Topic 6.4

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Topic 6.4 Gas Exchange
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6.4 (U1) Ventilation maintains concentration
gradients of oxygen and carbon dioxide between
air in alveoli and blood flowing in adjacent
capillaries.

• The human respiratory system works in
  collaboration with the transport system to ensure
  that oxygen is continually supplied to all body
  cells and carbon dioxide is removed from the
  body.
• This involves three process ventilation, gas
  exchange and cell respiration.

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Ventilation

• In humans, ventilation involves bringing fresh
  air to the alveoli and removing stale air.
• In other words, the process of ventilation
  involves the diffusion of CO2 out of the alveolus
  and the diffusion of O2 into the alveolus.
• These gases diffuse due to concentration
  gradients that exist between the alveoli and the
  blood in the capillaries.

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Gas Exchange

• Gas exchange can be defined as the uptake of
  oxygen molecules from the environment and the
  discharge of carbon dioxide into the environment.
  
• This happens in the alveoli of human lungs and
  the exchange occurs with the blood in the
  capillaries.
• In other words, CO2 is diffusing into body
  capillaries and then out of the lung capillary
  into the alveolus. Oxygen is diffusing into the
  lung capillary and then out of the capillary and
  into the body cells.

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6.4 (U2) Type I pneumocytes are extremely thin
alveolar cells that are adapted to carry out gas
exchange.

• The alveoli or air sacs of the lungs
  significantly increase the surface area of the
  lungs for exchange of gases.
• The outer layer of cells or epithelium of the
  alveoli are primarily composed of type I
  pneumocytes, flattened cells that are only 0.15
  µm of cytoplasm.
• The walls of the capillaries surrounding the
  alveoli are also only one cell layer thick for
  ease of diffusion of gases. The gases only need
  to diffuse a distance of 0.5 µm, an adaptation
  that increase rate of gas exchange.

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6.4 (U3) Type II pneumocytes secrete a solution
containing surfactant that creates a moist
surface inside the alveolus adhering to each
other by reducing surface tension.

• Type II pneumocytes make up 5 of the surface
  area of the aveoli and they secrete a fluid that
  coats the inner surface of the aveoli creating a
  film of moisture that the oxygen and carbon
  dioxide can dissolve into.
• The fluid contains a pulmonary surfactant that
  prevents the adhering of the walls of the alveoli
  to one another (prevents collapse of lung). The
  molecules of the surfactant are similar to
  phospholipids (hydrophillic heads and
  hydrophobic tails).

• Human babies born prematurely may suffer from
  infant respiratory distress syndrome due to the
  fact that they may be born without enough
  pulmonary surfactant. They will need oxygen and
  doses of surfactant.

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6.4 (U4) Air is carried to the lungs in the
trachea and bronchi and then to the alveoli in
bronchioles.

• Air is inhaled in the human body through the
  trachea and it travels down the bronchi to the
  bronchioles and into the alveoli.
• The trachea is held open by rings of catrillage
  and branches off into two primary bronchi (one
  per lung).
• Each bronchi branch off into smaller bronchioles
  which have the alveoli attached at the end. The
  bronchioles have walls of smooth muscle which
  means the width of the airways may change.

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6.4 (U5) Muscle contractions cause the pressure
changes inside the thorax that force air in and
out of the lungs to ventilate them.

• Air moves in and out of the lungs as changes in
  pressure inside the chest cavity occurs. Air
  will flow from regions of high pressure to low
  pressure.
• Muscle contractions cause the pressure inside the
  chest (thorax) to decrease lower than atmospheric
  pressure and air rushes into the lungs. Another
  set of muscle contractions cause the air pressure
  in the chest cavity to increase above atmospheric
  pressure and air rushes out of the lungs.
• The movement of air into and out of the lungs is
  known as inhalation and exhalation.

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.4 (U6) Different muscles are required for
inspiration and expiration and because muscles
only do work they contract.

• Muscles are always in one of two states,
  contracted or relaxed. Muscles can only do work
  when they are contracted and are often forced
  into relaxation by the contraction of another
  muscle that works in opposition.
• Muscles will often work in pairs to achieve
  movement. The muscles involved in inspiration and
  expiration work as antagonist pairs.

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6.4 (A1) External and internal intercostal
muscles, and diaphragm and abdominal are examples
of antagonistic muscle action.

• Ventilation relies on the work of antagonistic
  pairs of muscles in the thoracic cavity.
• The muscles change the volume of the chest cavity
  which changes the pressure inside the cavity
  which results in the movement of air into and out
  of the lungs.

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Inhalation

• During inhalation the external intercostals
  contract and this moves the rib cage up and out.
  At the same time the diaphragm is contracting
  which causes it to move down and flatten out.
• Both of these muscle contractions increase the
  volume of the thorax (chest) which in turn
  results in a decrease in pressure inside the
  chest.
• The decrease in pressure in the chest cavity
  causes air to rush in from outside until the
  pressure inside the lungs rises to match the
  atmospheric pressure outside.

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Exhalation

• During exhalation the internal intercostal
  muscles contract which move the rib cage down and
  in. At the same time the abdominal muscles
  contract and the diaphragm is pushed upward into
  a dome shape.
• The result of these muscles contractions is a
  decreased volume in the thorax and pressure rises
  above atmospheric pressure.
• The increased pressure in the chest cavity causes
  the air to flow out of the lungs until pressure
  inside the lungs falls back to atmospheric
  pressure.

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6.4 (A2) Cause and consequences of lung cancer

• See handout for assignment completed in class.

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6.4 (A3) Causes and consequences of emphysema

• See handout for assignment completed in class.