Module 14

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Module 14

• The Respiratory System

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• The respiratory system contains the nasal cavity,
  the paranasal sinuses, the pharynx, the larynx,
  the trachea, the bronchi, and the lungs.
• Generally, we split the respiratory system into
  two parts. The nasal cavity, paranasal sinuses,
  and the pharynx are a part of the upper
  respiratory tract.
• The larynx, trachea, bronchi, and lungs are a
  part of the lower respiratory tract.Upper
  respiratory tract The part of the respiratory
  system containing the nasal cavity, paranasal
  sinuses, and pharynxLower respiratory tract
  The part of the respiratory system containing the
  larynx, trachea, bronchi, and lungs

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Nasal Cavity

• Each of the parts of the respiratory system has
  its own function or functions.
• The nasal cavity, for example, has three
  functions.
• First, it provides for olfaction.
• The nasal cavity also conditions the air. There
  is a lot of surface area in the nasal cavity, and
  the tissue on that surface is very vascular.
• There is also a mucous membrane there that is
  moist and sticky. When you breathe in through the
  nasal cavity, then, the air is warmed by the
  blood vessels, moistened by the mucus, and it is
  cleansed of particles because the particles stick
  to the mucous membrane. So the air is moistened
  in the nasal cavity as well.

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• When the air passes out of your nasal cavity, it
  enters your pharynx.
• The superior (upper) part of the pharynx is
  called the nasopharynx. The nasopharynx ends at
  the uvula, a small process that hangs off of the
  soft palate. The uvula aids the soft palate in
  closing off the nasal cavity during deglutition.
• Two auditory orifices open into the nasopharynx,
  one from each ear. This allows air passage
  between the middle ear and the throat, which
  helps equalize the pressure between the middle
  ear and the atmosphere.

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• Inferior to the nasopharynx, we find the
  oropharynx.
• It extends from the uvula to the epiglottis.
• It is called the oropharynx because this is the
  place that the oral cavity opens to the pharynx.
• As a result, both food and air pass through the
  oropharynx. Two sets of tonsils are located in
  this region of the pharynx.

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• The bottom part of the pharynx is the
  laryngopharynx.
• It is actually posterior to the larynx.
• When the epiglottis is not blocking the larynx,
  air passes from the laryngopharynx to the larynx.
  
• When the epiglottis is closed, food passes from
  the laryngopharynx into the esophagus.

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• When air passes into the larynx, we encounter the
  second function of the respiratory system voice.
  
• As you probably already know, the larynx, often
  called the voice box, contains the vocal cords,
  which give us our ability to make intricate
  sounds so that we can speak, sing, and so on.

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• Once air travels down the larynx, it enters the
  trachea, which is also called the windpipe.
• The trachea consists of about 20 pieces of
  cartilage which are shaped like a C.
• Dense regular connective tissue and smooth muscle
  hold these pieces of cartilage together.
• Since the windpipe is a direct route to the
  lungs, it is a good place to mention another
  function of the respiratory system
  ventilation.Ventilation The process of
  getting air into the lungs and getting it back
  out

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• The trachea splits into two bronchi, which each
  carry air to and from a lung.
• A little more than half the air travels through
  the right branch into the right lung, and a
  little less than half travels through the
  slightly narrower left branch into the left lung.
  
• The right branch is called the right primary
  bronchus (bron' kus), and the left branch is
  called the left primary bronchus.
• Notice that the left lung and right lung are
  different from one another. Each lung is composed
  of lobes, but the right lung is made up of three
  lobes while the left lung is made up of only two.

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• Once in the lungs, the bronchi split into smaller
  tubes called secondary bronchi.
• Each secondary bronchus goes to one lobe. There
  are three secondary bronchi in the right lung and
  two in the left lung.
• These tubes then split into even smaller tubes
  called tertiary bronchi, and these split into
  even smaller tubes called bronchioles.
• The bronchioles continue to branch into even
  smaller tubes, until they become terminal
  bronchioles.
• These terminal bronchioles branch into tiny
  alveolar (al vee' oh lar) ducts, which open into
  many microscopic, balloon-like sacs called
  alveoli. The word alveoli actually means
  hollow cavity, which makes sense, since the
  alveoli are, indeed, hollow. The tissue of the
  alveoli is composed of simple squamous
  epithelium, which is lined with capillaries.

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• The alveoli begin the final function of the
  respiratory system respiration. This process
  involves three specific steps. First, there is
  external respiration.External respiration The
  process of O2 and CO2 exchange between the
  alveoli and the blood

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• Once external respiration has taken place, the
  next step of respiration, gas transport in the
  blood, takes place.
• Oxygenated blood leaves the lungs and goes to the
  heart and then to the tissues, where it can
  supply the body's cells with oxygen.
• At the same time, the blood picks up carbon
  dioxide from the cells, which is then transported
  back to the lungs so that it can be eliminated
  from the body.
• The final stage of respiration is internal
  respiration, which allows for the exchange of
  gases between the cells and the blood.
• Internal respiration The process of O2 and CO2
  exchange between the cells and the blood

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Voice

• One of the functions of the respiratory system is
  the voice.
• This is controlled by the vocal cords, which are
  located in the larynx.
• There are actually two types of vocal cords in
  the larynx true vocal cords (also called vocal
  folds) and false focal cords (also called
  vestibular folds).
• Both are mucosa-covered ligaments and both are
  found in the superior portion of the larynx. The
  false vocal cords are superior to the true vocal
  cords. These two sets of vocal cords have
  different jobs. The false vocal cords close the
  larynx to help prevent food from traveling down
  the wrong pipe during deglutition. They also
  close off your larynx when you hold your breath.
• The true vocal cords, on the other hand, actually
  produce sound.

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• The most striking feature is the thyroid
  cartilage, the hyaline cartilage framework of the
  Adam's apple.
• The middle illustration is a vertical cross
  section of the trachea. In this figure, you can
  see that the vestibular folds are superior to the
  vocal folds. Remember, the vestibular folds can
  completely close off the glottis, sealing the
  larynx. They do this by moving toward each other
  like a pair of sliding doors.
• The vocal folds do not close completely. They
  move toward each other during sound production,
  but the force of the air leaving the lungs
  blows them open, which causes them to vibrate.
  When we are silent, they are relaxed and form a
  small flap on either side of the larynx. The
  right-hand illustration is a look down the
  opening in the larynx. This opening is called the
  glottis. Note the muscle attachment, where the
  delicate muscles which control the vocal folds
  are anchored.

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• How is sound created?
• The vocal folds move together in the airway the
  air that passes through the vocal folds causes
  them to vibrate.
• This makes sound. Remember, sound is just a
  series of vibrations in the air. If a musician
  plucks a guitar string, it vibrates, making a
  sound.
• When air passes through the true vocal cords,
  they also vibrate, and that also makes a sound.

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• Of course, we can do a lot more than just make
  sound. We can vary the loudness of the sound as
  well as the pitch. How is that done?
• Well, the loudness of a sound is controlled by
  the displacement of the vibrations in the air.
• In the larynx, the more air that passes through
  the vocal cords, the larger their displacement.
  Thus, we control the volume of our voice by
  controlling how much air passes through the
  larynx.
• The more air we use, the louder our voice is. If
  you're going to yell at someone, you naturally
  take a deep breath, because you need a lot of air
  in order to raise the volume of your voice to
  that level.

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• Pitch is another important aspect to the sounds
  that we make. It is controlled by the frequency
  of the vibrations in the vocal cords.
• We vary the pitch of our voice in much the same
  way that the musician varies the pitch of the
  guitar strings.
• We can vary the thickness and the tension of our
  true vocal cords. The length of the vocal cords
  affects the pitch just as it does in guitar
  strings, but we cannot change that.
• When we are young, the true vocal cords are
  relatively short. This gives our voices a high
  pitch. As we mature, our vocal cords grow.
  Typically, boys' vocal cords grow more than
  girls' vocal cords. Thus, as a boy matures, his
  voice gets deeper, because the longer the vocal
  cords, the lower the pitch.
• Although girls' vocal cords also grow, they do
  not grow as much. Thus, a girl's voice will
  typically get deeper as she matures, but not
  nearly as noticeably as that of a boy.

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The Muscles and Mechanics of Ventilation
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• We divide the muscles of ventilation into three
  groups muscles of principal inspiration, muscles
  of forced inspiration, and muscles of forced
  expiration.
• The muscles of principal inspiration, the
  diaphragm and the external intercostals, are used
  when we breathe in normally.
• When we consciously take a deep breath or when we
  exercise vigorously, we use the muscles of forced
  inspiration, the sternocleidomastoid muscles, the
  pectoralis minor muscles, and the scalene
  muscles, as well.
• When we consciously force ourselves to exhale, we
  use the abdominal muscles and internal
  intercostal muscles (deep to the external
  intercostal muscles), which are the muscles of
  forced expiration.

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• How do these muscles control ventilation? Let's
  start with the diaphragm.
• The diaphragm is a long, sheet-like muscle and,
  when we exhale, it's relaxed.
• When we inhale, the diaphragm contracts, which
  causes it to shorten and pull downward.
• This pushes down on the abdominal organs below
  it, which increases the size of the cavity in the
  chest (the thoracic cavity). When this happens,
  the lungs expand, and that causes air to rush
  into them through the nasal cavity or the mouth.
• When the diaphragm relaxes, it gets longer, and
  the abdominal organs below it passively push it
  back up. This reduces the size of the thoracic
  cavity, which pushes in on the lungs.

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• The other set of muscles that work during normal
  inhalation are the external intercostals.
• When they contract, they pull the ribs up and
  out. Once again, this increases the size of the
  thoracic cavity, and the lungs expand as a
  result.
• Muscles of inhalation, then, increase the volume
  of the thoracic cavity.
• For a big forced breath, you have to use the
  muscles of forced inhalation, the
  sternocleidomastoid, pectoralis minor, and
  scalene muscles. These pull up either the ribs or
  the sternum when they contract, and that makes
  your thoracic cavity even larger. This increases
  the amount of air that you can inhale.

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• In order to exhale at rest, you just have to
  relax any of the inhalation muscles that were
  contracted.
• As we already discussed, when the diaphragm is
  relaxed, it gets longer.
• This reduces the size of the thoracic cavity. If
  any of the other muscles of inhalation are
  contracted, relaxing them will further reduce the
  size of the thoracic cavity.
• When that happens, the lungs get pushed in, and
  air is forced out.

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Factors That Aid Ventilation

• There are elastic fibers in the lungs.
• Thus, the lungs are a bit like balloons. They
  inflate because their elastic nature allows them
  to stretch.
• However, when nothing is forcing them to stretch
  what will they do? They will recoil back to their
  original size.
• The elasticity of the lungs, therefore, aids in
  expiration. When the diaphragm and intercostal
  muscles relax, the pressure inside the lungs
  increases. This forces the air out of the lungs.
• With nothing pushing against them anymore, the
  lungs naturally collapse, like a balloon that is
  losing air. This helps push air out of the lungs

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• Emphysema is a common lung disease which can be
  caused by smoking or being exposed to a lot of
  air pollution.
• In this disease, the walls of the tiny alveoli
  degenerate, and many tiny alveoli join together
  to make one large alveolus.
• This causes two problems. The rate at which
  oxygen can be exchanged with the blood in the
  alveoli depends on the surface area over which
  blood is exposed to oxygen. Many tiny alveoli
  have more surface area than one large alveolus.
  Thus, the decrease in surface area causes less
  oxygen to be exchanged with the blood. The second
  problem is that as the walls of the alveoli
  degenerate, the elastic tissue degenerates as
  well. Typically, it is replaced with scar tissue.
  As a result, the lungs lose their elasticity.
  This causes a person with emphysema to have
  difficulty exhaling.

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• There is another factor which aids expiration
  the surface tension of alveolar fluid.
• What does this mean?
• The wall of an alveolus is lined with simple
  squamous epithelium.
• This tissue is delicate, and that's why smoking
  or exposure to excess air pollution can destroy
  it. Now, like any mucous membrane or any of the
  internal membranes, this tissue is damp. It's not
  wet, but it's damp. The fluid which causes this
  dampness is called alveolar fluid, and it
  covers the entire surface of the alveolus.

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• Water is strongly attracted to other water
  molecules because of hydrogen bonding.
• In fact, it is more strongly attracted to other
  water molecules than to the molecules in the air.
  
• At the surface of any collection of water, this
  creates a surface tension. that is a powerful
  force.

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• The surface of the lungs is covered with a layer
  of tissue called the visceral pleura.
• There is a space between this tissue and another
  layer of tissue called the parietal pleura.
• The space in between these two layers of tissue
  is called the pleural cavity, and it is
  vacuum-sealed.
• What's in there? Just a tiny bit of watery fluid.
  Since there is not a lot in the pleural cavity,
  it is at a lower pressure than atmospheric
  pressure. We call that a negative pressure,
  since it is lower than the pressure to which you
  are accustomed.

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• Since the pleural cavity is at a pressure lower
  than atmospheric pressure, it puts external
  suction on the lung, holding it open. Thus, even
  though the elastic nature of the alveoli and the
  surface tension of the alveolar fluid keeps
  pulling the lungs closed, the negative pressure
  in the pleural cavity keeps them from collapsing
  completely.What happens if we lose the vacuum
  seal of the pleural cavity? Air will rush into
  the pleural cavity, giving it the same pressure
  as the atmosphere. When that happens, there is no
  longer a negative pressure holding the lung open,
  and the lung will collapse. That's called a
  pneumothorax.

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• The second process that aids inspiration and
  keeps the lungs from collapsing completely is a
  process that fights the surface tension of the
  alveolar fluid.
• Remember, this surface tension aids the alveoli
  in collapsing, and it is quite a powerful force.
  It turns out that this force must be reduced in
  order to allow for proper breathing. How is it
  reduced? It is reduced by a surfactant that is
  secreted by specialized cells in the
  lungs.Surfactant - A molecule with a
  hydrophilic end and a hydrophobic end

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• Airway resistance.
• Remember, the air flows into the lungs and
  ultimately to the alveoli through the bronchi,
  bronchioles, and alveolar ducts. Well, when air
  travels through a tube, the walls of the tube
  resist the flow.
• That's called airway resistance. Of course, the
  lower the airway resistance, the more healthy it
  is. It turns out that most of the airway
  resistance occurs in the bronchioles.
• Asthma is the classic disease that increases
  airway resistance.

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External Respiration
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• On the left-hand side of the figure, you see a
  cross section of a single alveolus with three
  capillaries near it. Notice the alveolus has a
  macrophage in it to destroy by phagocytosis any
  foreign invaders that make it to the alveolus.
  Air fills the lumen of the alveolus, and gases
  can be exchanged across the respiratory
  membrane.The right-hand side of the figure
  shows you the details of the respiratory
  membrane. The first layer is the alveolar fluid,
  which was mentioned above. This fluid, which has
  surfactant in it, coats the simple squamous
  epithelium of the alveolus. Notice the
  construction of this epithelium. The cells are
  spaced far from one another, and a thin membrane
  connects one cell to another. The gases in the
  air pass through this membrane on the way in and
  out of the alveolus.

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• Beyond the simple squamous epithelium of the
  alveolus, you find the basement membrane of the
  alveolar epithelium.
• Beyond that, there is a tiny interstitial space
  separating the alveolus and the capillary.
• That space ends with the basement membrane of the
  capillary endothelium.
• After that you find the simple squamous
  endothelium of the capillary, which leads to the
  lumen of the capillary.

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• When the alveolus fills with air, oxygen travels
  across the respiratory membrane and is picked up
  by the erythrocytes in the capillary.
• At the same time, carbon dioxide diffuses from
  the blood (mostly the blood plasma), across the
  respiratory membrane, and into the lumen of the
  alveolus.
• When the alveolus collapses, that carbon dioxide
  is forced out of the alveolus. When the alveolus
  is inflated again, fresh oxygen comes in with the
  new air.

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• Pneumonia is another way that the controlled
  relationship between ventilation and blood flow
  can be compromised.
• Pneumonia is a general term that refers to an
  infection of the lungs. The infections can be
  viral, bacterial, or even the result of protozoa.
  
• This infection usually leads to fluid collecting
  in the alveoli and poor ventilation of the lungs.
  Once again, then, there is not enough air in the
  alveoli for the blood that is pumping through
  them. As a result, the tissues do not get enough
  oxygen, and the results can be deadly!

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• A physiological process which aids in the
  efficiency of external respiration is the
  functional residual capacity of the alveoli.
• When you have exhaled, there is still air in your
  lungs. Typically, an adult's lungs have a little
  more than two liters of air in them after he or
  she has exhaled normally. When the person inhales
  at rest, usually the volume of air in the lungs
  increases by only half a liter or so. This is
  referred to as the tidal volume.Tidal volume -
  The volume of air inhaled or exhaled during
  normal, quiet breathing

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• Most healthy adults can breath in over 3
  additional liters of air when they forcefully
  inhale. Thus, the volume of air in the lungs can
  change from just over two liters to about five or
  six liters (depending on size) when we forcefully
  inhale. This maximum volume of air that the lungs
  can hold is referred to as the total lung
  capacity.Total lung capacity - The maximum
  volume of air contained in the lungs after a
  forceful inhalation

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• Just as you are able inhale more air than you
  usually do under normal conditions, you can also
  exhale more than you usually do under normal
  conditions. When you forcefully exhale, you expel
  as much air as possible from your lungs.
  Interestingly enough, you cannot empty your lungs
  of air. Even when you exhale as much as possible,
  a residual volume of air is left in your
  lungs.Residual volume - The volume of air left
  in the lungs after a forceful exhalation

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Gas Exchange During External and Internal
Respiration

• When dry, the air around us is 78 nitrogen and
  21 oxygen. The remaining 1 is a mixture of
  many, many gases, including carbon dioxide, which
  makes up about 0.04.
• Of course, we never really breathe dry air. The
  air that we breathe has some water vapor in it,
  and that reduces the percentages of nitrogen and
  hydrogen.
• Humidified air is typically 74 nitrogen, 20
  oxygen, 6 water vapor, and still about 0.04
  carbon dioxide. This air is typically at
  atmospheric pressure, which is near 760 mmHg.

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• What happens when we breathe this air into our
  lungs?
• The lungs have a functional residual capacity.
• Thus, there is old air in the lungs. When we
  breathe in, the new air that we are breathing
  in mixes with the old air that is already
  there. What do you think will happen to the
  partial pressures of oxygen and carbon dioxide as
  a result? Remember, this old air has been
  giving up its oxygen to the blood and has been
  taking carbon dioxide from the blood. Thus, it
  has less oxygen and more carbon dioxide than the
  new air.
• As a result, when the new air mixes with the
  old air, the partial pressure of oxygen will be
  lower than it was in the new air, and the
  partial pressure of carbon dioxide will be higher
  than it was.

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• When blood returns to the lungs, it has given up
  some of its oxygen to the tissues.
• As a result, the partial pressure of oxygen in
  the blood is low - about 40 mmHg. At the same
  time, this blood has picked up carbon dioxide
  from the tissues. Thus, the pressure of carbon
  dioxide is higher than normal - about 45 mmHg.
• Now let's compare these numbers to the partial
  pressures of oxygen and carbon dioxide in the
  alveoli. The partial pressure of oxygen in the
  alveoli is higher than the partial pressure of
  oxygen in the blood. As a result, diffusion
  dictates that the oxygen must travel out of the
  alveoli and into the blood. On the other hand,
  the partial pressure of carbon dioxide in the
  alveoli is smaller than the partial pressure of
  carbon dioxide in the blood. Thus, diffusion
  dictates that carbon dioxide must travel from the
  blood to the alveoli. In the end, then, the gases
  really don't know which way they are supposed
  to travel.

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• When the blood reaches the tissues, what happens?
  Well, the cells of the tissues have been using up
  their oxygen.
• Thus, the partial pressure of oxygen is low,
  averaging about 40 mmHg. At the same time, the
  process of using oxygen to convert food into
  energy (discussed in a later section) produces
  carbon dioxide. As a result, the partial pressure
  of carbon dioxide is high - about 45 mmHg. As the
  blood flows through the thin-walled capillaries,
  then, oxygen diffuses from the area of high
  partial pressure (the blood) to an area of low
  partial pressure (the cells).
• At the same time, the carbon dioxide diffuses
  from an area of high partial pressure (the cells)
  to an area of low partial pressure (the blood).
  This increases the partial pressure of carbon
  dioxide in the blood to 45 mmHg, and it lowers
  the partial pressure of oxygen in the blood to
  about 40 mmHg. The blood (darker red in color
  now) is then sent back to the heart and then to
  the lungs to start the process all over again.

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Respiratory Control

• Control of respiration takes place in the medulla
  and the pons of the brainstem.
• In the medulla, there are two groups of neurons
  called the dorsal respiratory group and the
  ventral respiratory group.
• In the pons, there is one group of neurons called
  the pontine respiratory group. The basic rhythm
  of breathing is thought to be set by the dorsal
  respiratory group, which controls the diaphragm.
• When the dorsal group sends action potentials,
  the diaphragm contracts, and you inhale. When the
  action potentials stop, the diaphragm relaxes,
  and you exhale. This gives us the basic rhythm of
  our respiration.

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• In addition to the automatic nature of
  ventilation, there is one really neat fail-safe
  mechanism.
• It is called the Hering-Breuer reflex, and it
  prevents us from over-stretching our lungs.
• If we inhaled really, really deeply, we could
  over-inflate our lungs, which could potentially
  tear the visceral pleura, which would be bad.
  However, this cannot happen because of the
  Hering-Breuer reflex.
• When we inspire deeply, we stretch receptors in
  our bronchioles. As the receptors stretch, they
  start sending action potentials to the medulla.
  If the action potentials get frequent enough,
  they will inhibit the dorsal and ventral
  respiratory groups. What will happen then? If the
  dorsal and ventral groups are inhibited,
  inspiration will stop and expiration will begin.
  This, then, is another example of negative
  feedback in the body. Receptors notice that you
  are inhaling too deeply, and action potentials
  are sent to stop the process of inhaling.

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• What does this chemical control center detect in
  order to determine the controls it needs? Oxygen,
  right? Wrong! The level of carbon dioxide is what
  guides this chemical process! This is so
  important, we want to emphasize itCarbon
  dioxide levels and pH are the major indicators in
  the control of ventilation.

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• As you learned in your chemistry course, this is
  called acid disassociation, and its effect is to
  lower the pH of the solution. Thus, we come to
  another important pointThe more CO2 in the
  blood plasma, the LOWER the pH of the blood.

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• The pH of blood can be used to control
  ventilation. If the pH is too low, the body
  increases ventilation to remove carbon dioxide.
  That raises the pH.
• If the pH is too high, the body decreases
  ventilation to retain carbon dioxide. That lowers
  the pH.
• How is this all controlled? Once again, the
  control starts in the medulla, where there is a
  group of chemoreceptors called the central
  chemoreceptors of the medulla.
• Now this is separate from the three respiratory
  groups of neurons discussed above. Those give us
  our basic rhythm of ventilation. The central
  chemoreceptors in the medulla affect the rate and
  depth of the ventilation.

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• There are also some other chemoreceptors in the
  aorta and in the carotid sinuses, but those are
  minor.
• The major set of chemoreceptors are in the
  medulla. When there is increased partial pressure
  of carbon dioxide in the body, blood pH drops.
  Since the blood supply feeds the cerebrospinal
  fluid, the pH of the CSF also goes down. That low
  pH caused by the increased carbon dioxide affects
  the central chemoreceptors of the medulla, and
  they stimulate more ventilation to get rid of
  carbon dioxide and therefore raise the pH of the
  CSF and the blood.

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Cellular Respiration

• However, why do the tissues need oxygen, and why
  do they make carbon dioxide?
• Cells burn food for energy in a process called
  cellular respiration. Although you learned about
  this process in your first-year biology course,
  we want to review it with you now and add a few
  more details.

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• The most efficient way for cells to produce
  energy is through aerobic respiration, which
  involves converting glucose (C6H12O6) into carbon
  dioxide and water. This process requires oxygen,
  and that is why it is called aerobic respiration.
  The overall chemical reaction for aerobic
  respiration is as follows
• C6H12O6 6O2 ? 6CO2 6H2O energy in the form
  of ATP

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• This reaction leads to the release of a lot of
  energy, which, as you know, is stored in the cell
  as adenosine triphosphate (ATP).
• This seemingly simple chemical reaction takes
  place in three enzyme-controlled stages
  glycolysis, the Krebs cycle, and the electron
  transport system.

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• Let's start with glycolysis, since it is the
  first stage of aerobic respiration. In order to
  get things going, the cell actually has to expend
  some energy in glycolysis. This energy is used to
  convert glucose into fructose diphosphate

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• Let's look at the reaction, then. Glucose is a
  molecule with 6 carbons. That's why there are six
  carbons shown below it.
• Fructose diphosphate also has 6 carbons, but in
  addition, it has a phosphate group (abbreviated
  as Pi) on each end of the carbon
• chain. That's what the symbol below fructose
  diphosphate means.
• Where did those phosphates come from? They came
  from ATP. Look at the equation. There are 2 ATPs
  on the reactants side, and there are two ADPs on
  the products side. What is the difference between
  ATP and ADP? The difference is a phosphate!
• Thus, the ATP break apart, releasing energy. This
  energy allows the phosphate that broke off of
  each ATP to add on to the carbon chain, making
  fructose diphosphate.

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• Now that we have fructose diphosphate, what
  happens? The next step is to break down the
  fructose diphosphate. Since this chemical has 6
  carbons with a phosphate on each end, what do you
  think it will be broken down into? It will be
  broken in half, making 2 molecules which have 3
  carbons each with one phosphate on each end. This
  three-carbon molecule, phosphogylceraldehyde, is
  lovingly called PGAL for short.

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• The next step is probably the most important step
  in glycolysis, at least from an energy point of
  view. The PGAL that is formed in the reaction
  above reacts with phosphate and a molecule called
  NAD. NAD is often called a hydrogen stealer,
  because it picks up hydrogen atoms from other
  molecules.

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• Once again, look what happened here. The two
  D-PGA each have two phosphates, for a total of 4
  phosphates. Those molecules give up all four of
  their phosphates to 4 ADP molecules. What does
  ADP plus a phosphate make? It makes ATP, which is
  a package of energy for the cell. When the D-PGAs
  lose their phosphates, they become the simple,
  3-carbon molecule pyruvate.
• Glycolysis requires a glucose and 2 ATPs. It
  makes 2 pyruvate molecules, 2 NADH molecules, and
  4 ATPs. This gives the cell a net gain of 2 ATPs

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• We want to point out a couple of things about
  glycolysis before we move on to the next stage of
  aerobic respiration. First of all, notice that no
  oxygen is used in any of the chemical reactions.
  Thus, oxygen is not required for glycolysis.
  Thus, glycolysis is an anaerobic step. In
  addition, you need to realize that all of this
  takes place in the cytoplasm of the cell.

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• Here is where we want to add another stage to
  cellular respiration.
• Technically, however, something happens in the
  mitochondrion to prepare the pyruvate for the
  Krebs cycle.
• We call that the oxidation of pyruvate. When
  pyruvate enters the mitochondrion, it is attacked
  by NAD, which steals a hydrogen atom.
• This causes the molecule to break down, and one
  of the carbons in pyruvate gets converted to CO2.
  This leaves us now with a 2-carbon chain, which
  is called an acetyl group. This acetyl group
  needs something with which to bond, however.
• Conveniently, there is an enzyme called
  coenzyme-A just waiting to be bonded to. When the
  acetyl group bonds to the coenzyme-A, the result
  is called acetyl coenzyme-A.

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• Now keep track of what went on here. The
  pyruvates each have 3 carbons. They each lose a
  hydrogen to NAD. That makes 2 NADH. They also
  each lose a carbon to make CO2. That leaves two
  molecules, each with 2 carbons. Those two-carbons
  molecules bond to coenzyme-A. This makes 2 acetyl
  coenzyme-A molecules. That's the oxidation of
  pyruvate, which prepares things for the Krebs
  cycle.

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• In the oxidation of pyruvate, each pyruvate loses
  a carbon to make carbon dioxide. The two-carbon
  molecules left (acetyl groups) attach to
  coenzyme-A. In addition, each pyruvate loses a
  hydrogen. Thus, 2 CO2 molecules are formed, 2
  NADH are formed, and 2 acetyl coenzyme-A
  molecules are formed.