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Respiration requires moist surfaces for gas exchange,
otherwise the oxygen molecules would simply bounce away unable to
diffuse into the tissues where they are needed.
A number of solutions to the problem of gas
exchange have evolved including:
- moist skin
- external gills
- fish gills
- tracheae (insects)
- simple gas bags ( amphibians)
- bird lungs with a one-way circuit
- mammal lungs with a two-way circuit aided by a
The Mammalian respiratory system includes the
- Nasal cavity and mouth
- pharynx and epiglottis
- larynx - voice box
- lungs which include:
- bronchial tubes
- alveoli - air sacs
Other Topics (under construction)
The Exchange of Gases
Carbon Dioxide Transport
The Control of Respiration
The exchange of gases and the path of oxygen is
shown in the diagram below
- Diffusion of oxygen from an area of relatively
high concentration to area of low concentration occurs in the
alveoli of the lungs. Specifically oxygen leaves the air and
dissolves into the moist film coating the air sac.
- Oxygen diffuses through the epithelial
membrane of the alveoli and into the cytoplasm of the alveoli
cells then exits through the other side all because the
concentration gradient is still favorable.
- The oxygen must pass through the fluid space
between the air sac epithelium and the capillary epithelium where
it will encounter the cell membrane of a capillary epithelium.
Once again net movement is toward the oxygen poor blood plasma
which lies just beyond.
- The oxygen readily diffuses through the
cytoplasm and the membrane on the far side of the capillary since
it is a small nonpolar molecule.
- Finally the oxygen enters the blood plasma
where it is picked up by a hemoglobin molecule imbedded in the
mosaic of a red blood cell membrane.