Respiratory System

Links to other lectures


Site under construction

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 diaphragm

The Mammalian respiratory system includes the following passages

  • Nasal cavity and mouth
  • pharynx and epiglottis
  • larynx - voice box
  • trachea
  • lungs which include:
    • bronchial tubes
    • bronchioles
    • alveoli - air sacs


Other Topics (under construction) include

Breathing Movements

The Exchange of Gases

Oxygen Transport

Carbon Dioxide Transport

The Control of Respiration


The exchange of gases and the path of oxygen is shown in the diagram below

  1. 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.
  2. 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.
  3. 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.
  4. The oxygen readily diffuses through the cytoplasm and the membrane on the far side of the capillary since it is a small nonpolar molecule.
  5. 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.