respiratory system · the respiratory system obtains oxygen by provid-ing conditions that allow the...

5RESPIRATORY

SYSTEM

The principal organs of the respiratory system arethe two lungs, which are in the right and left sidesof the chest (thoracic cavity) and are separatedfrom each other by the heart. Air passes into andout of the lungs through a series of passages andtubes called the upper airways. The flow of airdepends on other organs, including the musculardiaphragm and the muscles and bones that makeup the wall of the thoracic cavity (Fig. 5.1).

Part of each lung consists of tubes called thelower airways; they end in the microscopic sac-like alveoli, which make up most of the lungs.The lower airways transport air to and from thealveoli. Many pulmonary vessels transport bloodthroughout the lungs.

MAIN FUNCTIONS FORHOMEOSTASIS

Working in a coordinated fashion controlledmostly by the nervous system, these structuresperform the two functions of the respiratory sys-tem: gas exchange and sound production. Gas ex-change involves two processes: obtaining oxygenand eliminating carbon dioxide.

Gas Exchange

The respiratory system obtains oxygen by provid-ing conditions that allow the oxygen containedin air to pass into the blood flowing through thelungs. The circulatory system then transports theoxygen throughout the body. Oxygen (°2) mustbe supplied to body cells because it is a raw ma-terial used by mitochondria to obtain energy fromnutrients. This energy provides the power neededto perform all essential bodily activities.

The respiratory system eliminates carbon di-oxide (CO2)by providing conditions that allow itto move out of the blood in pulmonary vesselsand into the atmosphere. Carbon dioxide is trans-ported from body cells to the lungs by the circu-latory system.

Carbon dioxide must be eliminated because it

is a waste product from the series of chemical re-actions in mitochondria that release energy fromnutrients. When CO2 accumulates within thebody, it can interfere with body functions becauseCO2 combines with water to produce carbonicacid. The excess carbonic acid upsets the acid/base balance of the body. This disturbance canalter body proteins. Body structures and functions

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94 Human Aging: Biological Perspectives

FIGURE 5.1 The respiratory system and associated structures.

Vertebrae

Diaphragm

Abdominalmuscles

can be adversely affected, and serious illness ordeath can follow. Still, some acidic materials mustbe present for the body to achieve a normal acid/base balance, and a deficiency in acids can be asdisastrous as an excess. Furthermore, some CO2in the blood is used to make a buffer. Therefore,the respiratory system must eliminate some butnot too much CO2,

Many other acidic materials in the body con-tribute 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 main-tain acid/base balance by eliminating more CO2,This occurs in individuals whose kidneys do noteliminate acids adequately.

The rate of bodily activities changes from timeto time. These changes cause fluctuations in therates of °2 use and CO2 production and theamount of other acids. To maintain homeostasis,negative feedback systems employing the nervoussystem normally ensure that the rate of gas ex-

change by the respiratory system increases ordecreases to meet these fluctuations. This adap-tive mechanism occurs when a person begins tobreathe more heavily soon after beginning vigor-ous physical activity.

The maximum amount that gas exchange canbe increased to compensate for increases in bodilyactivity constitutes the reserve capacity of the res-piratory system. The limited nature of respiratorycapacity seems to contribute to setting a maxi-mum limit on how vigorously a person can exer-cise. This limit is experienced as the sensation offeeling completely out of breath while exercising.Limitations in the maximum functional capacitiesof the circulatory, nervous, and muscle systemsmay also playa role in establishing the maximumrate of physical activity attainable.

Three operations are involved in carrying outgas exchange. Ventilation (breathing) involvesmoving air through the airways into and out ofthe lungs. Perfusion of the lungs involves the

Nasal cavity-Nostril

Oral cavityI UpperPharynx airways

-Larynx-Trachea

-RibsSternum

Bronchus

Heart . Thoracic

LungI cavity

Chapter 5 - Respiratory System 95

movement of blood through the pulmonary ves-sels. Diffusion causes the °2 in inhaled air to moveinto the blood while CO2 exits into the air in thelungs.

Sound Production

Sound production, which is the second main func-tion of the respiratory system, is important be-cause it helps people communicate. A short sec-tion on sound production, including the effectscaused by aging, is presented at the end of thischapter.

VENTILATION

Ventilation involves two phases: inhaling (inspi-ration) and exhaling (expiration). Inspirationmoves air into the nostrils and down the airwaysto the deepest parts of the lungs, where the °2 itcontains can diffuse into the blood. Expirationmoves air containing CO2 from the innermostparts of the lungs up and out of the body.

To understand how ventilation occurs, onemust realize that air around the body is underatmospheric pressure. Materials normally movefrom areas of higher concentration or pressure toareas of lower concentration or pressure. For ex-ample, air moves into a balloon and inflates itwhen more air pressure is applied to its openingthan is already in the balloon. Conversely, releas-ing the opening of an inflated balloon results inair rushing out because the pressure within theballoon is higher than atmospheric pressure.

Inspiration

Inspiration occurs for the same reason that a bal-loon becomes inflated. Air moves into the bodywhen the air pressure outside the body is greaterthan that inside the respiratory system. A personcreates this difference in pressure by contractingmuscles 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 whoseedge is muscle and whose center is fibrous mate-rial. The muscular edge slants downward sharplyand is attached around its perimeter to the bodywall. When the muscle contracts, the roundedcentral region is pulled downward within thebody wall, moving like a piston downward in a

FIGURE 5.2 Inspiration.

Sternum raised

External intercostalmuscles (contracted)

Ribs raised

Diaphragmflattened

(contracted)

Abdominalmuscles(relaxed)

cylinder. When the central region is pulled down-ward and the diaphragm flattens somewhat, thepressure within the thoracic cavity decreases. Be-cause moisture on the outer surface of the lungscauses them, in effect, to adhere to the diaphragm,a similar decrease in pressure occurs within thelungs. Since the pressure in the lungs is then lowerthan atmospheric pressure, air flows through theairways and into the lungs, resulting in inspira-tion (Fig. 5.2).

The walls of the thoracic cavity contain manybones, including the ribs, the sternum, and somevertebrae. The cartilage and joints that connectthese bones allow them to move somewhat whenthe muscles attached to the bones contract. When

-96 Human Aging: Biological Perspectives

rIII

muscles move the ribs and sternum upward andoutward, pressure in the thoracic cavity decreases.The lungs, whose surfaces are stuck to the tho-racic walls by fluid, also have a decrease in inter-nal pressure, and inspiration occurs.

Inspiration usually involves simultaneousmovement of both the diaphragm and the bonesof the thorax. Some individuals rely mostly onmovement of the diaphragm (diaphragmaticbreathing), while in others movement of the ribs(costal breathing) makes the major contribution.Inspiration ends when parts of the body stop mov-ing and enough air has come in to raise the pres-sure in the lungs to atmospheric pressure. If themuscles are held in this position, no further airmovement occurs and the lungs remain inflated.People in this state are truly holding their breath.

Inspiration is called an active process becauseit requires the use of energy. Obtaining this en-ergy uses some of the oxygen that inspirationhelps obtain. The energy expended and the oxy-gen used are called the work of breathing. Usu-ally not more than 5 percent of the oxygen broughtin by inspiration is consumed in this process; therest is available for use by other body cells. Sincediaphragmatic breathing is more efficient and re-quires less energy than does costal breathing, itconsumes less oxygen. These differences leavemore of the oxygen obtained from inspiration foruse by other body cells.

Expiration

Expiration for a person who is resting and breath-ing quietly normally requires no muscle contrac-tion because the movements of inspiration set upconditions that allow it to occur automatically. Forexample, when the diaphragm moves downward,it pushes on the organs below it in the abdominalcavity, and this increases the pressure in the ab-dominal cavity. Also, the movements of the ribsand sternum stretch and bend elastic and springystructures in the thoracic wall such as ligaments,cartilage, and the ribs themselves. Finally, thelungs, which are elastic, are stretched outward.

As a result, as soon as the muscles of inspira-tion relax, the abdominal organs, structures in thethoracic wall, and the lungs start to spring backto their original positions. This elastic recoil in-creases the pressure in the lungs. The pressurequickly rises above atmospheric pressure, and

expiration occurs. The process is similar to whathappens when the opening of an inflated balloonis released. Each expiration is followed shortly bythe next inspiration (Fig. 5.3).

Since normal quiet expiration requires nomuscle contraction or energy, it is called a pas-sive process. However, when a person becomesvery active, passive expiration occurs too slowlyto meet the needs of the body. Then respiratorymuscles and energy can be used to perform ac-tive forced expiration. For example, abdominalmuscles can squeeze on the abdominal organs,causing them to push upward on the diaphragmmore forcefully, and chest muscles can pull theribs downward. The resulting increase in pressurein the lungs pushes air out of the respiratory sys-tem quickly (Fig. 5.4).

Rate of Ventilation (Minute Volume)

Ventilation usually occurs continuously to pro-vide ongoing replacement of the °2 being con-sumed and elimination of the CO2 being pro-duced. The rate of ventilation must be highenough to maintain homeostatic levels of thesegases in the body. The rate of ventilation is calledthe respiratory minute volume, the volume of airinspired per breath times the number of breathsper minute. The number of breaths per minute iscalled the respiratory rate. Minute volume can beexpressed mathematically:

Minute volume =volume per breath x breaths per minute

Lung Volumes The volume of air inspired equalsthe amount of air expired. When a person is atrest and breathing quietly, this volume is calledthe tidal volume (TV).

When a person is active and has to exchangegases more quickly, inspiratory and expiratoryvolumes can be increased considerably by increas-ing the distance the respiratory muscles contract.The extra amount a person can inspire is calledthe inspiratory reserve volume (IRV); the extraamount a person can expire is called the expira-tory reserve volume (ERV). The combination oftidal volume, inspiratory reserve volume, andexpiratory reserve volume is called the vital ca-pacity (VC). Vital capacity is the most air a personcan expire after taking the deepest possible inspi-

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FIGURE 5.3 Passive expiration: (a) Elasticrecoil collapses chest partially, causing expira-tion. (b) Elastic recoil collapses balloon, forcingair out.

Sternum lowered

Intercostal muscles(relaxed)

Ribs lowered

Diaphragmmoves

upward(relaxed)

ration. This can be expressed mathematically:

TV + IRV + ERV =VC

Besides increasing the volume of air respiredwith each breath, a person can increase the speedat which the air flows. This is accomplished by

FIGURE 5.4 Forced expiration: (a) Musclecontractions and elastic recoil collapse chestpartially, causing rapid expiration. (b) Pressurefrom hands and elastic recoil collapse balloon,forcing air out rapidly.

Sternum lowered more

Internal intercostal

muscles (contracted)

Ribs lowered more

Diaphragm pressingupward (relaxed)

increasing the speed and force of respiratorymuscle contractions, which can magnify pressurechanges in the lungs more than 25-fold.

A person who expires as much as possible stillhas some air left in the lungs. This volume is calledthe residual volume (RV). Thus, the total amountof air the lungs can hold equals TV + IRV + ERV +

Abdominal

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musclesI,(relaxed)I

Abdominal organSAbdominal musclesII

pushed upward,' flattened (contracted)\\

(a)(a)

I

Elastic Airrecoil flowscollapses outballoon

t -t(b) l'

(b)


RV and is called total lung capacity (TLC). Asmall amount of the TLC does not reach the al-

veoli but remains in the lower airways. This vol-ume of air-the dead space-cannot be used forgas exchange because only the alveoli are thinenough for this process to occur.

Respiratory Rate A normal person may have arespiratory rate of 15 to 20 breaths per minute,but this rate can change as needed. If the volumeper breath remains high, an increase in the rate ofrespiration increases the minute volume andtherefore the rate of gas exchange. Decreases inthe respiratory rate have the opposite effect onthe minute volume and the rate of gas exchange.Such adjustments in the respiratory rate occur aschanges in bodily activity alter the need for gasexchange.

When the rate of respiration increases, there isless time for each inspiration and expiration. If aperson does not increase the rate of airflow,breathing becomes rapid but shallow. Suchbreathing delivers little fresh air to the lungs forgas exchange.

REQUIREMENTSFORVENTILATION

The major features necessary for proper ventila-tion include (1) open airways for easy air move-ment, (2) defense mechanisms that assure that onlyclean, moist, warm air reaches the lungs, (3)proper pressure changes in the thoracic cavity andlungs to make the air move, (4) compliance in tho-racic and lung components so that pressurechanges cause them to expand easily to acceptincoming air, and (5) control systems that ensurethat the process occurs successfully and at thecorrect rate.

Contributions by Airways

The contributions made by muscles and skeletalcomponents to the first four of these requirementswere described previously. We will now considerthe ways by which the airways contribute to theserequirements.

Nasal Cavities Inspired air entering the nostrilspasses through the nasal cavities above the hardpalate. These cavities are held open by the bonesof the skull. As air passes through the nasal cavi-

ties, it is cleaned, moistened, warmed, and moni-tored so that it does not harm the delicate struc-

tures deep within the lungs (Fig. 5.5).The air is cleaned because dust and other par-

ticles are trapped by hairs inside the nostrils andby the sticky mucus that coats the inside of thecavities. Microscopic hairlike cilia on the cells lin-ing the cavities wave back and forth, causing themucus to glide back toward the throat. Then themucus and its trapped debris can be harmlesslyswallowed.

The air is moistened by the mucus to preventdrying of the lungs. Heat from the blood in thewalls of airways warms the air so that the lungsare not chilled. Finally, sensory nerve cells moni-tor the chemical contents of the air and send im-

pulses to the brain. The presence of such chemi-cals is perceived as aromas. The nervous systemmay cause inspiration to slow or stop if harmfulchemicals or particles are detected. Forced expi-ration (e.g., sneezing) may be initiated in an at-tempt to blow the noxious materials out of therespiratory system.

A person may inspire some or all of the air heor she breathes through the mouth rather thanthrough the nasal passages. This can increase therate of airflow, but it reduces the amount of clean-ing, moistening, and warming of inspired air. In-jury to the airways below the pharynx may re-sult. Inspiring through the mouth can also leadto excessive dryness of the oral cavity, which maycause oral discomfort and sores.

Nasopharynx Air in the nasal cavities movesbackward into the nasopharynx, which is abovethe soft palate. Bones and other firm tissues keepthis passage open except when one is swallow-ing, during which the tongue pushes upward onthe soft palate. The mucus, cilia, and blood in thenasopharynx further clean, moisten, and warmthe air.

Pharynx After passing through the nasophar-ynx, the air moves through the throat, or phar-ynx, into the opening in the voice box, or larynx.This opening is called the glottis. The pharynx isheld open by the firmness of the muscles andother tissues that make up its walls.

Since food and beverages in the oral cavity thatare being swallowed also pass through the phar-ynx, these materials can lodge in the pharynx orenter the glottis, blocking or injuring the airways.


FIGURE 5.5 Respiratory passages in the head and neck.

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Nasopharynx

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Soft palate

Pharynx

Epiglottis

Glottis

Esophagus

Two reflexes controlled by the nervous systemprevent these problems.

The swallowing reflex occurs whenever solidsor liquids are present in the pharynx behind thetongue. This reflex clears the pharynx by push-ing materials down into the esophagus. At thesame time, a flap called the epiglottis is movedover the glottis to prevent materials from enter-ing the larynx. The epiglottis is moved off the glot-tis after swallowing has been completed so thatventilation can begin again (Fig. 5.6).

The gag reflex is caused when irritating mate-rials enter the pharynx. This reflex causes musclesnear the pharynx to close the openings into thelarynx and esophagus. At the same time, musclecontractions in the abdomen raise the pressure inthe esophagus and trachea to prevent materialsfrom entering those passageways. A very stronggag reflex can result in vomiting.

Frontal sinus

Nasal cavity

External naris

Hard palateOral cavity

Tongue

LarynxVocalcords

Trachea

Larynx, Trachea, and Primary Bronchi Air pass-ing through the glottis moves through the larynx,down the windpipe (trachea), and through thetwo primary bronchi into the lower airways in thelungs. Plates and rings of springy cartilage withinthe walls of these airways provide support so thatthe airways stay open during ventilation.

Materials other than air that enter these air

passages initiate the cough reflex. During cough-ing, bursts of air that are expired rapidly forceforeign materials up and out of these airways.

The mucus, the cilia, and blood flow in thesestructures carry out further cleaning, moistening,and warming of the air. The cilia beat in an up-ward direction so that the mucus glides into thepharynx. Since the mucus carrying materialsslides upward in a smooth continuous stream, thismechanism is called the mucociliary escalator.Phagocytic macrophages and immune system

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FIGURE 5.6 The swallowing reflex: (a) Tongue pushes food back. (b) Soft palate elevates andepiglottis lowers to close airways. (c) Muscle contractions push food into esophagus. (d) Wave ofcontraction pushes food down to stomach.

SoftpalateFood

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Tongue

Epiglottis

Glottis

Esophagus

Trachea(a)

(b)

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II.

I r Esophagus

(e)Stomach

(c)


cells in the trachea and primary bronchi provideadditional defense against foreign materials.

Smaller Bronchi, Bronchioles, and AlveolarDucts As each primary bronchus enters a lung,it branches repeatedly, forming ever more numer-ous smaller bronchi and bronchioles and finallymicroscopic alveolar ducts.

The walls of these airways become thinner asthey branch and narrow. Cartilage in the smallerbronchi keeps them open during inspiration.There is no cartilage in the bronchioles or smallerairways. A peculiar helical structure of the col-lagen that coils around the airways and elastinfibers also support these smaller airways. Thecartilage and fibers provide the lungs with com-pliance. Like the trachea and bronchi, thesesmaller airways are protected by the cough reflexand defense cells and condition the entering air.

Smooth muscle cells in the airway walls allowfor appropriate adjustments in their diameter asthe amount of ventilation needed fluctuates. The

smaller airways provide most of this adaptabil-ity. The activity of the muscle is controlled by thenervous system, the endocrine system, and nearbychemicals.

As air is expired and the lungs decrease in size,the open passages in the airways become nar-rower. The walls of the smallest airways are sothin and weak that these airways close completelybefore all the air has escaped from the alveoli be-low them. This air remaining in the alveoli makesup part of the residual volume.

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Alveolar Sacs and Alveoli The inspired air inthe alveolar ducts passes into blind cup-shapedoutpocketings called alveoli. Most alveoli occurin clusters extending outward from slightly en-larged spaces at the ends of the alveolar ductscalled alveolar sacs. Each cluster may look like atightly packed bunch of plump grapes (Fig. 5.7).

There are about 300 million alveoli in the lungs.Because alveoli are hollow, filled with air, andvery small and because they make up most of thelungs, dried lungs have the consistency ofStyrofoam. The alveoli provide an amount of sur-face area equivalent to that of an area 30 feet longand 25 feet wide. The walls of the alveoli are verythin, allowing diffusion of °2 and CO2 betweenthe air and the blood to occur easily.

Special cells in the alveoli secrete a materialcalled surfactant. As surfactant spreads out, it

coats the inner surface of the alveoli and parts ofthe smaller airways. The surfactant greatly in-creases the compliance of the lungs by reducingthe attraction between the water molecules on the

inner surfaces of the lungs. Without surfactant,the attraction (surface tension) would be so greatthat the alveoli and small airways would collapse.The inner surfaces would stick together tightly,making it nearly impossible for them to separateand fill with air during inspiration. These char-acteristics can be compared to the difference be-tween the effort needed to inflate a new balloon

that contains a powdery surfactant and the effortneeded to inflate an old balloon that dried after

becoming damp.Surface tension is important because as it

makes the lungs collapse, it helps increase thepressure in the lungs and therefore assists in ex-piration. The combination of a moderate amountof surface tension in the alveoli and the large sur-face area they provide makes expiration mucheasier.

Control Systems

Nervous System Ventilation begins with inspi-ration, which requires the contraction of muscles.The nervous system signals activating thesemuscles originate in a region of the brain calledthe medulla oblongata and travel to the musclesthrough nerves. The medulla oblongata is insidethe region of the skull just above the neck. Thepart of it concerned with respiration is called therespiratory control center.

The respiratory control center starts inspirationwhen it detects an increase in CO2 levels or a de-crease in °2 levels in the blood flowing throughit. When sensory nerve cells from the aorta andarteries in the neck detect very high levels of CO2or very low levels of °2' these nerve cells alsostimulate the respiratory control center. Othersensory neurons in the lungs send impulses to therespiratory control center, telling it that the lungsare in a partially collapsed condition and areready for inspiration. Sensory nerves frommuscles and joints inform the respiratory centerwhen a person begins physical activity and willneed more gas exchange.

Nerves from the lungs inform the respiratorycenter and a nearby part of the brain called thepons when inspiration is complete. The brain then


FIGURE 5.7 Lower airways and alveoli.

Alveolar cavity

.~Frompulmonaryartery

'h,,~Topulmonaryvein

(d)

I/IJ":--- Frompulmonaryartery

(c)

Frompulmonaryartery

Respiratorybronchiole

Alveolarduct

Alveolar sac

Alveoli

Trachea

Bronchioles

Primary bronchi

(a)

Lymphaticvessels

Pulmonary venule

Pulmonary arteriole

Terminalbronchiole

Capillarynetwork

(b) Visceral pleura

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rstopstheimpulsesfor inspiration. As the musclesrelax,expiration begins because of elastic recoiltr the thorax and lungs. The respiratory center

rfcanalsosend impulses to the muscles indicatingfthata forceful expiration is needed.j Therespiratory center and the pons monitoritheirown impulses and are also informed byi nervesfrom the lungs when expiration is com-, plete. This triggers the beginning of the next in-

spiration.Therefore, the linking and repeating of:, twonegative feedback systems result in rhythmicj breathing.

Thedepth of breathing, the speed of airflow,andthe respiratory rate are adjusted when the res-piratorycenter detects that CO2 or °2 concentra-tionsand the acid/base balance in the blood are

Ii beginningto wander from proper levels. The ad-justmentsrestore appropriate gas levels and acid/basebalance so that homeostasis is maintained.

Ventilation is also modified by the swallowing,gag, and cough reflexes. In addition, upper re-gions of the brain such as the areas controllingemotionsand those controlling conscious actionscaninfluence the respiratory center. The consciouscontrolareas allow a person to voluntarily inspire,expire,stop ventilation, or modify ventilation foractions such as talking.

The nervous system also controls ventilationby adjusting the size of the lower airways. Im-pulses from the sympathetic nervous systemcauserelaxation of smooth muscles in the airways,permitting them to dilate and increasing minutevolume. Parasympathetic nerves cause thesmooth muscles to contract, constricting the air-ways and reducing minute volume. These changesallow the rate of gas exchange to maintain homeo-stasis for °2 and CO2,

Endocrine System Hormones from the endocrinesystem also help regulate ventilation. Norepineph-rine makes a main contribution. This hormone has

the same effect on the airways as do impulses fromsympathetic nerves.

AGE CHANGES AFFECTINGVENTILATION

Because everyone's airways are subjected to someair pollution and other environmental insults, itis difficult to know which age-related changes inairways are due to aging and which are due to


other factors. However, certain changes seem tooccur in all people. These universal changes areconsidered age changes and thus are included inthis section. Let us examine how age changes af-fect the five requirements for ventilation.

Open Airways

Mucus and Cilia All airways from the nasalcavities to the smallest bronchioles producemucus continuously. With aging, the mucusproduced is more viscous and therefore moredifficult to move. In addition, both the numberand the rate of motion of the cilia decrease. As

the clearing out of mucus slows, the accumu-lation of mucus narrows airways, and this in-hibits ventilation. When ventilation becomes

more difficult, the work of breathing increasesand extra CO2 is produced by the muscles ofventilation, making respiration less efficient.Narrower airways also reduce the rate of air-flow and therefore reduce the maximum possibleminute volume.

Airway Structure Age changes in the walls ofbronchioles cause them to become even narrower,amplifying the effect of mucus accumulation. Inaddition, the bronchioles close earlier during ex-piration, trapping more air in the smaller airwaysand in the alveoli, especially in the lower parts ofthe lung. One result is an increase in residual air.This causes the fresh air entering with each in-spiration to be mixed with a larger amount of staleresidual air, decreasing the rate of diffusion. A sec-ond result is uneven lung ventilation.

While the bronchioles become narrower, thelarger airways in the lungs and the alveolar ductsincrease in diameter. These changes compoundthe negative effects by increasing the dead space.Thus, fresh inspired air is mixed not only withmore residual air in the smallest airways and al-veoli but also with more dead space air. This fur-ther decreases the rate of diffusion.

The increase in tidal volume with age may helpminimize the expected drop in the diffusion rateduring tidal breathing by mixing more fresh airwith the increasing amounts of stale air remain-ing in the respiratory system. The rate of diffu-sion remains high because the °2 concentrationin the lungs is kept high while the CO2 levels arekept low.


Defense Mechanisms

Since age changes in the mucus and cilia causeslower movement of mucus, harmful materialssuch as microbes, particles, and noxious chemi-cals trapped by the mucus stay in the respiratorysystem longer. Aging also decreases the function-ing of other defense mechanisms, including re-flexes (see below), white blood cells, and the im-mune system. All these age changes cause an in-crease in the risk of developing respiratory infec-tions and other respiratory problems.

Proper Pressure Changes

Muscles As with most muscles, aging causesrespiratory muscles to become weaker. The de-crease in muscle strength is not enough to detractfrom performing tidal breathing or ventilating atmoderately increased minute volumes. However,it slowly decreases the maximum pressurechanges that the muscles can produce and thusdecreases the maximum rate of airflow attainable.

Skeletal System Age changes in the cartilage,bones, and joints of the thorax also reduce aperson's ability to produce large pressure changesin the thoracic cavity. The cartilage attaching theribs to the sternum becomes more calcified and

stiff, and the ribs become less elastic. Age changesin the cartilage and ligaments of other joints, suchas those between the ribs and the vertebrae, re-sult in decreases in the ease and range of motionof the bones they connect.

Aging also leads to slight alterations in thepositions of the bones of the chest. The chest be-comes deeper from front to back, making deepinspiration more difficult. Altered posture fromother changes in the skeletal system further re-duces a person's ability to inspire quickly andfully.

Because of these skeletal age changes, there is adecline in the maximum minute volume attainable

and an increase in the work of breathing. Olderpeople partially compensate for these effects by re-lying more on diaphragmatic breathing.

Lungs Though there are no important agechanges in the elastic fibers or surfactant, otherage changes in the lungs significantly affect pres-sure changes. For example, aging causes the coiled

collagen fibers in the lungs to become somewhatlimp and less resilient. Also, aging causes the al-veoli to become shallower, and this reduces theamount of surface area present. The resulting re-duction in surface tension decreases elastic recoil.

Both of these age changes reduce the maximumrate of expiration attainable and add to the workof breathing (Fig. 5.8).

Compliance

Aging causes the coiled collagen fibers to becomesomewhat limp and stretch more easily. Thesechanges increase the compliance of the lungs andtend to make inspiration easier. Note, however,that the increase in lung compliance is much lessthan the increase in chest stiffness caused by skel-etal age changes. Thus, there is a net increase instiffness of the respiratory system, resulting in adecreased ability to inspire.

Control Systems

Aging does not seem to affect the contributionsof the nervous system to rhythmic breathing un-der resting conditions. However, three types ofage change reduce the ability of the nervous sys-tem and endocrine system to cause adaptivechanges in ventilation:

1. Neurons monitoring °2' CO2, acid/base bal-ance, and muscle activity seem to become lesssensitive to changes in these parameters.

FIGURE 5.8 Effects of aging on alveoli: (a)Young alveoli. (b) Old alveoli.

(a) Young alveoli (b) Old alveoli

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Reductions in pressure changes caused by weaken-ing 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 whichair can flow in the system. Second, they make ven-tilation more difficult and therefore increase the

work of breathing. This reduces the amount ofavailable °2 and increases CO2 in the blood.

Although aging does not change the total lungcapacity, age changes affect the volumes of air that V,?lume.. (liters)can be moved. The more rapId closing of bron-chioles, together with stiffening of the system, 6causes a decrease in both inspiratory and expira- 5tory reserve volume. At the same time, tidal vol-ume increases somewhat, and the age changes 4cause an increase in residual volume both at rest 3

and during increased ventilation. These changes 2in volumes cause the vital capacity to decrease.

These changes in respiratory volumes have twoeffects. First, they further decrease maximumminute volume. Second, the decrease in vital ca-pacity, combined with the increase in residualcapacity, means that less fresh inspired air is mix-

2. There may be changes in the nervous pathwaysthrough which all their impulses are sent, re-sulting in altered ventilation.

3. The lungs become less sensitive to norepineph-rine from sympathetic nerves and the endo-crine system.

These age changes result in a slower andsmaller increase in minute volume when there is

a decrease in °2 or an increase in CO2, acids, orbody activity. As a result, individuals who beginvigorous activity feel out of breath and tire morequickly as they get older.

Age changes in other parts of the nervous sys-tem reduce its ability to provide the swallowing,gag, and cough reflexes that defend the respira-tory system (Chap. 6). Because of these changes,it takes a greater amount of material and a longertime to start a defensive reflex. Once it begins, theresponse is slower and weaker.

Therefore, older individuals must avoid situa-tions that raise the risk of choking. These includeeating quickly; talking or laughing while eating;eating while lying on one's back; and eating afterconsuming alcoholic beverages or medicationsthat slow the reflexes.

Consequences

ing with more stale air remaining in the lungs.This change decreases the rate of diffusion. Theproblem is compounded by the increase in deadspace. The age-related increase in tidal volumemay help compensate for this problem duringquiet breathing (Fig. 5.9).

The force of gravity on the lungs causes thelower bronchioles to close sooner than do those

in the upper regions. Therefore, the lower partshave a higher proportion of the residual air thando the upper regions. This unevenness in venti-lation increases with age. As seen below in thediscussion of perfusion and diffusion, this furtherdecreases the efficiency of the system. Thus, aspeople get older, they must ventilate more air toget the same amount of gas exchange, and thisadds to the work of breathing. Breathing moredeeply can partially overcome the deleterious ef-fects of uneven ventilation. Aging also reduces themaximum respiratory rate (breaths per minute)because of age changes that slow airflow and agechanges in the nervous system.

These decreases in maximum flow rate, maxi-mum volume per breath, and maximum respira-tory rate combine to cause a decrease in the maxi-mum minute volume. Many individuals can ex-pect their maximum minute volumes to declineby 50 percent as they pass from their twenties tovery old age. This change makes a major contri-bution to the decrease in the maximum rate of gasexchange as people age. Age changes in perfusionand diffusion, discussed below, cause additionaldecreases in gas exchange.

FIGURE 5.9 Age changes in respiratory volumes.

Inspiratoryreserve

Tidal volume

Expiratory reserve-Residualvolume

-

20 40 60 80Age in years


FIGURE 5.10 Pulmonary circulation and perfusion of the lungs.

Air in alveolus

Alveolar epithelium

Fused capillary andalveolar basementmembrane

Surfactant layer -

Capillary endothelium(wall)

Alveolar basementmembrane

Capillary basementmembrane

Red blood cells

External respiration (lungs)

PERFUSION

Recall from Chap. 4 that perfusion is the passageof blood through the vessels of body structures.Perfusion of the lungs proceeds as follows.

The right atrium receives blood from systemicveins from all parts of the body except the lungs.This blood has little oxygen because the oxygenwas removed and used as the blood flowed

through capillaries and past body cells. Therefore,~

Systemic arteries

Body tissues

this blood is called deoxygenated blood. It alsohas a high concentration of CO2, which diffusedinto the blood from the body cells (Fig. 5.10).

Deoxygenated blood from the right atriumflows into the right ventricle, which then pu~psit through the pulmonary arteries to the lungs.As these arteries enter and pass through the lungs,they branch into smaller vessels until they enterthe thin-walled pulmonary capillaries. These cap-illaries carry the blood close to the walls of the

lveolLThisallows gases to diffuse between the,loodin the capillaries and the air in the alveoli.~heblood then enters pulmonary veins, which!arrybloodto the left atrium. Since this blood hasnhighconcentration of °2, it is called oxygenatedblood.Italso has a low concentration of CO2,This,Ioodwillpass from the left atrium into the left ven-'ide,whichwill pump it through the systemic ar-

,:eriestoallparts of the body except the lungs.~;Likethe rate of ventilation, the rate of perfu-sionmustvary as a person's rate of activity, andlereforethe need for gas exchange, varies. Blood

flowtoanarea of the body can be changed by alter-~gthecardiac output and changing the diameterpfthearteriesdelivering blood to body structures.

AgeChanges in Perfusion

Thereare essentially no age changes that affect;thepulmonaryarteries and veins. Furthermore,:agingdoes not change cardiac output.

Thereason for the minimal change in pulmo-naryvesselscompared with other vessels in the,bodymay be that blood pressure in the pulmo-naryvesselsis much lower than that in the sys-temicvessels.When diseases such as emphysemacausea rise in pulmonary artery pressure, thesearteriesundergo changes that resemble athero-sclerosis.

Thoughpulmonary arteries and veins remainlargelyunchanged by aging, the pulmonary cap-illariesdecrease in number and accumulate some

fibrous material. Whether these are true agechangesor are due to the effects of air pollutionisuncertain.

Normally, the reduction in perfusion due tochangesin pulmonary vessels is slight. The effectonreducing gas exchange does not become ap-parentuntil the respiratory system is called on todeliverthe maximum rate of gas exchange. Eventhen,this causes only a small reduction in maxi-mumgas exchange.

However, heart disease and certain types ofpneumoniaand emphysema can reduce perfusionofthelungs. Such reductions decrease the rate ofgasexchange and therefore decrease the abilityofthe respiratory system to maintain homeosta-sisof °2' CO2, and acid/base balance. These ef-fectsare often noticed as the sensation of beingout of breath and being fatigued when one en-gagesin vigorous physical activity.


DIFFUSION

Recall that the alveoli are the destinations for in-

spired air. Their great numbers and deeply curvedsurfaces 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-walledcapillaries surround the alveoli. Only an exceed-ingly thin noncellular layer (basement membrane)separates the alveolar wall from the capillary wall.These structural features provide a thin surfacethrough which gases must pass. The secretionsfrom some alveolar cells keep their surfaces moist.Thus, the alveoli supply a large, thin, and moistsurface that is ideal for the diffusion of gases.Diffusion of °2 and CO2 in the lungs proceeds asfollows (Fig. 5.10).

The blood entering the pulmonary capillarieshas a very low concentration of °2 and a high con-centration of CO2, The air in the alveoli, by con-trast, has a high level of °2 and a low level of CO2because ongoing ventilation continuously re-freshes the alveolar air. Therefore, °2 diffusesfrom the alveolar air into the blood while CO2diffuses from the blood into the alveolar air.

Much of the CO2 that diffuses into the alveoliis removed with the next ventilation, which alsodelivers a new supply of °2' Only a very smallamount of the °2 that enters the blood can be car-ried by the plasma. Almost all the oxygen in theblood is bound to hemoglobin molecules, whichare contained in the red blood cells. Each hemo-

globin molecule can bind up to four molecules ofoxygen. When hemoglobin binds oxygen, the re-sult is oxyhemoglobin. Decreased CO2, increasedpH, or decreased temperature of the blood in-creases the amount of oxygen that can be boundto each hemoglobin molecule; the converse is alsotrue. Normally, this promotes complete oxygen-ation of blood in the lungs, where ventilationkeeps CO2levels and temperature low. It also pro-motes greater release of oxygen in other capillar-ies, where body cell activities keep CO2 levels andtemperature high. These characteristics of hemo-globin are sometimes displayed as oxyhemoglo-bin dissociation curves.

Continuous perfusion, coupled with continu-ous ventilation, keeps diffusion occurring in anuninterrupted fashion. Furthermore, alterationsin the rate of ventilation or perfusion can alter therate of diffusion to meet bodily needs. Increasingventilation (i.e., minute volume) or perfusion

-"


increases the differences in concentrations of °2and CO2between the blood and alveolar air. Thesechanges increase the rates of diffusion and gasexchange. Reducing ventilation or perfusion hasthe opposite effects.

Age Changes in Diffusion

Aging causes several changes that reduce themaximum minute volume of ventilation, increaseresidual volume, and cause uneven ventilation inthe lungs.

Age changes in the alveoli further decrease therate 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 whichfurther impair diffusion (Fig. 5.8).

EFFECTS FROM ALTERED GASEXCHANGE

Biological Effects

In summary, essentially all aspects of the respira-tory system involved in gas exchange are detri-mentally affected by aging, resulting in a drop inthe maximum rate of gas exchange. Furthermore,there is an overall decline in the efficiency of thissystem. Finally, the ability of the respiratory sys-tem to adjust the rate of gas exchange to meetbody needs declines. These changes occur at afairly steady rate throughout life.

As these changes occur, the maximum rate atwhich a person can perform physical activitiesdeclines, and a person who starts a vigorous ac-tivity such as running or climbing stairs will feeltired and out of breath sooner as age advances.Such an individual will not be able to perform attop speed for an extended period. Age changesin other systems, including the circulatory, skel-etal, muscle, and nervous systems, may contrib-ute to these decrements. The consequences ofthese effects can be reduced by raising one's pacegradually. Doing this provides extra time forrespiratory functioning to adapt to the increasedneed for gas exchange. Also, going at a more mod-erate pace lowers the required rate of gas exchange.

Although aging causes reductions in severalmaximum respiratory values, these age changesare observed only when people require that therespiratory system function at maximum capac-

ity. This system has such a great reserve capacitythat the decline in maximum values caused byaging has essentially no effect on a person whoengages in light or moderately vigorous activi-ties. Thus, unless a person engages in activitiessuch as very demanding physical work or highlycompetitive athletic events, age changes in respi-ratory functioning have little noticeable effect onhis or her lifestyle. The aging respiratory systemcan provide adequate service in all but the mostphysically demanding situations.

Factors other than aging alter gas exchange.Also, much can be done to minimize age-relatedreductions in the ability of the respiratory systemto satisfy the need for gas exchange. For example,a sedentary lifestyle further limits respiration,while regular exercise keeps the decline in maxi-mum minute volume small. Furthermore, incor-porating adequate vigorous physical activity intoone's lifestyle can restore much of the decline inrespiratory functioning caused by inactivity.

Another factor that adversely affects respira-tion is air pollution. Breathing polluted air seemsto increase both the speed and severity of essen-tially every age change in the respiratory systemmentioned thus far. People who smoke, live inareas where air quality is poor, or engage in oc-cupations where the air contains dust, fine par-ticles, or noxious chemicals have a much fasterand greater loss of respiratory functioning. Inaddition, these individuals are at higher risk fordeveloping respiratory diseases, including lungcancer, chronic bronchitis, and emphysema. Airpollutants can injure respiratory cells and tissuesin several ways including physically, chemically,and through free radicals, microbes and immuneresponses. Radon damages lung tissues throughthe radiation it causes and the free radicals it in-

duces. Breathing polluted air can be reduced byavoiding polluted areas; by not smoking; by wear-ing a protective mask; and by providing adequateventilation with clean air in living and workingareas.

Interactions

The biological effects of decreased gas exchangecan affect other aspects of life. The nature anddegree of these effects depend on the amount andimportance of physical activity in a person's life.Examples of people who may be affected moredramatically include people whose chief form of


recreation and social contact is competitive sportsand people whose jobs involve considerablephysical exertion.

Finally, changes in gas exchange can be affectedby other types of age changes. For example, uponretirement, a sedentary office worker may take upa physically demanding sport, which may providethe motivation to stop smoking. The result can bea slowing and even a temporary reversal of thedecline in respiratory capacity.

DISEASES OF THE RESPIRATORYSYSTEM

While age changes in the respiratory system haveonly a small impact on the ordinary activities ofdaily living, changes caused by disease can havea substantial effect. Respiratory diseases reducea person's speed and endurance in physical ac-tivities and cause significant disability. Treatmentcan extend for long periods and is often expen-sive. Furthermore, respiratory system diseases(not including cancer of the lungs) are the fourthleading cause of death for those over age 65. If lungcancer is added, respiratory disease ranks as thethird leading cause of death among the elderly.

The reasons for the high incidence of respira-tory diseases among older people are similar tothose for other diseases. They include reductionsin defense mechanisms; more time for the de-velopment of slowly progressing diseases; and in-creases in the number of exposures and the totaltime of exposure to disease-promoting factors.

There is one factor that contributes to the de-

velopment of virtually all these diseases: air pol-lution. One of the most common forms is smok-

ing and inhaling smoke from tobacco products.Though the proportion of smokers in the popula-tion has declined, the effects of smoking amongolder people will be evident for many years be-cause many older people have smoked for longperiods. The rate of decline of the respiratory sys-tem slows when a person stops smoking and theremay even be a period of improvement in gas ex-change. However, most effects from long-termsmoking are not reversible.

Other forms of air pollution include particu-late matter such as dust from coal mining, wood-working, farming, and the manufacture of fabrics.Fumes and vapors such as those from painting,chemical plants, and scientific laboratories canharm the lungs. Smog, automobile fumes, and

other types of air pollution associated with ur-ban environments are also significant risk factorsfor lung damage.

Reducing the inspiration of air pollutants cansignificantly reduce both the incidence and sever-ity of respiratory disease. Doing this will preservemuch of the capacity for gas exchange by the res-piratory system.

Respiratory diseases that are most commonamong people of advancing age include lung can-cer, chronic bronchitis, emphysema, pneumonia,and pulmonary embolism. These diseases and twoother abnormal conditions (sleep apnea and snor-ing) will be considered here. In examining theserespiratory diseases and abnormal conditions,keep in mind that the ability of hemoglobin tobind oxygen is affected by COz' pH, and tempera-ture. Respiratory diseases and conditions can re-duce ventilation, leaving more COz in the bloodand more warm air in the lungs. These changesreduce the ability to oxygenate blood not onlybecause the °z supply to the lungs is reduced. Theelevated COz reduces the pH in blood in thelungs, and the blood remains somewhat warmer.Under these conditions, the hemoglobin in bloodpassing through the lungs cannot pick up andhold as much oxygen. Therefore, the hemoglobincannot transport as much °z to body cells.

Lung Cancer

Normally, cells reproduce when the body needsmore of them; once the need is met, they stop re-producing. An example is the temporary rapidreproduction of skin cells that occurs until a cutin the skin heals. Cancer consists of cells that con-

tinue to divide and spread out in an uncontrolledfashion even when they are not needed. A clumpof these cells is called a tumor.

Some forms of cancer develop from lung cellsand are called primary lung cancer. These are thetypes caused primarily by smoking. Many othercancers of the lungs develop when the circulatorysystem moves cancer cells from another place inthe body to the lungs. Cancer that moves to an-other part of the body is called metastatic can-cer. Metastatic lung cancer often comes from thebreasts or the reproductive system.

A person with lung cancer may have from onetumor to very many tumors. Whether the canceris primary or metastatic, the effects on the lungsare similar. Ventilation becomes more difficult


because airways get blocked when tumors growinside them or squeeze them closed. Air volumesare reduced as alveoli become filled with cancer

cells. Occasionally the cancer becomes so large orstiffens the lungs so much that they cannot inflateor deflate adequately for ventilation. Cancer cellsin the alveoli may reduce diffusion by thickeningor replacing the respiratory membrane betweenthe air and the blood. Sometimes cancer will dis-tort, squeeze, or replace the pulmonary vesselsso that perfusion is reduced. Some blood vesselsare weakened, causing bleeding.

Several warning signs indicate that lung can-cer may be present. They include a persistentcough, coughing or spitting up blood, pain in thechest, difficulty swallowing, hoarseness, easy fa-tigability and the feeling of breathlessness, and aswelling of the fingertips. Any of these indicatorswarrant evaluation by a physician.

Though some forms of lung cancer can be curedif discovered early enough, most cases are notidentified until the cancer has grown so much thatit cannot be eliminated. The vast majority of casesof lung cancer result in death within a few years.The only effective "cure" is prevention: avoidingtobacco smoke and other forms of air pollution.

Chronic Bronchitis

To understand chronic bronchitis, recall that thetrachea and bronchi are lined with a thin layer ofmucus and that as the mucus is made, cilia sweepit up and out of the airways.

Development If a person inspires air with anexcess amount of harmful particles or noxiouschemicals, the cells lining the trachea and bron-chi become injured. The resulting inflammationcauses those cells to make mucus much faster, andthe lining of the airways becomes swollen. In ad-dition, the beating of cilia slows. The person nowhas bronchitis and will begin to cough to removethe extra mucus and pollutants.

If this person breathes the pollutants frequentlyand continuously, the airways remain inflamedfor a longer time, and the person then has chronicbronchitis. This condition is accompanied by ex-tra mucus production and coughing. After a whilethe cilia will be damaged and may completelydisappear.

Effects The major effect of chronic bronchitis isto reduce ventilation by making the airways nar-

-

row in two ways. First, mucus accumulates be-cause it is being produced more quickly and re-moved more slowly. Second, the lining of the air-ways swells inwardly. The effect on airflowthrough the trachea and bronchi is similar to thestuffed-up feeling that occurs when a head coldcauses swelling and the accumulation of mucusin nasal passages.

Expiration becomes especially difficult becausethe lower airways normally narrow during expi-ration. The additional narrowing from the mucusand swelling makes them so narrow that expira-tion can occur only very slowly. This decreasesthe minute volume, and so having enough gasexchange to meet the body's needs is quite diffi-cult. The problem is compounded because theperson will begin to rely more on forced expira-tion, increasing the work of breathing. The effortused in coughing raises the work of breathing tolevels that may leave the victim dizzy, breathless,and temporarily incapacitated.

The problem becomes very serious when theperson tries to do something physically active.Fatigue and the sense of being out of breath de-velop quickly and are rather severe. Some indi-viduals are disabled by this disease.

Fortunately, many cases of chronic bronchitisthat have not been allowed to progress too longcan be cured. The person need only eliminatebreathing polluted air. Eventually, mucus produc-tion will slow and the cilia will grow back andbegin to function as before.

Curing chronic bronchitis can be difficult ifsmoking is the source of the air pollution, how-ever, because tobacco smoke contains addictive

chemicals such as nicotine. Also, as the respira-tory system begins to clear itself, coughing in-creases temporarily. Smokers often experienceextra coughing in the morning because the clear-ing action began during the night, when they werenot smoking. After a period of abstention, smok-ing seems to help because it relieves the with-drawal symptoms and stops the clearing action,and thus stops the coughing. Of course, continu-ing to smoke only relieves certain symptomswhile the disease continues to destroy theperson's respiratory system.

Besides reducing directly the performance ofthe respiratory system, chronic bronchitis in-creases the risk of infection of the respiratory sys-tem because the accumulation and slow removalof mucus allow microbes to flourish in the air-


ways. It can also lead to emphysema, and thechronic coughing contributes to hemorrhoids.Long-term smoking is also a major risk factor fornonrespiratory diseases such as heart attack, ath-erosclerosis, and stroke.

Emphysema

Emphysema is a disease that involves actual de-struction of some parts of the lungs. There are twomain forms: centrilobar emphysema (CLE) andpanlobar emphysema (PLE). Both types will bepresent in most people with emphysema, thoughone type will predominate.

Centrilobar Emphysema Centrilobar emphy-sema most often develops along with or afterchronic bronchitis. It involves a thinning andweakening of the smallest bronchioles and theproduction of much mucus. Many results are simi-lar to those of chronic bronchitis. Additionally, thedamage to the bronchioles usually results in adecrease in the number of small blood vessels in

the lungs, decreasing perfusion. The reduction inblood vessels also makes it harder for the heart

to pump blood through the lungs, and the over-worked heart eventually becomes weaker. If CLEcontinues to progress, the victim eventually diesof respiratory failure, respiratory infection, orheart failure.

Panlobar Emphysema Panlobar emphysema isless common than CLE. Though the major causeis air pollution, some people inherit a tendency todevelop this type of emphysema.

PLE causes destruction of the walls of the al-

veoli and alveolar sacs. The results are like a highlyexaggerated version of age changes in the alveoli.Many walls between the alveoli shrink and disap-pear. Neighboring alveoli blend to form large air-filled spaces. The lungs change from having mi-croscopic spaces like those found in Styrofoam tohaving large spaces like those in a sponge. Thewall material that remains is weaker and less elas-

tic. All these changes reduce ventilation.With PLE, expiration becomes more difficult

and more residual air is left in the lungs. As pas-sive expiration decreases, forced expiration in-creases, increasing the work of breathing. Perfu-sion also decreases because the number of capil-laries 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 adecrease in the amount of surface area.

A complication of PLE is the partial or com-plete collapse of a lung. This occurs when a hol-low space developing close to the lung surfacebursts like a bubble. As escaping air separates thelung from the thoracic wall, the lung collapses likea balloon with a small leak. This condition is

called pneumothorax. Proper inspiration is impos-sible unless the leak heals and the body absorbsthe air from the thoracic cavity.

Overall Effects of Emphysema People in theearly stages of emphysema may hardly notice thedecline in their ability to perform physical activi-ties. As the disease progresses and devastatesmore of the lungs, gas exchange plummets. Even-tually, even walking at an ordinary pace becomesa challenge. Individuals with advanced cases areso disabled that they may be unable to get up, situp, or even roll over in bed without extreme fa-tigue. Mild exertion or a slight respiratory infec-tion can cause death. Among people over age 55,emphysema is the fifth leading cause of death formen and the seventh leading cause for women.

Pneumonia

Pneumonia is actually a group of related diseasesinvolving inflammation in the lungs. Severaltypes reduce a person's ability to inspire. Olderpeople are especially affected by pneumoniacaused by microbes (bacteria, viruses, and fungi)and by dust and chemical vapors. Pneumonia canalso result from aspirating stomach contents thathave moved up into the throat.

Microbial Pneumonia Reasons for the age-related increased susceptibility to microbial pneu-monia include age changes in the functioning ofthe mucociliary escalator, white blood cells, andthe immune system; the rising prevalence ofchronic bronchitis and emphysema; and otherdiseases that weaken the body and make it lessable to ward off infections.

Pneumonia caused by bacteria results in fill-ing of the airways and alveoli with fluids andcells from their walls. This material blocks the air-

ways. It usually becomes somewhat solid after1 to 2 days. If a person is otherwise healthy andreceives proper treatment, such as antibiotics, the

-- ;:]

- -.....112 Human Aging: Biological Perspectives

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. How-ever, some forms cause serious and permanentdamage that results in a reduction in respiratoryfunctioning and can cause death. These forms arethe ones most likely to occur in weakened or hos-pitalized individuals.

Viral pneumonia affects the walls of the alveoli,causing them to accumulate fluids and becomethicker, reducing gas exchange. If a person ishealthy and has a good immune system, the im-mune response will eliminate the virus in a fewdays and the lungs will return to normal func-tioning.

Because fungal pneumonia and tuberculosiscause death of the portions of the lungs they in-fect, they can be more serious than bacterial orviral pneumonia. Thus, after fungal and tubercu-lar infections are stopped, the lungs are left withregions that no longer function. Areas affected bytuberculosis are filled in with solid scar tissue

which, if calcified, can be detected on x-ray. Ifenough areas of the lungs are destroyed, gas ex-change and activity levels are permanently re-duced. More extensive damage results in death.

Unfortunately, many older individuals are nothealthy and do not have strong immune systemswhen they get pneumonia. Weakened personsmay have great difficulty combating the infection.Then the disease lasts longer and has a greaterimpact on the respiratory system. The proportionof individuals who survive microbial pneumoniadecreases rapidly with age. Those who surviveare often left weakened for long periods.

Dust and Vapors Some individuals breathe largeamounts of certain types of air pollution repeat-edly for long periods, usually because of theiroccupations. Examples include farmers, miners,textile mill workers, sandblasters, and woodwork-ers. The heavy exposure and the size and chemi-cal nature of such air pollutants cause the lungsto form large quantities of fibers and develop thecondition called pulmonary fibrosis.

With pulmonary fibrosis, the normal amountand rate of age changes in the lungs increase dra-matically, leading to a rapid decline in gas ex-change. Very severely affected people will becomequite disabled. Since the fibrosis is permanent,

affected individuals can recover little if any of thelost respiratory functioning even if they avoidfuture exposure to air pollution. The only solu-tion is to prevent pulmonary fibrosis by avoidingits causes.

Pulmonary Embolism

Pulmonary embolism (Chap. 4) is a disease con-dition in which blood clots have moved to the

lungs from the systemic veins or the heart. Con-ditions commonly promoting the formation ofsuch emboli in the elderly include varicose veins,congestive heart failure, and immobility. The eld-erly are especially prone to having conditions thatcause immobility. These include heart attack,stroke, hospitalization, recovery from surgery, .and fractures. The effects of pulmonary embolismdepend on the size and number of pulmonaryemboli.

Control Errors

Two age changes involving the control of ventila-tion that have not yet been discussed are sleepapnea and snoring.

Sleep Apnea Sleep apnea (SA) means having atleast five temporary cessations of ventilation perhour or exhibiting at least 10 occasions of de-pressed ventilation and cessation of ventilationper hour when asleep. The incidence of sleep ap-.nea increases with age up to age 65, after whichthe incidence plateaus. It is present in 4 percentof younger adults but in 25 percent to 30 percentof people over age 64. The male:female ratio forSA is approximately 3:1.

Sleep apnea may be caused by narrowing andcollapsing of the pharynx, especially when in asupine position (i.e., sleeping on one's back);be-cause the respiratory center becomes less sensi-tive; or because the center simply stops initiatinginspiration. Then blood levels of °2 and CO2change. These alterations in the blood may pro-vide the necessary stimulation to begin inspira-tion again. People with sleep apnea tend to snoreand to have frequent sudden awakenings withfeelings of respiratory distress.

Mild sleep apnea seems to have no deleteriousaffect on the body. However, frequent awakeningscan lead to fatigue, indications of sleep depriva.tion, and adverse alterations in mood and person-j


ality.Because sleep apnea causes significant fluc-tuations in °2' CO2, and blood pressure, seriouscasesincrease the risk of heart attack and stroke.

Treatmentsfor SA include avoiding sleeping in asupine position; using masks with pumps thatprovidepositive pressure into the respiratory sys-tem;reversal of obesity; medications; and surgi-calcorrection of the pharyngeal region.

Snoring Snoring, or making loud breathingsoundswhen asleep, is due to partially obstructedupper airways. Approximately half of all womenand well over half of all men above age 65 snore.Someindividuals who snore also have sleep apnea.

Snoring causes a variety of problems. Biologi-cally,it causes from mild to severe adverse effectsonblood °2 and CO2 levels and on circulation. Itcan also contribute to high blood pressure andheart disease. Since snoring disrupts normal sleeppatterns even if the person who is snoring doesnot awaken, it can result in fatigue and other in-dications of sleep deprivation.

Anyone who sleeps near a person who snorescanattest to some of the social implications. Theirresponses to the person who snores, together withthe multitude of jokes about snoring, can add tothe psychological impact produced by sleep dep-rivation. The fatigue felt by many snorers alsoaffects their social interactions and can impingeon their ability to carry out their jobs.

Though the causes of snoring and the role ofthe nervous system in snoring are not clear, re-search has provided methods of treatment for thiscondition.

SMOKING

Main consequences in the respiratory system fromsmoking have been mentioned. Smoking has ad-verse effects in other areas of the body, also. Ingeneral, smoking increases the formation of freeradicals and lipid peroxides while reducing theantioxidant actions of vitamin C, vitamin E, andp-carotenes. Smoking may increase free radicaldamage to DNA by 50 percent. In the skin, smok-ing speeds up and amplifies the effects from ag-ing and from photoaging. Smoking is associatedwith increased risks for most skin cancers. In the

circulatory system, smoking damages the endoth-elium; raises blood pressure; and increases sub-stantially the risk of blood clots, of atherosclero-sis, and of their complications. Effects on these

two systems are due partly to constriction of skinvessels and reductions in blood oxygen caused bysmoking. These two changes develop within min-utes of initiating smoking and can last for hours,long enough to light the next cigarette. The resultis continuous inadequate blood flow in the skinand elevated blood pressure. In the eyes, smok-ing is associated with a higher incidence of cata-racts and diseases of the retina. Smoking reducesestrogen levels in women and speeds up age-re-lated thinning of bones. Smoking doubles theproblems from non-insulin dependent diabetes;suppresses normal functioning of the immunesystem; promotes autoimmune diseases; and isassociated with higher rates of reproductive sys-tem and digestive system cancers. Cessation ofsmoking is associated with reduction or completereversal of these problems and risks.

SOUND PRODUCTION ANDSPEECH

The vocalizations produced by people involvewords and a variety of other sounds, such asmoans, grunts, whistles, cheers, and laughing.People use sound production for communication.Communication among individuals by sound andother means (e.g., visual signals) is important toa high quality of life and to survival because it isone of the three components in negative feedbacksystems. A common example of using vocaliza-tion as part of a negative feedback system is shout-ing a warning to a person in danger.

Human sound production can enhance life inother ways. Words and other vocalizations canmotivate and encourage positive actions such asbeginning a new career or hobby. They are alsoused in teaching, praising, consoling, expressingemotions, and many other human activities. Andwhat of the beauty of a poem or song? All theseare created by the sounds produced by the flowof air through airways.

Mechanisms

The respiratory system produces sound by pass-ing air through the upper airways and the mouth.Most of the sound people make is caused whenair passing through the larynx causes the vibra-tion of two flaps of tissue called the vocal cords(Fig. 5.5). The sound gets louder when more airflows through the larynx.


Different muscle contractions in the larynx con-trol the position and tension of the vocal cordsand thus alter the pitch of sounds. The soundsmade by the vocal cords are modified by the otherupper airways, especially the nasal passages andthe mouth. By changing the shape of these pas-sages and moving the tongue, a person can cre-ate a multitude of sounds and form words.

All the actions that produce and modify soundsfrom the respiratory system are controlled by thenervous system.

Age Changes

Many age changes that alter inspiration and es-pecially those which modify expiration affectsound production. Age-related stiffening of thelarynx, shrinkage of the vocal cords and itsmuscles, and changes in the mouth are also im-portant. Age changes in the nervous system arealso important since sound production dependson the coordinated action of many muscles. Evenage changes in hearing are important because theears provide feedback information so that thesounds a person produces can be adjusted to con-form to the sounds intended by that person.

Because of age changes in these areas, the voicebecomes more variable in pitch and volume dur-

ing speaking. Female voices often become lowerin pitch, while male voices often become higherin pitch. Other common changes include increasesin hoarseness, roughness, and extraneous soundswhile speaking. The voice often becomes weaker,and elders have declining abilities to speak veryquietly or with very loud volume. The ability tocontrol volume declines, and the precision ofword pronunciation diminishes.

Language fluency and vocabulary usually donot decline, and often increase. However, phrasesand sentences often become shorter, syllables andwords are repeated more often, and more wordsare pronounced incompletely. These trends inspeaking become more prominent in stressfulsituations. Of course, variability among eldersincreases with age, and some elders retain thevoice and speech of a young adult.

All these changes reduce the effectiveness ofvocalization in providing communication. Addi-tionally, some of the pleasure derived from thehuman voice may be lost. As a result, the contri-bution of the voice to happy and healthy survivaldiminishes. Age changes in the voice also alterthe way people respond socially to individualswho are getting older. These changes in turn af-fect aging individuals' responses and self-images.Therefore, biological aging of vocalization caninfluence nonbiological aspects of life.

respiratory system · the respiratory system obtains oxygen by provid-ing conditions that allow the...

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