1. Compact, complex animals have internal exchange surfaces that are extensively branched or folded, providing a large _______.
2. In the _, nutrients are absorbed across the many fingerlike projections of the lining of the intestine.
3. In the _, oxygen and carbon dioxide are exchanged across the epithelial linings of sacs at the tips of tiny air tubes.
4. In the _, wastes are removed from the blood across the epithelial linings of excretory tubes.
5. The _ transports materials between the exchange surfaces of other organ systems and body cells.
6. Body cells are bathed in _________, and exchange between body cells and the blood takes place through this fluid.

In higher complex animals, such as the vertebrates, the internal exchange surfaces are modified in such a way to facilitate the transfer of materials from cell to cell in the body. As animal size increases, diffusion distances increase and the ratio of surface area to volume drops. Larger organisms have had to evolve specialized systems that are made up of structures that provides a large surface area for internal exchange of materials. Examples of these systems include:

--> DIGESTIVE SYSTEM: The fingerlike projections of the lining of the intestine provide a large surface area for the absorption of nutrients.

--> RESPIRATORY SYSTEM: This ends with a sac called the alveolar sac. The anatomical arrangement of capillaries and alveoli produces a very large surface area that is available for gas exchange.

--> URINARY SYSTEM: wastes are easily removed from the blood across the epithelial linings of excretory tubes.

--> CIRCULATORY SYSTEM: transports materials between the exchange surfaces of other organ systems and body cells. An example is seen in the relationship between the alveoli and the capillaries surrounding it.

For any exchange of materials to occur between cells the fluid known as the INTERSTITIAL FLUID which fills the intercellular spaces allows dissolved materials to be exchanged between body cells and the blood