Human Ventilation and Gas Exchange

Aerobic Respiration:					Anaerobic Respiration:
Glucose + Oxygen => Water + Carbon Dioxide + Energy					Glucose => Carbon Dioxide + Lactic Acid + (little) Energy

Gas exchange surface for humans is the lungs. Contains a branching system stemming from the trachea and ending in the alveoli.

The Alveoli:

Each one is folded to increase surface area Each has a thin permeable surface They have a diffusion gradient: Ventilation brings in air rich in oxygen into the alveoli, and takes air with increased carbon dioxide away from alveoli. Blood flow/circulation maintains high carbon dioxide/low oxygen concentrations in the blood.

Note: Excess water loss from the lungs is prevented due to the limited contact with the outside air because the lungs are enclosed within the body cavity.

Maintenance of Breathing: At Rest (Nervous System)

Medulla controls breathing. Impulses from the inspiratory centre in the medulla cause contraction of breathing muscles. Stretch receptors are stimulated by increase in size of thorax/lungs. Impulses are then sent to the expiratory centre, which inhibits the inspiratory impulses.

During Exercise

Chemoreceptors (in the medulla/aortic bodies/carotid bodies) are sensitive to the rise in the carbon dioxide level in blood. Impulses are then sent to the inspiratory centre. This causes a more rapid rate of impulses to breathing muscles. This produces larger amounts of carbon dioxide so more oxygen is released; therefore a high rate of respiration is maintained with more haemoglobin free to act as a buffer.(A buffer is a substance, which can absorb hydrogen)

Breathing In:

Diaphragm contracts and flattens. Intercostal muscles contract, therefore ribs move up and out. The volume of the thorax increases, decreasing pressure below atmospheric pressure. Oxygen flows into large air passages i.e Trachea => Bronchi => largest Bronchioles Final pathway – oxygen diffuses into alveoli along the concentration gradient. In the alveoli, oxygen dissolves into a film of liquid, which then diffuses the short distance into the blood capillaries.

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