Body Plans and Environmental Exchange

In order to maintain fluidity of the cell (plasma) membrane, all cells are bathed in an aqueous solution.  Interaction with the environment occurs via diffusion (recall surface area : volume lesson...it only works if all the cells have access to this suitable environment).

The Hydra - Tapeworm - Complex Animal Solution

Hydra has two layers of cells only.  A tapeworm is extremely thin such that intestinal fluid bathes all of its cells.  Complex animals have specialized internal structures for interaction with the environment: digestive system, repiratory system, circulatory system and excretory system.  On a smaller scale the surface area of the small intestine is maximized through the presence of tiny extensions called microvilli.

Advantages of the Complex Animal Solution

Because of these innovations animals can live on land: the exterior of terrestrial animals does not need to be completely submerged in water.  The cellular environment is all internal; thereby allowing the organism to regulate its own environment.

What regulates the internal environment?

Part of the homeostasis concept of Claude Bernard is "interstitial fluid," the vertebrate internal environment.

A simple homeostatic control system: a receptor detects a change, the control center directs an effective response, an effector carries out the response to the change.  Negative feedback is also often used to prevent a small change in a variable concentration from becoming a large change.  One of the control centers is the hypothalamus which monitors body temperature among other things.  Positive feedback with when a small change in a variable concentration initiates an amplification of that change - or increasing of the change; child birth is an example: the baby's head pushes against the uterus causing more contractions to complete the birth.

Next:  "Ecology Review."