# Human Physiology/The respiratory system

The **Respiratory System** is vital to every human being. Without it, we
would cease to live outside of the womb. Let us begin by taking a look
at the structure of the respiratory system and how vital it is to life.
During inhalation or exhalation air is pulled towards or away from the
lungs, by several cavities, tubes, and openings.

The organs of the respiratory system make sure that **oxygen** enters
our bodies and **carbon dioxide** leaves our bodies.

The respiratory tract is the path of air from the nose to the lungs. It
is divided into two sections: **Upper Respiratory Tract** and the
**Lower Respiratory Tract**. Included in the upper respiratory tract are
the **Nostrils**, **Nasal Cavities**, **Pharynx**, **Epiglottis**, and
the **Larynx**. The lower respiratory tract consists of the **Trachea**,
**Bronchi**, **Bronchioles**, and the **Lungs**.

As air moves along the respiratory tract it is warmed, moistened and
filtered.

!The **lungs** flank the heart and great vessels in the chest cavity.
`<small>`{=html}(Source: *Gray\'s Anatomy of the Human Body*, 20th ed.
1918.)`</small>`{=html}"){width="230"}

## Functions

In this chapter we will discuss the four processes of respiration. They
are:

1.  **BREATHING** or ventilation
2.  **EXTERNAL RESPIRATION**, which is the exchange of gases (oxygen and
    carbon dioxide) between inhaled air and the blood.
3.  **INTERNAL RESPIRATION**, which is the exchange of gases between the
    blood and tissue fluids.
4.  **CELLULAR RESPIRATION**

In addition to these main processes, the respiratory system serves for:

-   **REGULATION OF BLOOD pH**, which occurs in coordination with the
    kidneys, and as a
-   **DEFENSE AGAINST MICROBES**
-   **Control of body temperature** due to loss of evaporate during
    expiration

## Breathing and Lung Mechanics

**Ventilation** is the exchange of air between the external environment
and the alveoli. Air moves by bulk flow from an area of high pressure to
low pressure. All pressures in the respiratory system are relative to
atmospheric pressure (760mmHg at sea level). Air will move in or out of
the lungs depending on the pressure in the alveoli. The body changes the
pressure in the alveoli by changing the volume of the lungs. As volume
increases pressure decreases and as volume decreases pressure increases.
There are two phases of ventilation; inspiration and expiration. During
each phase the body changes the lung dimensions to produce a flow of air
either in or out of the lungs.

The body is able to stay at the dimensions of the lungs because of the
relationship of the lungs to the thoracic wall. Each lung is completely
enclosed in a sac called the pleural sac. Two structures contribute to
the formation of this sac. The parietal pleura is attached to the
thoracic wall where as the visceral pleura is attached to the lung
itself. In-between these two membranes is a thin layer of intrapleural
fluid. The intrapleural fluid completely surrounds the lungs and
lubricates the two surfaces so that they can slide across each other.
Changing the pressure of this fluid also allows the lungs and the
thoracic wall to move together during normal breathing. Much the way two
glass slides with water in-between them are difficult to pull apart,
such is the relationship of the lungs to the thoracic wall.

The rhythm of ventilation is also controlled by the \"Respiratory
Center\" which is located largely in the medulla oblongata of the brain
stem. This is part of the autonomic system and as such is not controlled
voluntarily (one can increase or decrease breathing rate voluntarily,
but that involves a different part of the brain). While resting, the
respiratory center sends out action potentials that travel along the
phrenic nerves into the diaphragm and the external intercostal muscles
of the rib cage, causing inhalation. Relaxed exhalation occurs between
impulses when the muscles relax. Normal adults have a breathing rate of
12-20 respirations per minute.

### The Pathway of Air

When one breathes air in at sea level, the inhalation is composed of
different gases. These gases and their quantities are Oxygen which makes
up 21%, Nitrogen which is 78%, Carbon Dioxide with 0.04% and others with
significantly smaller portions.

right\|framed\|Diagram of the
**Pharynx**.

In the process of breathing, air enters into the nasal cavity through
the nostrils and is filtered by coarse hairs (*vibrissae*) and mucous
that are found there. The vibrissae filter macroparticles, which are
particles of large size. Dust, pollen, smoke, and fine particles are
trapped in the mucous that lines the **nasal cavities** (hollow spaces
within the bones of the skull that warm, moisten, and filter the air).
There are three bony projections inside the nasal cavity. The
**superior, middle, and inferior nasal conchae**. Air passes between
these conchae via the nasal meatuses.

Air then travels past the nasopharynx, oropharynx, and laryngopharynx,
which are the three portions that make up the pharynx. The
**pharynx** is a funnel-shaped tube that
connects our nasal and oral cavities to the larynx. The **tonsils**
which are part of the lymphatic system, form a ring at the connection of
the oral cavity and the pharynx. Here, they protect against foreign
invasion of antigens. Therefore, the respiratory tract aids the immune
system through this protection. Then the air travels through the
**larynx**. The larynx closes at the epiglottis to prevent the passage
of food or drink as a protection to our trachea and lungs. The larynx is
also our voicebox; it contains vocal cords, in which it produces sound.
Sound is produced from the vibration of the vocal cords when air passes
through them.

The **trachea**, which is also known as our windpipe, has ciliated cells
and mucous secreting cells lining it, and is held open by C-shaped
cartilage rings. One of its functions is similar to the larynx and nasal
cavity, by way of protection from dust and other particles. The dust
will adhere to the sticky mucous and the cilia helps propel it back up
the trachea, to where it is either swallowed or coughed up. The
**mucociliary escalator** extends from the top of the trachea all the
way down to the **bronchioles**, which we will discuss later. Through
the trachea, the air is now able to pass into the bronchi, bronchioles
and finally alveoli before entering the pulmonary capillaries. There is
lots of oxygen and then there is less carbon dioxide when the air comes
in, but when it diffuses, the amounts exchange. All of this happens in
seconds.

### Inspiration

**Inspiration is initiated by contraction of the diaphragm** and in some
cases the intercostals muscles when they receive nervous impulses.
During normal quiet breathing, **the phrenic nerve stimulates the
diaphragm to contract and move downward into the abdomen**. This
downward movement of the diaphragm enlarges the thorax. When necessary,
the intercostal muscles also increase the thorax by contacting and
drawing the ribs upward and outward.

As the diaphragm contracts inferiorly and thoracic muscles pull the
chest wall outwardly, the volume of the thoracic cavity increases. The
lungs are held to the thoracic wall by negative pressure in the pleural
cavity, a very thin space filled with a few millilitres of lubricating
pleural fluid. The negative pressure in the pleural cavity is enough to
hold the lungs open in spite of the inherent elasticity of the tissue.
Hence, as the thoracic cavity increases in volume the lungs are pulled
from all sides to expand, causing a drop in the pressure (a partial
vacuum) within the lung itself (but note that this negative pressure is
still not as great as the negative pressure within the pleural
cavity\--otherwise the lungs would pull away from the chest wall).
Assuming the airway is open, air from the external environment then
follows its pressure gradient down and expands the alveoli of the lungs,
where gas exchange with the blood takes place. As long as pressure
within the alveoli is lower than atmospheric pressure air will continue
to move inwardly, but as soon as the pressure is stabilized air movement
stops.

### Expiration

During quiet breathing, expiration is normally a passive process and
does not require muscles to work (rather it is the result of the muscles
relaxing). When the lungs are stretched and expanded, stretch receptors
within the alveoli send inhibitory nerve impulses to the medulla
oblongata, causing it to stop sending signals to the rib cage and
diaphragm to contract. The muscles of respiration and the lungs
themselves are elastic, so when the diaphragm and intercostal muscles
relax there is an elastic recoil, which creates a positive pressure
(pressure in the lungs becomes greater than atmospheric pressure), and
air moves out of the lungs by flowing down its pressure gradient.

Although the respiratory system is primarily under involuntary control,
and regulated by the medulla oblongata, we have some voluntary control
over it also. This is due to the higher brain function of the cerebral
cortex.

When under physical or emotional stress, more frequent and deep
breathing is needed, and both inspiration and expiration will work as
active processes. Additional muscles in the rib cage forcefully contract
and push air quickly out of the lungs. In addition to deeper breathing,
when coughing or sneezing we exhale forcibly. Our abdominal muscles will
contract suddenly (when there is an urge to cough or sneeze), raising
the abdominal pressure. The rapid increase in pressure pushes the
relaxed diaphragm up against the pleural cavity. This causes air to be
forced out of the lungs.

Another function of the respiratory system is to sing and to speak. By
exerting conscious control over our breathing and regulating flow of air
across the vocal cords we are able to create and modify sounds.

### Lung Compliance

**Lung Compliance** is the magnitude of the change in lung volume
produced by a change in pulmonary pressure. Compliance can be considered
the opposite of stiffness. A low lung compliance would mean that the
lungs would need a greater than average change in intrapleural pressure
to change the volume of the lungs. A high lung compliance would indicate
that little pressure difference in intrapleural pressure is needed to
change the volume of the lungs. More energy is required to breathe
normally in a person with low lung compliance. Persons with low lung
compliance due to disease therefore tend to take shallow breaths and
breathe more frequently.

**Determination of Lung Compliance** Two major things determine lung
compliance. The first is the elasticity of the lung tissue. Any
thickening of lung tissues due to disease will decrease lung compliance.
The second is surface tensions at air water interfaces in the alveoli.
The surface of the alveoli cells is moist. The attractive force, between
the water cells on the alveoli, is called surface tension. Thus, energy
is required not only to expand the tissues of the lung but also to
overcome the surface tension of the water that lines the alveoli.

To overcome the forces of surface tension, certain alveoli cells (Type
II pneumocytes) secrete a protein and lipid complex called
\"\"Surfactant"", which acts like a detergent by disrupting the hydrogen
bonding of water that lines the alveoli, hence decreasing surface
tension.

### Control of respiration

#### Central control

The medulla oblongata is the primary respiratory control center

Its main function is to send signals to the muscles that control
respiration to cause breathing to occur.

#### Peripheral control

CO~2~ is converted to HCO~3~; most CO~2~ produced at the tissue cells is
carried to lungs in the form of HCO~3~

-   CO~2~ & H~2~O form carbonic acid (H~2~CO~3~)
-   changes to H CO~3~ & H+ ions
-   result is H+ ions are buffered by plasma proteins

## Respiratory System: Upper and Lower Respiratory Tracts

For the sake of convenience, we will divide the respiratory system in to
the upper and lower respiratory tracts:

### Upper Respiratory Tract

The upper respiratory tract consists of the nose and the pharynx. Its
primary function is to receive the air from the external environment and
filter, warm, and humidify it before it reaches the delicate lungs where
gas exchange will occur.

Air enters through the nostrils of the nose and is partially filtered by
the nose hairs, then flows into the nasal cavity. The nasal cavity is
lined with epithelial tissue, containing blood vessels, which help warm
the air; and secrete mucous, which further filters the air. The
endothelial lining of the nasal cavity also contains tiny hairlike
projections, called cilia. The *cilia* serve to transport dust and other
foreign particles, trapped in mucous, to the back of the nasal cavity
and to the pharynx. There the mucus is either coughed out, or swallowed
and digested by powerful stomach acids. After passing through the nasal
cavity, the air flows down the pharynx to the larynx.

### Lower Respiratory Tract

The lower respiratory tract starts with the larynx, and includes the
trachea, the two bronchi that branch from the trachea, and the lungs
themselves. This is where gas exchange actually takes place.

1.  Larynx

The larynx (plural larynges), colloquially known as the voice box, is an
organ in our neck involved in protection of the trachea and sound
production. The larynx houses the vocal cords, and is situated just
below where the tract of the pharynx splits into the trachea and the
esophagus. The larynx contains two important structures: the epiglottis
and the vocal cords.

The epiglottis is a flap of cartilage located at the opening to the
larynx. During swallowing, the larynx (at the epiglottis and at the
glottis) closes to prevent swallowed material from entering the lungs;
the larynx is also pulled upwards to assist this process. Stimulation of
the larynx by ingested matter produces a strong cough reflex to protect
the lungs. Note: choking occurs when the epiglottis fails to cover the
trachea, and food becomes lodged in our windpipe.

The vocal cords consist of two folds of connective tissue that stretch
and vibrate when air passes through them, causing vocalization. The
length the vocal cords are stretched determines what pitch the sound
will have. The strength of expiration from the lungs also contributes to
the loudness of the sound. Our ability to have some voluntary control
over the respiratory system enables us to sing and to speak. In order
for the larynx to function and produce sound, we need air. That is why
we can\'t talk when we\'re swallowing.

1.  Trachea
2.  Bronchi
3.  Lungs

## Homeostasis and Gas Exchange

!Gas exchange{width="200"} Homeostasis is
maintained by the respiratory system in two ways: gas exchange and
regulation of blood pH. Gas exchange is performed by the lungs by
eliminating carbon dioxide, a waste product given off by cellular
respiration. As carbon dioxide exits the body, oxygen needed for
cellular respiration enters the body through the lungs. ATP, produced by
cellular respiration, provides the energy for the body to perform many
functions, including nerve conduction and muscle contraction. Lack of
oxygen affects brain function, sense of judgment, and a host of other
problems.

### Gas Exchange

Gas exchange in the lungs and in the alveoli is between the alveolar air
and the blood in the pulmonary capillaries. This exchange is a result of
increased concentration of CO2, and a decrease of oxygen. This process
of exchange is done through diffusion.

### External Respiration

External respiration is the exchange of gas between the air in the
alveoli and the blood within the pulmonary capillaries. A normal rate of
respiration is 12-25 breaths per minute. In external respiration, gases
diffuse in either direction across the walls of the alveoli. Oxygen
diffuses from the air into the blood and carbon dioxide diffuses out of
the blood into the air. Most of the carbon dioxide is carried to the
lungs **in plasma** as bicarbonate ions (HCO3-). When blood enters the
pulmonary capillaries, the bicarbonate ions and hydrogen ions are
converted to carbonic acid (H2CO3) and then back into carbon dioxide
(CO2) and water. This chemical reaction also uses up hydrogen ions. The
removal of these ions gives the blood a more neutral pH, allowing
hemoglobin to bind up more oxygen. De-oxygenated blood \"blue blood\"
coming from the pulmonary arteries, generally has an oxygen partial
pressure (pp) of 40 mmHg and CO2 pp of 45 mmHg. Oxygenated blood leaving
the lungs via the pulmonary veins has an O2 pp of 100 mmHg and CO2 pp of
40 mmHg. It should be noted that alveolar O2 pp is 105 mmHg, and not 100
mmHg. The reason why pulmonary venous return blood has a lower than
expected O2 pp can be explained by \"Ventilation Perfusion Mismatch\".

### Internal Respiration

Internal respiration is the exchanging of gases at the cellular level.

#### The Passage Way From the Trachea to the Bronchioles

There is a point at the inferior portion of the trachea where it
branches into two directions that form the right and left primary
bronchus. This point is called the **Carina** which is the keel-like
cartilage plate at the division point. We are now at the **Bronchial
Tree**. It is named so because it has a series of respiratory tubes that
branch off into smaller and smaller tubes as they run throughout the
lungs.

#### Right and Left Lungs

right\|framed\|Diagram of the
**lungs** The **Right Primary
Bronchus** is the first portion we come to, it then branches off into
the **Lobar (secondary) Bronchi**, **Segmental (tertiary) Bronchi**,
then to the **Bronchioles** which have little cartilage and are lined by
simple cuboidal epithelium (See fig. 1). The bronchi are lined by
pseudostratified columnar epithelium. Objects will likely lodge here at
the junction of the Carina and the Right Primary Bronchus because of the
vertical structure. Items have a tendency to fall in it, where as the
Left Primary Bronchus has more of a curve to it which would make it hard
to have things lodge there.

The **Left Primary Bronchus** has the same setup as the right with the
lobar, segmental bronchi and the bronchioles.

The lungs are attached to the heart and trachea through structures that
are called the **roots of the lungs**. The roots of the lungs are the
bronchi, pulmonary vessels, bronchial vessels, lymphatic vessels, and
nerves. These structures enter and leave at the **hilus** of the lung
which is \"the depression in the medial surface of a lung that forms the
opening through which the bronchus, blood vessels, and nerves pass\"
(medlineplus.gov).

There are a number of **terminal bronchioles** connected to
**respiratory bronchioles** which then advance into the **alveolar
ducts** that then become **alveolar sacs**. Each bronchiole terminates
in an elongated space enclosed by many air sacs called **alveoli** which
are surrounded by blood capillaries. Present there as well, are
**Alveolar Macrophages**, they ingest any microbes that reach the
alveoli. The **Pulmonary Alveoli** are **microscopic**, which means they
can only be seen through a microscope, membranous air sacs within the
lungs. They are units of respiration and the site of gas exchange
between the respiratory and circulatory systems.

### Cellular Respiration

First the oxygen must diffuse from the alveolus into the capillaries. It
is able to do this because the capillaries are permeable to oxygen.
After it is in the capillary, about 5% will be dissolved in the blood
plasma. The other oxygen will bind to red blood cells. The red blood
cells contain hemoglobin that carries oxygen. Blood with hemoglobin is
able to transport 26 times more oxygen than plasma without hemoglobin.
Our bodies would have to work much harder pumping more blood to supply
our cells with oxygen without the help of hemoglobin. Once it diffuses
by osmosis it combines with the hemoglobin to form oxyhemoglobin.

Now the blood carrying oxygen is pumped through the heart to the rest of
the body. Oxygen will travel in the blood into arteries, arterioles, and
eventually capillaries where it will be very close to body cells. Now
with different conditions in temperature and pH (warmer and more acidic
than in the lungs), and with pressure being exerted on the cells, the
hemoglobin will give up the oxygen where it will diffuse to the cells to
be used for cellular respiration, also called aerobic respiration.
Cellular respiration is the process of moving energy from one chemical
form (glucose) into another (ATP), since all cells use ATP for all
metabolic reactions.

It is in the mitochondria of the cells where oxygen is actually consumed
and carbon dioxide produced. Oxygen is produced as it combines with
hydrogen ions to form water at the end of the electron transport chain
(see chapter on cells). As cells take apart the carbon molecules from
glucose, these get released as carbon dioxide. Each body cell releases
carbon dioxide into nearby capillaries by diffusion, because the level
of carbon dioxide is higher in the body cells than in the blood. In the
capillaries, some of the carbon dioxide is dissolved in plasma and some
is taken by the hemoglobin, but most enters the red blood cells where it
binds with water to form carbonic acid. It travels to the capillaries
surrounding the lung where a water molecule leaves, causing it to turn
back into carbon dioxide. It then enters the lungs where it is exhaled
into the atmosphere.

## Lung Capacity

!450 px\|right The normal volume
moved in or out of the lungs during quiet breathing is called **tidal
volume**. When we are in a relaxed state, only a small amount of air is
brought in and out, about 500 mL. You can increase both the amount you
inhale, and the amount you exhale, by breathing deeply. Breathing in
very deeply is **Inspiratory Reserve Volume** and can increase lung
volume by 2900 mL, which is quite a bit more than the tidal volume of
500 mL. We can also increase expiration by contracting our thoracic and
abdominal muscles. This is called **expiratory reserve volume** and is
about 1400 ml of air. **Vital capacity** is the total of tidal,
inspiratory reserve and expiratory reserve volumes; it is called vital
capacity because it is vital for life, and the more air you can move,
the better off you are. There are a number of illnesses that we will
discuss later in the chapter that decrease vital capacity. Vital
Capacity can vary a little depending on how much we can increase
inspiration by expanding our chest and lungs. Some air that we breathe
never even reaches the lungs! Instead it fills our nasal cavities,
trachea, bronchi, and bronchioles. These passages aren\'t used in gas
exchange so they are considered to be **dead air space**. To make sure
that the inhaled air gets to the lungs, we need to breathe slowly and
deeply. Even when we exhale deeply some air is still in the lungs,(about
1000 ml) and is called **residual volume**. This air isn\'t useful for
gas exchange. There are certain types of diseases of the lung where
residual volume builds up because the person cannot fully empty the
lungs. This means that the vital capacity is also reduced because their
lungs are filled with useless air.

## Stimulation of Breathing

There are two pathways of motor neuron stimulation of the respiratory
muscles. The first is the control of voluntary breathing by the cerebral
cortex. The second is involuntary breathing controlled by the medulla
oblongata.

There are chemoreceptors in the aorta, the carotid body of carotid
arteries, and in the medulla oblongata of the brainstem that are
sensitive to pH. As carbon dioxide levels increase there is a buildup of
carbonic acid, which releases hydrogen ions and lowers pH. Thus, the
chemoreceptors do not respond to changes in oxygen levels (which
actually change much more slowly), but to pH, which is dependent upon
plasma carbon dioxide levels. **In other words, CO2 is the driving force
for breathing**. The receptors in the aorta and the carotid sinus
initiate a reflex that immediately stimulates breathing rate and the
receptors in the medulla stimulate a sustained increase in breathing
until blood pH returns to normal.

This response can be experienced by running a 100-meter dash. During
this exertion (or any other sustained exercise) your muscle cells must
metabolize ATP at a much faster rate than usual, and thus will produce
much higher quantities of CO2. The blood pH drops as CO2 levels
increase, and you will involuntarily increase breathing rate very soon
after beginning the sprint. You will continue to breathe heavily after
the race, thus expelling more carbon dioxide, until pH has returned to
normal. Metabolic acidosis therefore is acutely corrected by respiratory
compensation (hyperventilation).

## Regulation of Blood pH

Many of us are not aware of the importance of maintaining the acid/base
balance of our blood. It is vital to our survival. Normal blood pH is
set at 7.4, which is slightly alkaline or \"basic\". If the pH of our
blood drops below 7.2 or rises above 7.6 then very soon our brains would
cease functioning normally and we would be in big trouble. Blood pH
levels below 6.9 or above 7.9 are usually fatal if they last for more
than a short time. Another wonder of our amazing bodies is the ability
to cope with every pH change -- large or small. There are three factors
in this process: the lungs, the kidneys and buffers.

So what exactly is pH? pH is the concentration of hydrogen ions (H+).
Buffers are molecules which take in or release ions in order to maintain
the H+ ion concentration at a certain level. When blood pH is too low
and the blood becomes too acidic (acidosis), the presence of too many H+
ions is to blame. Buffers help to soak up those extra H+ ions. On the
other hand, the lack of H+ ions causes the blood to be too basic
(alkalosis). In this situation, buffers release H+ ions. Buffers
function to maintain the pH of our blood by either donating or grabbing
H+ ions as necessary to keep the number of H+ ions floating around the
blood at just the right amount.

The most important buffer we have in our bodies is a mixture of carbon
dioxide (CO2) and bicarbonate ion (HCO3). CO2 forms carbonic acid
(H2CO3) when it dissolves in water and acts as an acid giving up
hydrogen ions (H+) when needed. HCO3 is a base and soaks up hydrogen
ions (H+) when there are too many of them. In a nutshell, blood pH is
determined by a balance between bicarbonate and carbon dioxide.

**Bicarbonate Buffer System**. With this important system our bodies
maintain homeostasis. (Note that H2CO3 is Carbonic Acid and HCO3 is
Bicarbonate)

CO2 + H2O \<\-\--\> H2CO3 \<\-\--\> (H+) + HCO3

-   If pH is too high, carbonic acid will donate hydrogen ions (H+) and
    pH will drop.
-   If pH is too low, bicarbonate will bond with hydrogen ions (H+) and
    pH will rise.

Too much CO2 or too little HCO3 in the blood will cause acidosis. The
CO2 level is increased when hypoventilation or slow breathing occurs,
such as if you have emphysema or pneumonia. Bicarbonate will be lowered
by ketoacidosis, a condition caused by excess fat metabolism (diabetes
mellitus).

Too much HCO3 or too little CO2 in the blood will cause alkalosis. This
condition is less common than acidosis. CO2 can be lowered by
hyperventilation.

So, in summary, if you are going into respiratory acidosis the above
equation will move to the right. The body\'s H+ and CO2 levels will rise
and the pH will drop. To counteract this the body will breathe more and
release H+. In contrast, if you are going into respiratory alkalosis the
equation will move to the left. The body\'s H+ and CO2 levels will fall
and the pH will rise. So the body will try to breathe less to release
HCO3. You can think of it like a leak in a pipe: where ever there is a
leak, the body will \"fill the hole\".

## Problems Associated With the Respiratory Tract and Breathing

The environment of the lung is very moist, which makes it a hospitable
environment for bacteria. Many respiratory illnesses are the result of
bacterial or viral infection of the lungs. Because we are constantly
being exposed to harmful bacteria and viruses in our environment, our
respiratory health can be adversely affected. There are a number of
illnesses and diseases that can cause problems with breathing. Some are
simple infections, and others are disorders that can be quite serious.

**Carbon Monoxide Poisoning**: caused when carbon monoxide binds to
hemoglobin in place of oxygen. Carbon monoxide binds much tighter,
without releasing, causing the hemoglobin to become unavailable to
oxygen. The result can be fatal in a very short amount of time.

:   Mild Symptoms: flu like symptoms, dizziness, fatigue, headaches,
    nausea, and irregular breathing
:   Moderate Symptoms: chest pain, rapid heart beat, difficulty
    thinking, blurred vision, shortness of breath and unsteadiness
:   Severe Symptoms: seizures, palpitations, disorientation, irregular
    heart beat, low blood pressure, coma and death.

**Pulmonary Embolism:** blockage of the pulmonary artery (or one of its
branches) by a blood clot, fat, air or clumped tumor cells. By far the
most common form of pulmonary embolism is a thromboembolism, which
occurs when a blood clot, generally a venous thrombus, becomes dislodged
from its site of formation and embolizes to the arterial blood supply of
one of the lungs.

:   Symptoms may include difficulty breathing, pain during breathing,
    and more rarely circulatory instability and death. Treatment,
    usually, is with anticoagulant medication.

## Upper Respiratory Tract Infections

The upper respiratory tract consists of our nasal cavities, pharynx, and
larynx. Upper respiratory infections (URI) can spread from our nasal
cavities to our sinuses, ears, and larynx. Sometimes a viral infection
can lead to what is called a secondary bacterial infection.**\"Strep
throat\"** is a primary bacterial infection and can lead to an upper
respiratory infection that can be generalized or even systemic (affects
the body as a whole). Antibiotics aren\'t used to treat viral
infections, but are successful in treating most bacterial infections,
including strep throat. The symptoms of strep throat can be a high
fever, severe sore throat, white patches on a dark red throat, and
stomach ache.

Sinusitis:An infection of the cranial sinuses is called **sinusitis**. Only about 1-3% of URI\'s are accompanied by sinusitis. This \"sinus infection\" develops when nasal congestion blocks off the tiny openings that lead to the sinuses. Some symptoms include: post nasal discharge, facial pain that worsens when bending forward, and sometimes even tooth pain can be a symptom. Successful treatment depends on restoring the proper drainage of the sinuses. Taking a hot shower or sleeping upright can be very helpful. Otherwise, using a spray decongestant or sometimes a prescribed antibiotic will be necessary.

```{=html}
<!-- -->
```

Otitis Media:Otitis media in an infection of the middle ear. Even though the middle ear is not part of the respiratory tract, it is discussed here because it is often a complication seen in children who has a nasal infection. The infection can be spread by way of the *\'auditory (Eustachian) tube* that leads form the nasopharynx to the middle ear. The main symptom is usually pain. Sometimes though, vertigo, hearing loss, and dizziness may be present. Antibiotics can be prescribed and tubes are placed in the eardrum to prevent the buildup of pressure in the middle ear and the possibility of hearing loss.

!Photo of
**Tonsillitis**.{width="300"}

Tonsillitis:Tonsillitis occurs when the tonsils become swollen and inflamed. The tonsils located in the posterior wall of the nasopharynx are often referred to as adenoids. If you suffer from tonsillitis frequently and breathing becomes difficult, they can be removed surgically in a procedure called a tonsillectomy.

```{=html}
<!-- -->
```

Laryngitis:An infection of the larynx is called laryngitis. It is accompanied by hoarseness and being unable to speak in an audible voice. Usually, laryngitis disappears with treatment of the URI. Persistent hoarseness without a URI is a warning sign of cancer, and should be checked into by your physician.

{{-}}

## Lower Respiratory Tract Disorders

Lower respiratory tract disorders include infections, restrictive
pulmonary disorders, obstructive pulmonary disorders, and lung cancer.

### Lower Respiratory Infections

Acute bronchitis:An infection that is located in the primary and secondary bronchi is called bronchitis. Most of the time, it is preceded by a viral URI that led to a secondary bacterial infection. Usually, a nonproductive cough turns into a deep cough that will expectorate mucus and sometimes pus.\
Pneumonia:A bacterial or viral infection in the lungs where the bronchi and the alveoli fill with a thick fluid. Usually it is preceded by influenza. Symptoms of pneumonia include high fever & chills, with headache and chest pain. Pneumonia can be located in several lobules of the lung and obviously, the more lobules involved, the more serious the infection. It can be caused by a bacteria that is usually held in check, but due to stress or reduced immunity has gained the upper hand.

### Restrictive Pulmonary Disorders

Pulmonary Fibrosis:Vital capacity is reduced in these types of disorders because the lungs have lost their elasticity. Inhaling particles such as sand, asbestos, coal dust, or fiberglass can lead to **pulmonary fibrosis**, a condition where fibrous tissue builds up in the lungs. This makes it so our lungs cannot inflate properly and are always tending toward deflation. Pulmonary fibrosis can be synonymous with interstitial lung disease (ILD), or interstitial pneumonia or pneumonitis.

### Obstructive Pulmonary Disorders

!400 px\|Diagram of the lungs during an **asthma
attack**.

Asthma:Asthma is a respiratory disease of the bronchi and bronchioles. The symptoms include wheezing, shortness of breath, and sometimes a cough that will expel mucus. The airways are very sensitive to irritants which can include pollen, dust, animal dander, and tobacco. Even being out in cold air can be an irritant. When exposed to an irritant, the smooth muscle in the bronchioles undergoes spasms. Most asthma patients have at least some degree of bronchial inflammation that reduces the diameter of the airways and contributes to the seriousness of the attack.

```{=html}
<!-- -->
```

Emphysema:Emphysema is a type of chronic obstructive pulmonary disease. Typically characterized by a loss of elasticity and surfactant in the alveoli, a loss of surface area decreases the gas exchange in the lungs. These patients have difficulty with too little expiratory pressure, not retaining inspired air long enough for sufficient gas exchange to happen.

```{=html}
<!-- -->
```

Chronic Bronchitis:Another type of chronic obstructive pulmonary disease, Chronic Bronchitis is caused by overproduction of mucus in the airways, causing an inadequate expiration of inspired air. Retention of air in the lungs reduces gas exchange at the alveoli, and can lead to a hypoxic drive. These patients are known as \"blue bloaters\", vulnerable to cyanosis and often have increased thoracic diameters.

## Respiratory Distress Syndrome

Pathophysiology

At birth the pressure needed to expand the lungs requires high
inspiratory pressure. In the presence of normal surfactant levels the
lungs retain as much as 40% of the residual volume after the first
breath and thereafter will only require far lower inspiratory pressures.
In the case of deficiency of surfactant the lungs will collapse between
breaths, this makes the infant work hard and each breath is as hard as
the first breath. If this goes on further the pulmonary capillary
membranes become more permeable, letting in fibrin rich fluids between
the alveolar spaces and in turn forms a hyaline membrane. The hyaline
membrane is a barrier to gas exchange, this hyaline membrane then causes
hypoxemia and carbon dioxide retention that in turn will further impair
surfactant production.

Etiology

Type two alveolar cells produce surfactant and do not develop until the
25th to the 28th week of gestation, in this, respiratory distress
syndrome is one of the most common respiratory disease in premature
infants. Furthermore, surfactant deficiency and pulmonary immaturity
together leads to alveolar collapse. Predisposing factors that
contribute to poorly functioning type II alveolar cells in a premature
baby are if the child is a preterm male, white infants, infants of
mothers with diabetes, precipitous deliveries, cesarean section
performed before the 38th week of gestation. Surfactant synthesis is
influenced by hormones, this ranges form insulin and cortisol. Insulin
inhibits surfactant production, explaining why infants of mothers with
diabetes type 1 are at risk of development of respiratory distress
syndrome. Cortisol can speed up maturation of type II cells and
therefore production of surfactant. Finally, in the baby delivered by
cesarean section are at greater risk of developing respiratory distress
syndrome because the reduction of cortisol produced because the lack of
stress that happens during vaginal delivery, hence cortisol increases in
high stress and helps in the maturation of type II cells of the alveoli
that cause surfactant.

Treatment

Today to prevent respiratory distress syndrome are animal sources and
synthetic surfactants, and administrated through the airways by an
endotracheal tube and the surfactant is suspended in a saline solution.
Treatment is initiated post birth and in infants who are at high risk
for respiratory distress syndrome.

## Sleep Apnea

!CPAP is the most common treatment for obstructive sleep
apnea.{width="200"}
Sleep apnea or sleep apnoea is a sleep disorder characterized by pauses
in breathing during sleep. These episodes, called apneas (literally,
\"without breath\"), each last long enough so one or more breaths are
missed, and occur repeatedly throughout sleep. The standard definition
of any apneic event includes a minimum 10 second interval between
breaths, with either a neurological arousal (3-second or greater shift
in EEG frequency, measured at C3, C4, O1, or O2), or a blood oxygen
desaturation of 3-4 percent or greater, or both arousal and
desaturation. Sleep apnea is diagnosed with an overnight sleep test
called polysomnogram. One method of treating central sleep apnea is with
a special kind of CPAP, APAP, or VPAP machine with a Spontaneous Time
(ST) feature. This machine forces the wearer to breathe a constant
number of breaths per minute.

(CPAP), or continuous positive airway pressure, in which a controlled
air compressor generates an airstream at a constant pressure. This
pressure is prescribed by the patient\'s physician, based on an
overnight test or titration.

## Nutrition for COPD (Chronic Obstructive Pulmonary Disease) Patients

Nutrition is particularly important for ventilator-dependent patient.
When metabolizing macronutrients carbon dioxide and water are produced.
The respiratory quotient (RQ) is a ratio of produced carbon dioxide to
amount consumed. Carbohydrates metabolism produces the most amount of
carbon dioxide so they have the highest (RQ). Fats produce the least
amount of carbon dioxide along with proteins. Protein has a slightly
higher RQ ratio. It is recommended that this kind of patient not exceed
a 1.0 respiratory quotient (RQ). Lowering carbohydrates and
supplementing fat or protein in the diet might not result in maintaining
the desired outcome because, excess amounts fat or protein may also
result in a respiratory quotient (RQ) higher than 1.0.

-   Please reference source and fact accuracy. It seems like by
    definition, it is impossible to exceed a respiratory quotient (RQ)
    of 1.0. \*

## Case Study

**Cystic Fibrosis**

This disease is most common in Caucasians and will happen to 1 in every
2500 people. It is most known for its effects on the respiratory tract
although it does effect other systems as well. The respiratory passages
become clogged with a thick mucus that is difficult to expel even with
vigorous coughing. Breathing becomes difficult and affected individuals
run the risk of choking to death on their own secretions unless
strenuous effort is made to clear the lungs multiple times every day.
Victims frequently will die in the 20\'s of pneumonia. All of us secrete
mucus by certain cells in the epithelium that line the respiratory
passageways. In normal cases the cells also secrete a watery fluid that
will dilute the mucus making it easier to pass through the airways. In
cystic fibrosis that secretion of watery fluid is impaired. This makes
the mucus thicker and difficult to clear from the passageways. A 1989
discovery found that cystic fibrosis is caused by defects in a type of
anion channel found in apical membranes of epithelial cells in the
respiratory system and elsewhere[^1]. The defects directly impede the
chloride ion transport, which will then indirectly effect other
exchanges of ions in the cells. This causes the epithelium to not create
the osmotic gradient necessary for water secretion. It has been known
for a long time that cystic fibrosis is caused by a recessive gene
inheritance. This gene codes for the chloride channel protein, which can
malfunction in a variety of ways, each with specific treatment required.

## Glossary

## References

-   Mader, Sylvia S. *Human Biology*. McGraw Hill Publishing, Burr
    Ridge, IL. 2004.
-   Van De Graaff, Kent M. *Human Anatomy*. McGraw Hill Publishing, Burr
    Ridge, IL. 2002.
-   Department of Environmental Biology, University of Adelaide,
    Adelaide, South Australia
-   Wikipedia:Lung
-   Medlineplus.gov. *Hilus*.
    <http://www2.merriam-webster.com/cgi-bin/mwmednlm?book=Medical&va=hilum+>
-   <http://www.an-attorney-for-you.com/legal/carbon-monoxide.htm>
-   www.ineedtoknow.com
-   \'\'\"The respiratory system\"Authors Mary Kitteredge,intro. by C.
    Everett Koop, M.D.,SC.D., foreword by Sandra Thurman

## External Resources

-   Eastern Kentucky University Gary Ritchison Lecture Notes: Human
    Respiration (includes
    videos)
-   In depth anatomy and physiology of the pulmonary
    system.

pt:Fisiologia/Sistema
respiratório

[^1]: Riordan,J.R., Rommens,J.M., Kerem,B., Alon,N., Rozmahel,R.,
    Grzelczak,Z., Zielenski,J., Lok,S., Plavsic,N. and Chou,J.L. (1989)
    Identification of the cystic fibrosis gene: cloning and
    characterization of complementary DNA. Science, 245, 1066--73.
