Internal exchange surfaces of complex animals.
Many animals have organ systems with finely branched or folded surfaces specialized for exchange.
- Digestive system:
the small intestine is lined with fingerlike projections for nutrient absorption.
- Respiratory system:
the spongelike surface of the lung anables gas exchange.
- Excretory system:
a kidney is composed of microscopic tubules that exhange chemicals with blood flowing through tiny capillaries.