Animal body structures reflect their functions.

• 20.1 Most animal bodies are organized in a hierarchy from cells to tissues, organs, and organ systems.
• 20.2 Connective tissue provides support.
• 20.3 Epithelial tissue covers most interior and exterior surfaces of the body.
• 20.4 Muscle tissue enables movement.
• 20.5 Nervous tissue transmits information.
• 20.6 Each organ system performs special tasks.

Animals have an internal environment.

• 20.7 Our bodies function best within a narrow range of internal conditions.
• 20.8 Animals regulate their internal environment through homeostasis.

How does homeostasis work?

• 20.9 Negative and positive feedback systems influence homeostasis.
• 20.10 Temperature control is a component of homeostasis.
• 20.11 This is how we do it: Why do we yawn?
• 20.12 Animals must balance their water content within a narrow range.
• 20.13 In humans, the kidneys regulate water balance.