[6.4] Gas Exchange

6.4.1 - Distinguish between the following -

  • 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.
Gas Exchange:
external image gas_exchange.gifThis 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

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

    6.4.2 The need for a ventilation system - explain
    • 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.

      6.4.3 Features of the alveoli - describe
external image alveoli.gif
      • 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

        6.4.3 Structure of the ventilation system - draw and label
        external image thorax.gif
(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

External intercostals contract for inspiration: Internal intercostals contract for expiration
      • 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
    • 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
    • *** More on gas exchange

**Lung and breathing complications:

    • Cor pulmonale
      Cor pulmonale
    • Bronchi bring air from trachea to broncioles and alveoli. Mucous accumulation and inflammation along the bronchi tubes can cause a condition called bronchitis.
      external image 17099.jpg

      external image bronchitis_diagram.jpg

      Reading a polysomnograph showing moments of non-brathing, called apnea.
      external image Figure%201%20Respiration%20with%20CR%20belt.JPG