**Respiratory Surfaces**
* Structure depends mainly on:
* ___ of the organism
* Whether it lives in water or on land
* _____ demands
* Ex: An endotherm requires a larger area of respiratory surface than a similar-sized ectotherm
* In some relatively simple animals, such as
sponges, cnidarians, and flatworms, the plasma membrane of every cell in the body is close enough to the _____ environment for gasses to diffuse in and out.
* In most animals, however, the bulk of the body lacks direct access to the respiratory medium.
* The respiratory surface in these animals is a thin, moist epithelium that constitutes a respiratory ____.
* Some animals, such as earthworms and some amphibians, use the ___ as a respiratory organ.
* Just below the moist skin is a dense net of capillaries.
* Because the respiratory surface must be moist, however, the possible habitats of these animals are limited to water or damp places.
* For most other animals, the general body surface is not sufficiently large to exchange gasses for the entire body.
* The solution is a respiratory organ that is extensively _____ or branched, thus enlarging the surface area for gas exchange