Body Systems: Circulatory System

Animals possess a circulatory system that delivers food, oxygen, and other necessary substances to all cells, and that removes waste products. Materials are transferred between individual cells and their internal environment through the cell membrane by diffusion, osmosis, and active transport; during diffusion and osmosis, molecules move from a higher concentration to a lower concentration, while during active transport, carrier molecules push or pull substances across the cell membrane, using adenosine triphosphate (ATP) for energy.