Internal exchange surfaces of complex animals.

Many animals have organ systems with finely branched or folded surfaces specialized for exchange.

  1. Digestive system: the small intestine is lined with fingerlike projections for nutrient absorption.

  2. Respiratory system: the spongelike surface of the lung anables gas exchange.

  3. Excretory system: a kidney is composed of microscopic tubules that exhange chemicals with blood flowing through tiny capillaries.