the
inside of the cavities. Microscopic hairlike cilia on the
cells lining the cavities wave back and forth, causing the mucus to glide back
toward the throat. Then the mucus and its trapped debris can be harmlessly
swallowed.
The air is moistened by the mucus to prevent drying of the
lungs. Heat from the blood in the walls of airways warms the air so that the
lungs are not chilled. Finally, sensory nerve cells monitor the chemical
contents of the air and send impulses to the brain. The presence of such
chemicals is perceived as aromas. The nervous system may cause inspiration to
slow or stop if harmful chemicals or particles are detected. Forced expiration
(e.g., sneezing) may be initiated in an attempt to blow the noxious materials
out of the respiratory system.
A person may inspire some or all of the air he or she
breathes through the mouth rather than through the nasal passages. This can
increase the rate of airflow, but it reduces the amount of cleaning,
moistening, and warming of inspired air. Injury to the airways below the pharynx
may result. Inspiring through the mouth can also lead to excessive dryness of
the oral cavity, which may cause oral discomfort and sores.
Nasopharynx Air in the nasal cavities moves backward
into the nasopharynx, which is above the soft palate. Bones and
other firm tissues keep this passage open except when one is swallowing, during
which the tongue pushes upward on the soft palate. The mucus, cilia, and blood
in the nasopharynx further clean, moisten, and warm the air.
Pharynx After passing through the nasopharynx, the
air moves through the throat, or pharynx, into the opening in the
voice box, or larynx. This opening is called the glottis.
The pharynx is held open by the firmness of the muscles and other tissues that
make up its walls.
Since food and beverages in the oral cavity that are being
swallowed also pass through the pharynx, these materials can lodge in the
pharynx or enter the glottis, blocking or injuring the airways. Two reflexes
controlled by the nervous system prevent these problems.
The swallowing reflex occurs whenever solids
or liquids are present in the pharynx behind the tongue (Fig.
5.6). This reflex clears the pharynx by pushing materials down into the
esophagus. At the same time, a flap called the epiglottis is
moved over the glottis to prevent materials from entering the larynx. The
epiglottis is moved off the glottis after swallowing has been completed so that
ventilation can begin again.
The gag reflex is caused when irritating
materials enter the pharynx. This reflex causes muscles near the pharynx to close
the openings into the larynx and esophagus. At the same time, muscle
contractions in the abdomen raise the pressure in the esophagus and trachea to
prevent materials from entering those passageways. A very strong gag reflex can
result in vomiting.
Larynx, Trachea, and Primary Bronchi Air passing through
the glottis moves through the larynx, down the windpipe (trachea),
and through the two primary bronchi into the lower airways in the
lungs. Plates and rings of springy cartilage within the walls of these airways
provide support so that the airways stay open during ventilation.
Materials other than air that enter these air passages
initiate the cough reflex. During coughing, bursts of air that
are expired rapidly force foreign materials up and out of these airways.
The mucus, the cilia, and blood flow in these structures
carry out further cleaning, moistening, and warming of the air. The cilia beat
in an upward direction so that the mucus glides into the pharynx. Since the
mucus carrying materials slides upward in a smooth continuous stream, this
mechanism is called the mucociliary
escalator. Phagocytic macrophages and immune system cells in the
trachea and primary bronchi provide additional defense against foreign
materials.
Smaller Bronchi, Bronchioles, and Alveolar Ducts As each primary bronchus enters a lung, it branches
repeatedly, forming ever more numerous smaller bronchi and bronchioles
and finally microscopic alveolar ducts.
The walls of these airways become thinner as they branch and
narrow. Cartilage in the smaller bronchi keeps them open during inspiration.
There is no cartilage in the bronchioles or smaller airways. A peculiar helical
structure of the collagen that coils around the airways and elastin fibers also
support these smaller airways. The cartilage and fibers provide the lungs with
compliance. Like the trachea and bronchi, these smaller airways are protected
by the cough reflex and defense cells, and they condition the entering air.
Smooth muscle cells in the airway walls allow for
appropriate adjustments in their diameter as the amount of ventilation needed
fluctuates. The smaller airways provide most of this adaptability. The activity
of the muscle is controlled by the nervous system, the endocrine system, and
nearby chemicals.
As air is expired and the lungs decrease in size, the open
passages in the airways become narrower. The walls of the smallest airways are
so thin and weak that these airways close completely before all the air has
escaped from the alveoli below them. This air remaining in the alveoli makes up
part of the residual volume.
Alveolar Sacs and Alveoli The inspired air in the alveolar ducts
passes into blind cup‑shaped outpocketings called
alveoli. Most alveoli occur in clusters extending outward from
slightly enlarged spaces at the ends of the alveolar ducts, called alveolar
sacs. Each cluster may look like a tightly packed bunch of plump grapes
(Fig.
5.7a and
Fig.
5.7b).
There are about 300 million alveoli in the lungs. Because
alveoli are hollow, filled with air, and very small, and because they make up
most of the lungs, dried lungs have the consistency of Styrofoam. The alveoli
provide an amount of surface area equivalent to that of an area 30 feet long
and 25 feet wide. The walls of the alveoli are very thin, allowing diffusion of
O2 and CO2 between the air and the blood to occur easily.
Special cells in the alveoli secrete a material called surfactant.
As surfactant spreads out, it coats the inner surface of the alveoli and parts
of the smaller airways. The surfactant greatly increases the compliance of the
lungs by reducing the attraction between the water molecules on the inner
surfaces of the lungs. Without surfactant, the attraction (surface
tension) would be so great that the alveoli and small airways would
collapse. The inner surfaces would stick together tightly, making it nearly
impossible for them to separate and fill with air during inspiration. These
characteristics can be compared to the difference between the effort needed to
inflate a new balloon or to put a hand into a new rubber glove that contains a
powdery surfactant, and the effort needed to inflate an old balloon or to put a
hand into an old rubber glove that dried after becoming damp.
Surface tension is important because as it makes the lungs
collapse, it helps increase the pressure in the lungs, and therefore assists in
expiration. The combination of a moderate amount of surface tension in the
alveoli and the large surface area they provide makes expiration much easier.
Nervous System Ventilation begins with inspiration, which
requires the contraction of muscles. The nervous system signals activating
these muscles originate in a region of the brain called the medulla
oblongata and travel to the muscles through nerves. The medulla
oblongata is inside the region of the skull just above the neck (Fig.
6.8). The part of it concerned with respiration is called the respiratory
control center.
The respiratory control center starts inspiration when it
detects an increase in CO2 levels or a decrease in O2 levels
in the blood flowing through it. When sensory nerve cells from the aorta and
arteries in the neck detect very high levels of CO2 or very low
levels of O2, these nerve cells also stimulate the respiratory
control center. Other sensory neurons in the lungs send impulses to the
respiratory control center, telling it that the lungs are in a partially
collapsed condition and are ready for inspiration. Sensory nerves from muscles
and joints inform the respiratory center when a person begins physical activity
and will need more gas exchange.
Nerves from the lungs inform the respiratory center and a
nearby part of the brain called the pons when inspiration is
complete. The brain then stops the impulses for inspiration. As the muscles
relax, expiration begins because of elastic recoil of the thorax and lungs. The
respiratory center can also send impulses to the muscles indicating that a
forceful expiration is needed.
The respiratory center and the pons monitor their own
impulses and are also informed by nerves from the lungs when expiration is
complete. This triggers the beginning of the next inspiration. Therefore, the
linking and repeating of two negative feedback systems result in rhythmic
breathing.
The depth of breathing, the speed of airflow, and the
respiratory rate are adjusted when the respiratory center detects that CO2
concentrations, O2 concentrations, or the acid/base balance in the blood
are beginning to wander from proper levels. The adjustments restore appropriate
gas levels and acid/base balance so that homeostasis (i.e., continuing good health) is maintained.
Ventilation is also modified by the swallowing, gag, and
cough reflexes. In addition, upper regions of the brain such as the areas
controlling emotions and those controlling conscious actions can influence the
respiratory center. The conscious control areas allow voluntary modifications
of ventilation including inspiration, expiration, stopping ventilation, and
modifying ventilation for actions such as talking.
The nervous system also controls ventilation by adjusting
the size of the lower airways. Impulses from the sympathetic
nervous system cause relaxation of smooth muscles in the airways, permitting
them to dilate and increasing minute volume. Parasympathetic
nerves cause the smooth muscles to contract, constricting the airways and
reducing minute volume. These changes allow the rate of gas exchange to
maintain homeostasis for O2 and CO2.
Endocrine System Hormones from the endocrine system also help
regulate ventilation. Norepinephrine makes a main contribution.
This hormone has the same effect on the airways as do impulses from sympathetic
nerves.
Age Changes Affecting Ventilation
Because everyone's airways are subjected to some air
pollution and other environmental insults, it is difficult to know which age‑related
changes in airways are due to aging and which are due to other factors.
However, certain changes seem to occur in all people. These universal changes
are considered age changes and thus are included in this section. Let us
examine how age changes affect the five requirements for ventilation.
Mucus and Cilia All airways from the nasal cavities to the
smallest bronchioles produce mucus continuously. With aging, the mucus produced
is more viscous and therefore more difficult to move. In addition, both the
number and the rate of motion of the cilia decrease. As the clearing out of
mucus slows, the accumulation of mucus narrows airways, and this inhibits
ventilation. When ventilation becomes more difficult, the work of breathing
increases and extra CO2 is produced by the muscles of ventilation,
making respiration less efficient. Narrower airways also reduce the rate of airflow
and therefore reduce the maximum possible minute volume.
Airway Structure Age changes in the walls of bronchioles
cause them to become even narrower, amplifying the effect of mucus
accumulation. In addition, the bronchioles close earlier during expiration,
trapping more air in the smaller airways and in the alveoli, especially in the
lower parts of the lung. One result is an increase in residual air. This causes
the fresh air entering with each inspiration to be mixed with a larger amount
of stale residual air, decreasing the rate of diffusion. A second result is
uneven lung ventilation.
While the bronchioles become narrower, the larger airways in
the lungs and the alveolar ducts increase in diameter. These changes compound
the negative effects by increasing the dead space. Thus, fresh inspired air is
mixed not only with more residual air in the smallest airways and alveoli but
also with more dead space air. This further decreases the rate of diffusion.
The increase in tidal volume with age may help minimize the
expected drop in the diffusion rate during tidal breathing by mixing more fresh
air with the increasing amounts of stale air remaining in the respiratory
system. The rate of diffusion remains high because the O2
concentration in the lungs is kept high while the CO2 levels are
kept low.
Since age changes in the mucus and cilia cause slower
movement of mucus, harmful materials such as microbes, particles, and noxious
chemicals trapped by the mucus stay in the respiratory system longer. Aging
also decreases the functioning of other defense mechanisms, including reflexes
(see below), white blood cells, and the immune system. All these age changes
cause an increase in the risk of developing respiratory infections and other
respiratory problems.
Muscles As with most muscles, aging causes
respiratory muscles to become weaker. The decrease in muscle strength is not
enough to detract from performing tidal breathing or ventilating at moderately
increased minute volumes. However, it slowly decreases the maximum pressure
changes that the muscles can produce and thus decreases the maximum rate of
airflow attainable.
Skeletal System Age changes in the cartilage, bones, and
joints of the thorax also reduce a person's ability to produce large pressure
changes in the thoracic cavity. The cartilage attaching the ribs to the sternum
becomes more calcified and stiff, and the ribs become
less elastic. Age changes in the cartilage and ligaments of other joints, such
as those between the ribs and the vertebrae, result in decreases in the ease
and range of motion of the bones they connect.
Aging also leads to slight alterations in the positions of
the bones of the chest. The chest becomes deeper from front to back, making
deep inspiration more difficult. Altered posture from other changes in the
skeletal system further reduces a person's ability to inspire quickly and
fully.
Because of these skeletal age changes, there is a decline in
the maximum minute volume attainable and an increase in the work of breathing.
Older people partially compensate for these effects by relying more on
diaphragmatic breathing.
Lungs Though there are no important age changes in
the elastic fibers or surfactant, other age changes in the lungs significantly
affect pressure changes. For example, aging causes the coiled collagen fibers
in the lungs to become somewhat limp and less resilient. Also, aging causes the
alveoli to become shallower, and this reduces the amount of surface area
present. The resulting reduction in surface tension decreases elastic recoil.
Both of these age changes reduce the maximum rate of expiration attainable and
add to the work of breathing (Fig.
5.8).
Aging causes the coiled collagen fibers to become somewhat
limp and stretch more easily. These changes increase the compliance of the
lungs and tend to make inspiration easier. Note, however, that the increase in
lung compliance is much less than the increase in chest stiffness caused by
skeletal age changes. Thus, there is a net increase in stiffness of the
respiratory system, resulting in decreased ability to inspire.
Aging does not seem to affect the contributions of the
nervous system to rhythmic breathing under resting conditions. However, three
types of age change reduce the ability of the nervous system and endocrine
system to cause adaptive changes in ventilation:
1. Neurons monitoring O2, CO2, acid/base
balance, and muscle activity seem to become less sensitive to changes in these
parameters.
2. There may be changes in the nervous pathways through which
all their impulses are sent, resulting in altered ventilation.
3. The lungs become less sensitive to norepinephrine from
sympathetic nerves and the endocrine system.
These age changes result in a slower and smaller increase in
minute volume when there is a decrease in O2 or an increase in CO2,
acids, or body activity. As a result, individuals who begin vigorous activity
feel out of breath and tire more quickly as they get older.
Age changes in other parts of the nervous system reduce its
ability to provide the swallowing, gag, and cough reflexes that defend the
respiratory system (Chap. 6). Because of these changes, it takes a greater
amount of material and a longer time to start a defensive reflex. Once it
begins, the response is slower and weaker.
Therefore, older individuals must avoid situations that
raise the risk of choking. These include eating quickly; talking or laughing
while eating; eating while lying on one's back; and eating after consuming
alcoholic beverages or medications that slow the reflexes.
Reductions in pressure changes caused by weakening of
muscles, stiffening of the respiratory system, and decreased alveolar surface
area combine with narrowing of the airways to produce two effects. First, they
cause a decrease in the rate at which air can flow in the system. Second, they
make ventilation more difficult and therefore increase the work of breathing.
This reduces the amount of available O2 and increases CO2
in the blood.
Although aging does not change the total lung capacity, age
changes affect the volumes of air that can be moved. The more rapid closing of
bronchioles, together with stiffening of the system, causes a decrease in both
inspiratory and expiratory reserve volume. At the same time, tidal volume increases
somewhat, and the age changes cause an increase in residual volume both at rest
and during increased ventilation. These changes in volumes cause the vital
capacity to decrease.
These changes in respiratory volumes have two effects.
First, they further decrease maximum minute volume. Second, the decrease in
vital capacity, combined with the increase in residual capacity, means that
less fresh inspired air is mixing with more stale air remaining in the lungs.
This change decreases the rate of diffusion. The problem is compounded by the
increase in dead space. The age‑related increase in tidal volume may help
compensate for this problem during quiet breathing (Fig.
5.9).
The force of gravity on the lungs causes the lower
bronchioles to close sooner than do those in the upper regions. Therefore, the
lower parts have a higher proportion of the residual air than do the upper
regions. This unevenness in ventilation increases with age. As seen below in
the discussion of perfusion and diffusion, this further decreases the
efficiency of the system. Thus, as people get older, they must ventilate more
air to get the same amount of gas exchange, and this adds to the work of
breathing. Breathing more deeply can partially overcome the deleterious effects
of uneven ventilation. Aging also reduces the maximum respiratory rate (breaths
per minute) because of age changes that slow airflow and age changes in the
nervous system.
These decreases in maximum flow rate, maximum volume per
breath, and maximum respiratory rate combine to cause a decrease in the maximum
minute volume. Many individuals can expect their maximum minute volumes to
decline by 50 percent as they pass from their twenties to very old age. This
change makes a major contribution to the decrease in the maximum rate of gas
exchange as people age. Age changes in perfusion and diffusion, discussed below,
cause additional decreases in gas exchange.
Recall from Chap. 4 that perfusion is the passage of blood
through the vessels of body structures. Perfusion of the lungs proceeds as
follows.
The right atrium receives blood from systemic veins
from all parts of the body except the lungs. This blood has little oxygen
because the oxygen was removed and used as the blood flowed through capillaries
and past body cells. Therefore, this blood is called deoxygenated blood.
It also has a high concentration of CO2, which diffused into the
blood from the body cells (Fig.
5.10(a), Fig.
5.10(b), Fig.
5.10(c)).
Deoxygenated blood from the right atrium flows into the
right ventricle, which then pumps it through the pulmonary arteries
to the lungs. As these arteries enter and pass through the lungs, they branch
into smaller vessels until they enter the thin‑walled pulmonary
capillaries. These capillaries carry the blood close to the walls of
the alveoli. This allows gases to diffuse between the blood in the capillaries
and the air in the alveoli. The blood then enters pulmonary veins,
which carry blood to the left atrium. Since this blood has a high concentration
of O2, it is called oxygenated blood. It also has a
low concentration of CO2. This blood will pass from the left atrium
into the left ventricle, which will pump it through the systemic arteries
to all parts of the body except the lungs.
Like the rate of ventilation, the rate of perfusion must
vary as a person's rate of activity, and therefore the need for gas exchange,
varies. Blood flow to an area of the body can be changed by altering the
cardiac output and changing the diameter of the arteries delivering blood to
body structures.
There are essentially no age changes that affect the
pulmonary arteries and veins. Furthermore, aging does not change cardiac
output.
The reason for the minimal change in pulmonary vessels compared
with other vessels in the body may be that blood pressure in the pulmonary
vessels is much lower than that in the systemic vessels. When
diseases such as emphysema cause a rise in pulmonary artery pressure, these
arteries undergo changes that resemble atherosclerosis.
Though pulmonary arteries and veins remain largely unchanged
by aging, the pulmonary capillaries decrease in number and accumulate some
fibrous material. Whether these are true age changes or are due to the effects
of air pollution is uncertain.
Normally, the reduction in perfusion due to changes in
pulmonary vessels is slight. The effect on reducing gas exchange does not
become apparent until the respiratory system is called on to deliver the
maximum rate of gas exchange. Even then, this causes only a small reduction in
maximum gas exchange.
However, heart disease and certain types of pneumonia and
emphysema can reduce perfusion of the lungs. Such reductions decrease the rate
of gas exchange and therefore decrease the ability of the respiratory system to
maintain homeostasis of O2, CO2, and acid/base balance.
These effects are often noticed as the sensation of being out of breath and being
fatigued when one engages in vigorous physical activity.
Recall that the alveoli are the destinations for inspired
air. Their great numbers and deeply curved surfaces provide a great deal of
surface area.
The walls of the alveoli are only one cell thick, and the
cells are flat and very thin. Thin-walled capillaries surround the alveoli.
Only an exceedingly thin noncellular layer (basement membrane)
separates the alveolar wall from the capillary wall. These structural features
provide a thin surface through which gases must pass. The secretions from some
alveolar cells keep their surfaces moist. Thus, the alveoli supply a large,
thin, and moist surface that is ideal for the diffusion of gases. Diffusion of
O2 and CO2 in the lungs proceeds as follows (Fig.
5.10(c)).
The blood entering the pulmonary capillaries has a very low
concentration of O2 and a high concentration of CO2. The
air in the alveoli, by contrast, has a high level of O2 and a low
level of CO2 because ongoing ventilation continuously refreshes the
alveolar air. Therefore, O2 diffuses from the alveolar air into the
blood while CO2 diffuses from the blood into the alveolar air.
Much of the CO2 that diffuses into the alveoli is
eliminated with the next expiration, and the following inspiration delivers a
new supply of O2 into the alveoli. Only a very small amount of the O2
that enters the blood can be carried by the plasma. Almost all the oxygen in
the blood is bound to hemoglobin molecules, which are contained in the red
blood cells. Each hemoglobin molecule can bind up to four molecules of oxygen.
When hemoglobin binds oxygen, the result is oxyhemoglobin.
Decreased CO2, increased pH, or decreased temperature of the blood
increases the amount of oxygen that can be bound to each hemoglobin molecule;
the converse is also true. Normally, this promotes complete oxygenation of
blood in the lungs, where ventilation keeps CO2 levels and
temperature low. It also promotes greater release of oxygen in other
capillaries, where body cell activities keep CO2 levels and
temperature high. These characteristics of hemoglobin are sometimes displayed
as oxyhemoglobin dissociation curves.
Continuous perfusion, coupled with continuous ventilation, keeps
diffusion occurring in an uninterrupted fashion. Furthermore, alterations in
the rate of ventilation or perfusion can alter the rate of diffusion to meet
bodily needs. Increasing ventilation (i.e., minute volume) or perfusion
increases the differences in concentrations of O2 and CO2
between the blood and alveolar air. These changes increase the rates of
diffusion and gas exchange. Reducing ventilation or perfusion has the opposite
effects.
Aging causes several changes that reduce the maximum minute
volume of ventilation, increase residual volume, and cause uneven ventilation
in the lungs.
Age changes in the alveoli further decrease the rate of
diffusion. The alveoli become flatter and shallower, decreasing the amount of surface
area. The alveolar membrane that remains becomes thicker and undergoes chemical
changes which further impair diffusion (Fig.
5.8).
Effects from Altered Gas Exchange
In summary, essentially all aspects of the respiratory
system involved in gas exchange are detrimentally affected by aging, resulting
in a drop in the maximum rate of gas exchange. Furthermore, there is an overall
decline in the efficiency of this system. Finally, the ability of the
respiratory system to adjust the rate of gas exchange to meet body needs
declines. These changes occur at a fairly steady rate throughout life.
As these changes occur, the maximum rate at which a person
can perform physical activities declines, and a person who starts a vigorous
activity such as running or climbing stairs will feel tired and out of breath
sooner as age advances. Such an individual will not be able to perform at top
speed for an extended period. Age changes in other systems, including the
circulatory, skeletal, muscle, and nervous systems, may contribute to these
decrements. The consequences of these effects can be reduced by raising one's
pace gradually. Doing this provides extra time for respiratory functioning to
adapt to the increased need for gas exchange. Also, going at a more moderate
pace lowers the required rate of gas exchange.
Although aging causes reductions in several maximum
respiratory values, these age changes are observed only when people require
that the respiratory system function at maximum capacity. This system has such
a great reserve capacity that the decline in maximum values caused by aging has
essentially no effect on a person who engages in light or moderately vigorous
activities. Thus, unless a person engages in activities such as very demanding
physical work or highly competitive athletic events, age changes in respiratory
functioning have little noticeable effect on his or her lifestyle. The aging
respiratory system can provide adequate service in all but the most physically
demanding situations.
Factors other than aging alter gas exchange. Also, much can
be done to minimize age‑related reductions in the ability of the
respiratory system to satisfy the need for gas exchange. For example, a
sedentary lifestyle further limits respiration, while regular exercise keeps
the decline in maximum minute volume small. Furthermore, incorporating adequate
vigorous physical activity into one’s lifestyle can restore much of the decline
in respiratory functioning caused by inactivity.
Another factor that adversely affects respiration is air
pollution. Breathing polluted air seems to increase both the speed and severity
of essentially every age change in the respiratory system mentioned thus far.
People who smoke, live in areas where air quality is poor, or engage in
occupations where the air contains dust, fine particles, or noxious chemicals
have a much faster and greater loss of respiratory functioning. In addition,
these individuals are at higher risk for developing respiratory diseases,
including lung cancer, chronic bronchitis, and emphysema. Air pollutants can
injure respiratory cells and tissues in several ways including physically,
chemically, and through free radicals, microbes and immune responses. Radon
damages lung tissues through the radiation it causes and the free radicals it
induces. Breathing polluted air can be reduced by avoiding polluted areas; by
not smoking; by wearing a protective mask; and by providing adequate
ventilation with clean air in living and working areas.
The biological effects of decreased gas exchange can affect
other aspects of life. The nature and degree of these effects depend on the
amount and importance of physical activity in a person’s life. Examples of
people who may be affected more dramatically include people whose chief form of
recreation and social contact is competitive sports and people whose jobs
involve considerable physical exertion.
Finally, changes in gas exchange can be affected by other
types of age changes. For example, upon retirement, a sedentary office worker
may take up a physically demanding sport, which may provide the motivation to
stop smoking. The result can be a slowing and even a temporary reversal of the
decline in respiratory capacity.
Diseases of the Respiratory System
While age changes in the respiratory system have only a
small impact on the ordinary activities of daily living, changes caused by
disease can have a substantial effect. Respiratory diseases reduce a person's
speed and endurance in physical activities and cause significant disability.
Treatment can extend for long periods and is often expensive. Furthermore,
respiratory system diseases (not including cancer of the lungs) are the third
leading cause of death for those over age 65. If lung cancer is added,
respiratory disease ranks as the second leading cause of death among the
elderly. (Suggestion:
Chap 05 - 109-1-3)
The reasons for the high incidence of respiratory diseases
among older people are similar to those for other diseases. They include
reductions in defense mechanisms; more time for the development of slowly
progressing diseases; and increases in the number of exposures and the total
time of exposure to disease‑promoting factors.
There is one factor that contributes to the development of
virtually all these diseases: air pollution. One of the most
common forms is smoking and inhaling smoke from tobacco products. Though the
proportion of smokers in the population has declined, the effects of smoking
among older people will be evident for many years because many older people
have smoked for long periods. The rate of decline of the respiratory system
slows when a person stops smoking, and there may even be a period of
improvement in gas exchange. However, most effects from long‑term smoking
are not reversible.
Other forms of air pollution include particulate matter such
as dust from coal mining, woodworking, farming, and the manufacture of fabrics.
Fumes and vapors such as those from painting, chemical plants, and scientific
laboratories can harm the lungs. Smog, automobile fumes, and other types of air
pollution associated with urban environments are also significant risk factors
for lung damage.
Reducing the inspiration of air pollutants can significantly
reduce both the incidence and severity of respiratory disease. Doing this will
preserve much of the capacity for gas exchange by the respiratory system.
Respiratory diseases that are most common among people of
advancing age include lung cancer, chronic bronchitis, emphysema, pneumonia,
and pulmonary embolism. These diseases and two other abnormal conditions (sleep
apnea and snoring) will be considered here. In examining these respiratory
diseases and abnormal conditions, keep in mind that the ability of hemoglobin
to bind oxygen is affected by CO2, pH, and temperature. Respiratory
diseases and conditions can reduce ventilation, leaving more CO2 in
the blood and more warm air in the lungs. These changes reduce the ability to
oxygenate blood not only because the O2 supply to the lungs is
reduced. The elevated CO2 reduces the
pH in blood in the lungs, and the blood remains somewhat warmer. Under these
conditions, the hemoglobin in blood passing through the lungs cannot pick up
and hold as much oxygen. Therefore, the hemoglobin in the blood passing through
the lungs cannot transport as much O2 to body cells.
Normally, cells reproduce when the body needs more of them;
once the need is met, they stop reproducing. An example is the temporary rapid
reproduction of skin cells that occurs until a cut in the skin heals. Cancer
consists of cells that continue to divide and spread out in an uncontrolled
fashion even when they are not needed. A clump of these cells is called a tumor.
Some forms of cancer develop from lung cells and are called primary lung cancer. These are the types caused primarily by smoking. Many other cancers of the lungs develop when the circulatory system moves cancer cells from another place in the body to the lungs. Cancer that moves to another part of the body is called metastatic cancer. Metastatic lung cancer often comes from the breasts or the reproductive system. (Suggestion 109.02.03)
A person with lung cancer may have from one tumor to very
many tumors. Whether the cancer is primary or metastatic, the effects on the
lungs are similar. Ventilation becomes more difficult because airways get
blocked when tumors grow inside them or squeeze them closed. Air volumes are
reduced as alveoli become filled with cancer cells. Occasionally the cancer
becomes so large or stiffens the lungs so much that they cannot inflate or
deflate adequately for ventilation. Cancer cells in the alveoli may reduce
diffusion by thickening or replacing the respiratory membrane between the air and
the blood. Sometimes cancer will distort, squeeze, or replace the pulmonary
vessels so that perfusion is reduced. Some blood vessels are weakened, causing
bleeding.
Several warning signs indicate that lung cancer may be
present. They include a persistent cough, coughing or spitting up blood, pain
in the chest, difficulty swallowing, hoarseness, easy fatigability and the
feeling of breathlessness, and a swelling of the fingertips. Any of these
indicators warrant evaluation by a physician.
Though some forms of lung cancer can be cured if discovered
early enough, most cases are not identified until the cancer has grown so much
that it cannot be eliminated. The vast majority of cases of lung cancer result
in death within a few years. The only effective "cure" is prevention:
avoiding tobacco smoke and other forms of air pollution.
To understand chronic bronchitis, recall that the trachea and
bronchi are lined with a thin layer of mucus and that as the mucus is made,
cilia sweep it up and out of the airways.
Development If a person inspires air with an excess amount of harmful particles or noxious chemicals, the cells
lining the trachea and bronchi become injured. The resulting inflammation
causes those cells to make mucus much faster, and the lining of the airways
becomes swollen. In addition, the beating of cilia slows. The person now has
bronchitis and will begin to cough to remove the extra mucus and pollutants.
If this person breathes the pollutants frequently and
continuously, the airways remain inflamed for a longer time, and the person
then has chronic bronchitis. This condition is accompanied by
extra mucus production and coughing. After a while the cilia will be damaged
and may completely disappear.
Effects The major effect of chronic bronchitis is to
reduce ventilation by making the airways narrow in two ways. First, mucus
accumulates because it is being produced more quickly and removed more slowly.
Second, the lining of the airways swells inwardly. The effect on airflow
through the trachea and bronchi is similar to the stuffed‑up feeling that
occurs when a head cold causes swelling and the accumulation of mucus in nasal
passages.
Expiration becomes especially difficult because the lower
airways normally narrow during expiration. The additional narrowing from the
mucus and swelling makes them so narrow that expiration can occur only very
slowly. This decreases the minute volume, and so having enough gas exchange to
meet the bodys needs is quite difficult. The problem is compounded
because the person will begin to rely more on forced expiration, increasing the
work of breathing. The effort used in coughing raises the work of breathing to
levels that may leave the victim dizzy, breathless, and temporarily
incapacitated.
The problem becomes very serious when the person tries to do
something physically active. Fatigue and the sense of being out of breath
develop quickly and are rather severe. Some individuals are disabled by this
disease.
Fortunately, many cases of chronic bronchitis that have not
been allowed to progress too long can be cured. The person need only eliminate
breathing polluted air. Eventually, mucus production will slow and the cilia
will grow back and begin to function as before.
Curing chronic bronchitis can be difficult if smoking is the
source of the air pollution, however, because tobacco smoke contains addictive
chemicals such as nicotine. Also, as the respiratory system begins to clear
itself, coughing increases temporarily. Smokers often experience extra coughing
in the morning because the clearing action began during the night, when they
were not smoking. After a period of abstention, smoking seems to help because
it relieves the withdrawal symptoms and stops the clearing action, and thus
stops the coughing. Of course, continuing to smoke only relieves certain
symptoms while the disease continues to destroy the person's respiratory
system.
Besides reducing directly the
performance of the respiratory system, chronic bronchitis increases the risk of
infection of the respiratory system because the accumulation and slow removal
of mucus allow microbes to flourish in the airways. It can also lead to emphysema,
and the chronic coughing contributes to hemorrhoids. Long‑term smoking is
also a major risk factor for nonrespiratory diseases
such as heart attack, atherosclerosis, and stroke.
Emphysema
is a disease that involves actual destruction of some parts of the lungs. There
are two main forms: centrilobar
emphysema (CLE) and panlobar
emphysema (PLE). Both types will be present in most
people with emphysema, though one type will predominate.
Centrilobar Emphysema Centrilobar
emphysema most often develops along with or after chronic bronchitis. It
involves a thinning and weakening of the smallest bronchioles and the
production of much mucus. Many results are similar to those of chronic
bronchitis. Additionally, the damage to the bronchioles usually results in a
decrease in the number of small blood vessels in the lungs, decreasing
perfusion. The reduction in blood vessels also makes it harder for the heart to
pump blood through the lungs, and the overworked heart eventually becomes
weaker. If CLE continues to progress, the victim eventually dies of respiratory
failure, respiratory infection, or heart failure.
Panlobar Emphysema Panlobar emphysema
is less common than CLE. Though the major cause is air pollution, some people
inherit a tendency to develop this type of emphysema.
PLE causes destruction of the walls of the alveoli and
alveolar sacs. The results are like a highly exaggerated version of age changes
in the alveoli. Many walls between the alveoli shrink and disappear.
Neighboring alveoli blend to form large air‑filled spaces. The lungs
change from having microscopic spaces like those found in Styrofoam to having
large spaces like those in a sponge. The wall material that remains is weaker
and less elastic. All these changes reduce ventilation.
With PLE, expiration becomes more difficult and more
residual air is left in the lungs. As passive expiration decreases, forced
expiration increases, increasing the work of breathing. Perfusion also
decreases because the number of capillaries declines as the alveolar walls are
destroyed. Besides reducing gas exchange, this overworks the heart,
occasionally leading to heart failure. Finally, diffusion is reduced because
there is a decrease in the amount of surface area.
A complication of PLE is the partial or complete collapse of
a lung. This occurs when a hollow space developing close to the lung surface
bursts like a bubble. As escaping air separates the lung from the thoracic
wall, the lung collapses like a balloon with a small leak. This condition is
called pneumothorax. Proper inspiration is impossible unless the
leak heals and the body absorbs the air from the thoracic cavity.
Overall Effects of Emphysema People in the early
stages of emphysema may hardly notice the decline in their ability to perform
physical activities. As the disease progresses and devastates more of the
lungs, gas exchange plummets. Eventually, even walking at an ordinary pace
becomes a challenge. Individuals with advanced cases are so disabled that they
may be unable to get up, sit up, or even roll over in bed without extreme
fatigue. Mild exertion or a slight respiratory infection can cause death. Among
people over age 55, emphysema is the fifth leading cause of death for men and
the seventh leading cause for women.
Pneumonia
is actually a group of related diseases involving inflammation in the lungs.
Several types reduce a person's ability to inspire. Older people are especially
affected by pneumonia caused by microbes (bacteria, viruses, and fungi) and by
dust and chemical vapors. Pneumonia can also result from aspirating stomach
contents that have moved up into the throat.
Microbial Pneumonia Reasons for the age‑related increased
susceptibility to microbial pneumonia include age changes in the functioning of
the mucociliary escalator, white blood cells, and the
immune system; the rising prevalence of chronic bronchitis and emphysema; and
other diseases that weaken the body and make it less able to ward off
infections.
Pneumonia caused by bacteria results in filling of the
airways and alveoli with fluids and cells from their walls. This material
blocks the airways. It usually becomes somewhat solid after 1 to 2 days. If a
person is otherwise healthy and receives proper treatment, such as antibiotics,
the infection can be overcome and the material will be cleared away after about
a week.
Many types of bacteria that cause pneumonia leave the lungs
with no residual damage. However, some forms cause serious and permanent damage
that results in a reduction in respiratory functioning and can cause death.
These forms are the ones most likely to occur in weakened or hospitalized
individuals.
Viral pneumonia affects the walls of the alveoli, causing
them to accumulate fluids and become thicker, reducing gas exchange. If a
person is healthy and has a good immune system, the immune response will
eliminate the virus in a few days and the lungs will return to normal
functioning.
Because fungal pneumonia, Coronavirus COVID-19 and
tuberculosis cause death of the portions of the lungs they infect, they can be
more serious than bacterial or viral pneumonia. Thus, after fungal, COVID-19
and tubercular infections are stopped, the lungs are left with regions that no
longer function. Areas affected by tuberculosis are filled in with solid scar
tissue which, if calcified, can be detected on x‑ray. If enough areas of
the lungs are destroyed, gas exchange and activity levels are permanently
reduced. More extensive damage results in death.
Unfortunately, many older individuals are not healthy and do
not have strong immune systems when they get pneumonia. Weakened persons may
have great difficulty combating the infection. Then the disease lasts longer
and has a greater impact on the respiratory system. The proportion of
individuals who survive microbial pneumonia decreases rapidly with age. Those
who survive are often left weakened for long periods.
Dust and Vapors Some individuals breathe large amounts of
certain types of air pollution repeatedly for long periods, usually because of
their occupations. Examples include farmers, miners, textile mill workers,
sandblasters, and woodworkers. The heavy exposure and the size and chemical
natures of such air pollutants cause the lungs to form large quantities of
fibers and develop the condition called pulmonary fibrosis.
With pulmonary fibrosis, the normal amount and rate of age
changes in the lungs increase dramatically, leading to a rapid decline in gas
exchange. Very severely affected people will become quite disabled. Since the
fibrosis is permanent, affected individuals can recover little if any of the
lost respiratory functioning even if they avoid future exposure to air
pollution. The only solution is to prevent pulmonary fibrosis by avoiding its
causes.
Pulmonary embolism
(Chap. 4) is a disease condition in which blood clots have moved to the lungs
from the systemic veins or the heart. Conditions commonly promoting the
formation of such emboli in the elderly include varicose veins, congestive
heart failure, and immobility. The elderly are especially
prone to having conditions that cause immobility. These include heart attack,
stroke, hospitalization, recovery from surgery, and fractures. The effects of
pulmonary embolism depend on the size and number of pulmonary emboli.
Main consequences in the respiratory system from smoking
have been mentioned. Smoking has adverse effects in other areas of the body,
also. In general, smoking increases the formation of free radicals and lipid
peroxides while reducing the antioxidant actions of vitamin C, vitamin E, and
β carotenes. Smoking may increase free radical damage to DNA by 50
percent. In the skin, smoking speeds up and amplifies the effects from aging
and from photoaging. Smoking is associated with increased risks for most skin
cancers. In the circulatory system, smoking damages the endothelium; raises
blood pressure; and increases substantially the risk of blood clots, of
atherosclerosis, and of their complications. Effects on these two systems are
due partly to constriction of skin vessels and reductions in blood oxygen
caused by smoking. These two changes develop within minutes of initiating
smoking and can last for hours, long enough to light the next cigarette. The
result is continuous inadequate blood flow in the skin and elevated blood pressure.
In the eyes, smoking is associated with a higher incidence of cataracts and
diseases of the retina. Smoking reduces estrogen levels in women and speeds up
age-related thinning of bones. Smoking doubles the problems from non-insulin
dependent diabetes; suppresses normal functioning of the immune system;
promotes autoimmune diseases; reduces the sense of taste, the benefits from
some vitamins, and liver function; and is associated with higher rates of
reproductive system and digestive system cancers. Cessation of smoking is
associated with reduction or complete reversal of these problems and risks.
Two age changes involving the control of ventilation that
have not yet been discussed are sleep apnea and snoring.
Sleep Apnea A person has sleep apnea
(SA) if he or she exhibits at least five temporary cessations of
ventilation per hour or exhibiting at least 10 occasions of depressed
ventilation and cessation of ventilation per hour when asleep. The incidence of
sleep apnea increases with age up to age 65, after which the incidence
plateaus. It is present in 4 percent of younger adults but in 25 percent to 30
percent of people over age 64. The male:female ratio
for SA is approximately 3:1.
Sleep apnea may be caused by narrowing and collapsing of the
pharynx, especially when in a supine position (i.e., sleeping on one's back);
because the respiratory center becomes less sensitive; or because the center
simply stops initiating inspiration. Then blood levels of O2 and CO2
change. These alterations in the blood may provide the necessary stimulation to
begin inspiration again. People with sleep apnea tend to snore and to have
frequent sudden awakenings with feelings of respiratory distress.
Mild sleep apnea seems to have no deleterious affect on the body. However, frequent awakenings can lead
to fatigue, indications of sleep deprivation, and adverse alterations in mood
and personality. Because sleep apnea causes significant fluctuations in O2,
CO2, and blood pressure, serious cases increase the risk of heart
attack and stroke. Treatments for SA include avoiding sleeping in a supine
position; using masks with pumps that provide positive pressure into the
respiratory system; reversal of obesity; medications; and surgical correction
of the pharyngeal region.
Snoring Snoring, or making loud breathing sounds when asleep, is due to
partially obstructed upper airways. Approximately half of all women and well
over half of all men above age 65 snore. Some individuals who snore also have
sleep apnea.
Snoring causes a variety of problems. Biologically, it
causes from mild to severe adverse effects on blood O2 and CO2
levels and on circulation. It can also contribute to high blood pressure and
heart disease. Since snoring disrupts normal sleep patterns even if the person
who is snoring does not awaken, it can result in fatigue and other indications
of sleep deprivation.
Anyone who sleeps near a person who snores can attest to
some of the social implications. Their responses to the person who snores,
together with the multitude of jokes about snoring, can add to the
psychological impact produced by sleep deprivation. The fatigue felt by many
snorers also affects their social interactions and can impinge on their ability
to carry out their jobs.
Though the causes of snoring and the role of the nervous
system in snoring are not clear, research has provided methods of treatment for
this condition.
The vocalizations produced by people involve words and a
variety of other sounds, such as moans, grunts, whistles, cheers, and laughing.
People use sound production for communication. Communication among individuals
by sound and other means (e.g., visual signals) is important to a high quality
of life and to survival because it is one of the three components in negative
feedback systems. A common example of using vocalization as part of a negative
feedback system is shouting a warning to a person in danger.
Human sound production can enhance life in other ways. Words
and other vocalizations can motivate and encourage positive actions such as
beginning a new career or hobby. They are also used in teaching, praising,
consoling, expressing emotions, and many other human activities. And what of
the beauty of a poem or a song? All these are created by the sounds produced by
the flow of air through airways.
The respiratory system produces sound by passing air through
the upper airways and the mouth. Most of the sound people make is caused when
air passing through the larynx causes the vibration of two flaps of tissue
called the vocal cords (Fig.
5.5). The sound gets louder when more air flows through the larynx.
Different muscle contractions in the larynx control the
position and tension of the vocal cords and thus alter the pitch of sounds. The
sounds made by the vocal cords are modified by the other upper airways,
especially the nasal passages and the mouth. By changing the shape of these
passages and moving the tongue, a person can create a multitude of sounds and
form words.
All the actions that produce and modify sounds from the
respiratory system are controlled by the nervous system.
Many age changes that alter inspiration and especially those
which modify expiration affect sound production. Age-related stiffening of the
larynx, shrinkage of the vocal cords and its muscles, and changes in the mouth
are also important. Age changes in the nervous system are also important since
sound production depends on the coordinated action of many muscles. Even age
changes in hearing are important because the ears provide feedback information
so that the sounds a person produces can be adjusted to conform to the sounds
intended by that person.
Because of age changes in these areas, the voice becomes
more variable in pitch and volume during speaking. Female voices often become
lower in pitch, while male voices often become higher in pitch. Other common
changes include increases in hoarseness, roughness, and extraneous sounds while
speaking. The voice often becomes weaker, and elders have declining abilities
to speak very quietly or with very loud volume. The ability to control volume
declines, and the precision of word pronunciation diminishes.
Language fluency and vocabulary usually do not decline, and
often increase. However, phrases and sentences often become shorter, syllables
and words are repeated more often, and more words are pronounced incompletely.
These trends in speaking become more prominent in stressful situations. Of
course, variability among elders increases with age, and some elders retaining
the voice and speech of a young adult.
All these changes reduce the effectiveness of vocalization
in providing communication. Additionally, some of the pleasure derived from the
human voice may be lost. As a result, the contribution of the voice to happy
and healthy survival diminishes. Age changes in the voice also alter the way
people respond socially to individuals who are getting older. These changes in
turn affect aging individuals' responses and self‑images. Therefore,
biological aging of vocalization can influence nonbiological aspects of life.
©
Copyright 2020: Augustine G. DiGiovanna, Ph.D.,
Salisbury University, Maryland
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