1. Exchange with the environment occurs as dissolved substances diffuse across the plasma membranes between the cells and their aqueous surroundings. For animals with complex internal organization and relatively small surface to area volume ratio, internal surfaces are specialized for exchange with the environment. Folding or branching gives these moist internal membranes expansive surface area. Materials are shuttled between all these exchange surfaces by the circulatory system.
             2. Interstitial fluid fills the space between the cells of the internal environment of vertebrates. This fluid exchanges nutrients and wastes with the blood contained in microscopic vessels called capillaries.
             3. Homeostasis – The steady-state physiological condition of the body.
             4. Negative feedback is a mechanism of homeostasis, whereby a change in a physiological variable that is being monitored triggers a response that counteracts the initial fluctuation. It prevents small changes from becoming too large. An example is a thermometer detecting a temperature above the set point and the thermostat switching the heater off. Positive feedback involves a change in some variable that triggers mechanisms that amplify rather than reverse the change. During childbirth, the pressure of the baby's head against sensors near the opening of the uterus stimulates uterine contractions, which cause greater pressure against the uttering opening, heightening the contractions, which causes still greater pressure.
             5. Suspension feeders are mostly aquatic animals that sift small food particles from the water. Substrate-feeders, such as leaf miners, live in or on their food source, eating their way through the food. Fluid-feeders make their living by sucking nutrient-rich fluids from a living host. Bulk-feeders eat relatively large pieces of food by either killing their prey or tearing off pieces of meat or vegetation by us
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