Human Anatomy and Physiology:Layout 1

HUMAN ANATOMY AND PHYSIOLOGY
AN INTRODUCTION TO RESPIRATION
In this essay we will consider a few major aspects of res-
direction. However, he had achieved a good deal.
piration. We shall first consider the interesting history The next developments came with Joseph Priestly
of the study of respiration before moving on to our
(1733-1804) but perhaps Lavoisier had the clearest ex-
modern understanding of respiration. We will look at the
pressions of oxygen and respiration. Finally it was Gas-
structure and function of the respiratory system includ-
ton Magnus, by use of the mercuric air pump that
ing the upper and lower respiratory tracts with a note
showed that both arterial and venous blood contain
on the control system. Secondly we will consider the
both CO2 and O2, though in different proportions. Thus
physiology of respiration. Thirdly we will discuss some it took many centuries for the links between air, oxygen,
of the major common disorders and diseases which carbon dioxide, respiration and blood to be unraveled.
affect the system with a special focus on asthma.
Let us now look at modern views of respiration.
A BRIEF HISTORY OF THE STUDY OF RESPIRATION
THE STRUCTURE AND FUNCTION
Hippocrates "counted air as an instrument of the body"
OF THE RESPIRATORY SYSTEM
just as food was eaten. Galen (129-200) felt that respi-
The respiratory system is concerned with the exchange
ration served a triple purpose: 1) breathing cooled of oxygen and carbon dioxide between the blood and
the heart, 2) air needed for production of vital spirits the lungs. This is essential for the provision of energy for
3) respiration got rid of the friligimous or products of the
cellular metabolic function. Most energy extractions can
innate fire burning in the heart. Basically Galen's ideas
only take place in the presence of oxygen and the route
were followed for many centuries until the era of of oxygen intake into the body is via the respiratory sys-
the 17th century. Robert Boyle (1627-1691) who was pri-
tem. Basically inspiration brings in oxygen and expiration
marily a physicist was interested in the weight and pres-
excretes carbon dioxide. This is known as the exchange
sure of the air and in 1660 he showed that air was
of gases. Before we consider this in more detail let’s look
essential to both life and combustion by placing a can-
at the main organs of the respiratory system.
dle and a small animal in a vessel. Robert Hooke (1635-
1703) showed that after the thorax of a dog had been
Firstly we should consider the nasal cavity. As air is
opened life could be prolonged by artificial respiration
breathed in it is warmed and moistened while dust and
- this proved that the whole of the essential business debris are filtered out by cilia or hairs and mucus pro-
of respiration takes place in the lungs.
ducing goblet cells. Air which passes through the nasal
cavity joins air which is brought in through the mouth.
Richard Lower (1631-1691) upset the old ideas that the
The nasal cavity itself is divided by the septum. There
change in blood colour took place in the heart rather
are openings to the nostrils at the front and to the phar-
than in the lungs. He also demonstrated the necessity ynx at the back. In addition there are small openings
of fresh air in life rather than air generally and that in
to the maxillary, frontal and ethmoidal sinuses.
fact 'where a fire burns readily there we can easily
breathe'. He was close to conceptualizing oxygen here
Now we can move on to the pharynx. At the base
but this had to wait until Priestley’s work or perhaps the
of the pharynx there are two openings - one leading to
research of Lavoisier (1743-1794). However, John
the larynx and the rest of the respiratory system and the
Mayow (1641-1679) an English chemist had come very
other to the oesophagus and the digestive system. The
close to this conclusion in his experiments. He said that
epiglottis prevents both openings being open simulta-
air entered the lungs during inspiration simply because
neously. The larynx is located between the pharynx and
the pressure or elastic force of the atmosphere drove the trachea. This is commonly known as the voice box.
it in to fill in the increased space afforded by the en-
The trachea is a tube which extends from the larynx
larged and dilated thorax. Mayow had also in reality
and divides to form the bronchi. It is lined with cilia with
come near to discovering oxygen but did not isolate further warms and filters the air. The bronchi lead to the
it or realise that carbon dioxide passed in the reverse
lungs. The lungs are a pair of organs located on either

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side of the thoracic cavity and enclosed in double
the number of hydrogen ions in the blood, which
membranes or pleura. The bronchi enter the lungs at
decreases the pH of the blood. This is as a direct con-
a point known as the helium. They divide into smaller
sequence of increase in carbon dioxide concentra-
and smaller branches known as bronchioles and tion because carbon dioxide becomes carbonic
finally alveolar ducts and alveoli. These are sack like
acid in an aqueous environment. The response is that
structures and this is the site of gas exchange. Their
the inspiratory centre in the medulla sends nervous im-
total surface area is approxi-
pulses to the external intercostal
mately 70 square meters.
muscles and diaphragm via
the phrenic nerve to increase
Lets us consider the actual
breathing rate and volume of lung
mechanism of breathing for
inhalation. There are two main
a moment. Inspiration involves
kinds of chemoreceptors with
movement of the diaphragm
affect breathing rates. Firstly the
and the internal and external in-
central chemoreceptors located
tercostal muscles. On inspiration
on the medulla oblongata detect
the diaphragm contracts pulling downwards on changes in the pH of the cerebrospinal fluid and
the lung and drawing air into the lungs. The ribs move
secondly the peripheral chemoreceptors in the Aortic
upwards and outwards at the same time. Expiration body detects changes in blood oxygen and carbon
is said to be a passive event which arises from the re-
dioxide, but not blood pH (those in the carotid body
laxation of the diaphragm and intercostals. The vol-
detect all levels).
ume of air breathed in and out is known as the tidal
In addition there are pulmonary stretch receptors
volume and is normally about half a litre. However
which are mechanoreceptors found in the lungs.
total lung capacity includes inspiratory reserve vol-
When the lung expands the receptors imitate
ume, expiratory reserve volume and residual volumes
the Hering-Breuer reflex which reduces respiratory rate
so bringing the overall lung capacity to about 6 litres
and increases production of pulmonary surfactant.
for men and 4.2 for women.
We have now considered the structure and functions
THE CONTROL SYSTEM FOR RESPIRATION
of the respiratory system as well as its mechanics and its
The control unit of ventilation consists of a processor control systems. We should now consider the physiol-
or breathing centre in the brain which integrates
ogy of the system starting with the exchange of gases.
emotional, chemical and physical stimuli inputs and
controls an effector - in this case the lungs via motor
THE EXCHANGE OF GASES
nerves from the spinal cord. Ventilation is normally au-
AND PHYSIOLOGY OF RESPIRATION
tonomic with a limited voluntary override. Ondine's
In basic terms the exchange of the gases (oxygen
curse is the exception to this where the autonomic
and carbon dioxide) occurs in the alveoli. These are
control is lost. The mechanism of generation is not
surrounded by a network of small blood vessels with
completely understood but involves the integration of
extremely thin walls. Blood which enters these vessels
neural signals by respiratory control centres in has a high level of carbon dioxide picked up from the
the medulla and pons. In the medulla we have the
body tissues. The carbon dioxide leaves the blood,
ventral respiratory group i.e. nucleus retroambigualis,
passes through the walls of the blood vessels into the
nucleus ambiguus, nucleus parambigualis and alveoli and into the lungs. Oxygen then passes the
the pre-Botzinger complex. This group controls volun-
other way into the blood and the blood, now rich in
tary forced exhalation and also works to increase oxygen, leaves the lungs and travels to the heart.
the force of inspiration. The medulla also contains On a slightly more detailed level it can be said that
O
the dorsal respiratory group consisting mainly the diffusion of oxygen and carbon dioxide is a con-
of the nucleus tractus solitarius and this controls mostly
tinuous process but dependent on atmospheric pres-
inspiratory movements and their timing. The pons con-
sure compared to that of blood and tissues. Normal
tains the pneumotaxic centre which is involved with
atmospheric pressure at sea level is 101.3 kilopascals
2
the fine tuning of the respiration rate and the ap-
(KPa). Air that is inspired contains nitrogen, oxygen,
neustic centre. In addition there is further integration
carbon dioxide, water vapour and inert gases.
in the anterior horn cells of the spinal cord.
Exchange of gases occurs when there is a difference
The actual breathing rate of a human is controlled in
in the partial pressures across the semi-permeable
the following way. Chemoreceptors detect the levels
membrane (in this case alveolar membrane).
of carbon dioxide in the blood by monitoring The movement by diffusion is from the higher con-

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centration to the lower until equilibrium is achieved.
blood. The cardiac cycle is otherwise known as the
For example the pressure of alveolar content oxygen
heart beat and in general this lasts about 0.8 seconds
is 13.3 KPa and in deoxygenated blood it is 5.3 KPa. in duration. The contraction of the heart (systole) oc-
In contrast the pressure of alveolar content carbon
curs in the left and right atria, blood is forced through
dioxide is 5.3 KPa and deoxygenated blood is 5.8KPa.
the valves into the ventricles, from there into the aorta
The pressure of nitrogen and other inert gases remains
and then distributed throughout the body. Following
unchanged as they are not used by the body. In ad-
the ventricular contraction the heart relaxes (diastole)
dition trace gases present
and the heart is prepared for the
in breath at extremely low levels
next cycle. A healthy heart beats
are ammonia, acetone and iso-
60-80 times per minute on aver-
prene. The primary force
age with variations for sex, age
in the respiratory tract is atmos-
and fitness levels.
pheric pressure - this explains
In more detail it should be noted
why it is harder to breath at
that the vena cava transports
higher altitudes for example. The
deoxygenated blood into the
above process is known as external respiration.
right atrium as the four pulmonary veins bring oxy-
Internal respiration is the process by which diffused
genated blood into the left atrium. The valves and
oxygen reaches the cells and this is achieved by vessels of the heart open and close in response to
a lower partial pressure concentration in body tissue
changing pressures in the heart chambers and this
than in capillary fluid. Furthermore oxygen is released
ensures that blood flows only in one direction. The
from oxyhaemoglobin because of low tissue or cellu-
sinoatrial node (SA node) and the atrioventricular
lar oxygen levels, low pH, and rise in temperatures.
node (AV node) are very important in the cycle. Elec-
Cell metabolism also increases the release of carbon
trical changes within the heart can be recorded by
dioxide into blood encouraging the disassociation of
electrocardiogram or ECG.
the oxygen molecules from red blood cells.
Finally another related aspect of the physiology of res-
In fact is it is difficult to discuss respiration without some
piration are the differences in inhaled and exhaled
reference to red blood cells and the cardiac cycle.
air. The major differences in composition between in-
Let us take each in turn briefly:
haled and exhaled air are as follows. Nitrogen con-
tent: in inhaled air this is about 70% and in exhaled
In fact one of the main functions of blood is the trans-
79%. Oxygen content is approximately 20% in inhaled
portation of oxygen from the lungs to the tissues and
air and 16% in exhaled. Carbon Dioxide content is
the removal of carbon dioxide from the blood to the
about 0.03% in inhaled air and 4% in exhaled. Water
lungs for excretion (in addition to transportation of
vapour varies in inhaled air but in exhaled air there is
hormones to organs and tissues, carrying of protec-
saturation. Finally the temperature of inhaled air varies
tive substances such as antibodies, blood also con-
with atmospheric conditions but exhaled air is close
tains clotting factors to minimize blood loss in case of
to body temperature.
injury). As we stated above the oxygen is associated
with the red blood cells or RBC.
We have now discussed the structure and function of
the respiratory systems, some of the mechanics in-
Red blood cells (erythrocytes) have the following
volved and the basic physiology. We should now turn
structure. They are bi-concave, have no nucleus, are
to the third topic adumbrated in our introduction -
produced in bone marrow, live about 7 months, re-
namely a review of some of the main disorders and dis-
sponsible for exchange of gases and are flexible to
eases of the respiratory system. Here we will focus on
help transportation. They contain haemoglobin which
asthma as this is currently on the increase worldwide.
is rich in iron and responsible for gas transportation
and are broken down in the spleen, bone marrow
DISORDERS AND DISEASES OF THE RESPIRATORY SYSTEM
and liver. Blood grouping is determined by different
Respiratory disease is really an umbrella term for dis-
antigens, the main groups are A, B, O, AB and some
eases of the lung, bronchial tubes, trachea and phar-
contain either a positive or negative Rhesus factor.
ynx and they range from mild like the common cold
Red blood cells are involved in oxygen transportation
to life threatening diseases such as bacterial pneu-
and oxyhaemoglobin is formed when the blood is sat-
monia and pulmonary embolism. Respiratory diseases
urated with oxygen and the iron atoms or binding sites
can be classified as either obstructive - conditions
are full. We also need to look briefly at the cardiac
that impeded the rate of flow into and out of the
cycle as this drives the transportation of oxygenated
lungs e.g. asthma or restrictive - conditions that cause

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a reduction in the functional volume of the lungs e.g.
can not empty properly and this may lead to hyper-
pulmonary fibrosis. A further example of a restrictive
inflation. Severe attacks can lead to respiratory fail-
disease would be emphysema.
ure. Much more research needs to be done in the
area of asthma. It may be caused by a complex in-
Emphysema results in the permanent destruction of
teraction of genetic and environmental factors and
the alveoli because of over distension. Predisposing
these can also influence how severe a person’s
factors can include smoking and also exposure to
asthma is and how they respond to medication. We
toxic chemicals in addition to inflammation of the
could divide the causes into environmental, genetic
bronchi and alveoli, increases in pressures due to
or a combination of both. Let’s look at some environ-
coughing and enzymatic deficiency. The damage
mental causes first.
can not be reversed, only slowed down and can lead
to shortage of breath and low levels of circulating
It is thought that some of the contributory factors to-
oxygen and high levels of circulating carbon dioxide.
wards asthma attacks are low air temperatures, air
Emphysema can develop alongside chronic bronchi-
pollution and cigarette smoke, respiratory tract infec-
tis and be known as a form of chronic obstructive pul-
tions, stress and exercise. In children viral illnesses such
monary disease or COPD. The sufferer can have
as the common cold may trigger asthma too. Asthma
cyanosis or lack of oxygen. Diagnoses can be by
is actually on the increase in the developing world -
spirometry and diffusion testing, X rays, high resolution
especially among children. In fact there are a
spiral chest CT scan, bronchoscopy, blood tests and
plethora of possible causes of asthma ranging from
pulse. Treatments currently can only really slow the dis-
allergens such as house dust mites, cockroaches,
ease down unless there is a lung transplant although
pollen, mold and pets. Perfumes, hairsprays and
the most important measure is to slow down the pro-
volatile organic compounds can trigger asthma. As-
gression by stopping the patient smoking and avoid
pirin, beta blockers and penicillin can start an attack.
all exposure to smoke and lung irritants. Also pul-
Food allergies can contribute. Use of fossils fuels,
monary rehabilitation can be helpful in optimizing
smog, sulfur dioxide cause asthma is urban areas per-
quality of life. We have mentioned emphysema as
haps. Industrial compounds such as sulfites, chlori-
one disease of the lungs but there are of course many
nated swimming pools may induce. Hormonal triggers
more such as asbestosis and other dust caused dis-
in adolescent girls can trigger asthma as can emo-
eases, black lung disease, cystic fibrosis, farmer's lung
tional stress. In one genetic study of asthma over 100
and other mold diseases, hay fever, Legionnaire's dis-
genes were found to associate with asthma in some
ease, lung cancer, pleurisy, pneumonia, sarcoidosis,
way, however some of these are only influential in
tuberculosis and whooping cough ranging from the
combination with environmental factors and as
mild e.g. common cold and hay fever to the lethal
stated previously much more research needs to be
such as asbestosis. Let us look at one particular lung
done in this area.
complaint that is growing in the current era: Asthma.
However, one thing is apparent. Asthma is on the in-
Asthma has been chosen here because of its in-
crease worldwide. According to one study there has
creasing importance. According to the World Health
been a 75% increase in asthma in US children in re-
Organization (WHO) there are about 300 million suf-
cent decades. More than 6% of children in the USA
ferers of asthma and in 2005 255,000 died of this.
have asthma and this increases dramatically to 40% in
Asthma is the most common chronic disease among
the urban areas. According to the Centres for Disease
children. Asthma deaths will increase by over 20% in
Control some 9% of children in the US had asthma in
the next ten years if urgent action is not taken and fi-
2001 compared with just 3.6% in 1980. A study by the
nally asthma is under-diagnosed and under-treated
WHO reports that some 8% of the Swiss population suf-
CO
creating a substantial burden to individuals and fam-
fers from asthma compared to only 2% some 25-
ilies and restricting individuals’ activities for a lifetime. 30 years ago. Asthma is more
common in affluent countries but it
Basically asthma is an inflammatory disease of the air
is not restricted to them. WHO es-
passages which is associated with the inter
2
mittent
timate that there are up to 20
overactivity of the smooth muscles associated with
million asthmatics in India
the respiratory tract. The mucous membrane and
alone. Also there is a socio-
muscle layers of the bronchi thicken, mucous glands
economic pattern to asthma.
enlarge and airflow is restricted. During an attack the
The incidence of asthma is
bronchi contract and excessive mucous is produced.
highest among lower income
Due to the narrowing of the air passages the lungs
populations and these also

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tend to be disproportionately ethnic minorities in
today's environment. As we seek to come to terms
some Western countries and are also more likely to
with greenhouse gas emissions and pollution we can
live nearer industrial areas.
see the effects they have on humanity - asthma is just
one clear illustration of this. We continue to pollute the
The good news is that asthma is preventable in many
Earth's atmosphere both indoors and outdoors and
cases. Inhaled glucocorticoids are the most common
both have their influence on asthma. Indoor pollution
preventative medication and usually come in inhaler
has a lower profile than outdoor but is also significant.
devices. However, long term use of these devices can
Volatile organic compounds such as perfumes, soap,
have side effects including redistribution of fat, blood
dishwashing liquid, detergent, fabric conditioner,
glucose problems, weight gain and even osteoporosis.
paper tissues, shampoo, hairsprays, gel, cosmetics,
Leukotriene modifiers can be used and Mast cell sta-
creams, deodorants, cologne, shaving cream, air
bilizers. Antimuscarinics can be used as a mixed re-
freshener, candles and paint can all trigger asthma.
liever and preventer. Antihistamines can be used to
Higher profile outdoor pollution from fossils fuels, nitro-
treat allergic symptoms that may underlie the chronic
gen dioxide, sulfur dioxide are thought to cause
inflammation or allergy shots can be used. Methotrex-
asthma in urban areas. As we continue to pollute we
ate can be used in some difficult to treat patients. The
can expect to see incidence of asthma rise. However,
most effective treatment for asthma is simply to identify
despite the research of organizations like the World
the triggers such as smoke, pets, cockroaches, chem-
Health Organization many more studies will need to
icals, exercise, aspirin and limit the exposure to them.
be conducted into the links between pollution and
asthma. However, asthma is only one of a plethora of
CONCLUSION
disorders and diseases that are influenced by the
In the discussion above we have explored the early
quality of the air we breathe. We can therefore con-
history of the study of respiration. This leads to modern
clude that a knowledge of respiration, its systems and
studies and explanations of the respiration with a look
disorders is a vital basis for decisions we take on
at the structure, function, mechanics and control sys-
the future of the planet.
tems of the respiratory system. This was followed by a
more detailed discussion of the physiology of respira-
tion with a final note on respiratory system diseases
Dr Simon Harding
and disorders. This is of the utmost importance in
and Lidia Starzyƒska

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