Blood vessel

Chapter Forty-Two: Circulation and Gas Exchange Preface All animals must exchange substances with the environment. Unicellular animals may directly exchange with the environment. Nutrients and oxygen enter through the plasma membrane to the cytoplasm while carbon dioxide and metabolic by-products exit from the cytoplasm through the plasma membrane. Multicellular organisms cannot exchange material at the cellular level and therefore rely on specialized systems that carry out exchange with the environment and transport the material from the sites of exchange to the rest of the body. Concept One: Circulatory systems link exchange surfaces with cells throughout the body