Gas Exchange

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Gas Exchange

• IB Biology

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6.4.1 Ventilation, gas exchange and cell
respiration.

• 1. Ventilation
• The flow of air in and out of the alveoli is
  called ventilation and has two stages
  inspiration (or inhalation) and expiration (or
  exhalation).
• Lungs are not muscular and cannot ventilate
  themselves, but instead the whole thorax moves
  and changes size, due to the action of two sets
  of muscles the intercostal muscles and the
  diaphragm.

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2. Gas Exchange

• This is the diffusion of gases (oxygen and carbon
  dioxide)
• There are two sites for gas exchange
• (a)Alveoli Oxygen diffuses into the blood from
  the alveoli and carbon dioxide diffuses from the
  blood into the alveoli
• (b)Tissues Oxygen diffuses from blood into the
  cells and carbon dioxide diffuses from cells to
  the blood

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3. Cell Respiration

• Aerobic respiration uses oxygen in the
  mitochondria and produces carbon dioxide
• Anaerobic respiration does not use oxygen but
  still produces carbon dioxide

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6.4.2 The need for a ventilation system.

• A ventilation system is needed to maintain
  concentration gradients in the alveoli
• The steep concentration gradient across the
  respiratory surface is maintained in two ways by
  blood flow on one side and by air flow on the
  other side. The ventilation system replaces
  diffuses oxygen (keeping the concentration high)
  and removes carbon dioxide (keeping the
  concentration low).
• This means oxygen can always diffuse down its
  concentration gradient from the air to the blood,
  while at the same time carbon dioxide can diffuse
  down its concentration gradient from the blood to
  the air.

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6.4.3 Features of the alveoli.

• Large surface area due to the combined spherical
  shape (600 million alveoli 80 m2)
• Flattened epithelial cells of alveoli and close
  association with capillaries
• Short diffusion distance from alveoli to blood
  (0.5-1.0 um)
• Dense capillary network
• Moist surface for the solution of gases

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6.4.4 Structure of the ventilation system.

• (a) Trachea
• (b) Cartilage ring support
• (c) Bronchi (plural) Bronchus (single)
• (d) Lung
• (e) Heart
• (f) Sternum
• (g) Rib cage
• (h) Bronchioles (j) Alveoli (k) Diaphragm

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6.4.5 Mechanism of ventilation.

• The diaphragm contracts and flattens downwards.
• The external intercostal muscles contract,
  pulling the ribs up and out
• this increases the volume of the thorax this
  increases the lung and alveoli volume
• this decreases the pressure of air in the alveoli
  below atmospheric (Boyle's law)air flows in to
  equalise the pressure

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Ventilation, contd

• The diaphragm relaxes and curves upwards
• the external intercostal muscles relax, allowing
  the ribs to fall
• this decreases the volume of the thorax
• this decreases the lung and alveoli volume
• this increases the pressure of air in the alveoli
  above atmospheric (Boyle's law)air flows out to
  equalise the pressure.
• The abdominal muscles contract, pushing the
  diaphragm upwards
• The internal intercostal muscles contract,
  pulling the ribs downward
• This gives a larger and faster expiration, used
  in exercise