Introduction

Every living cell depends on a steady supply of nutrients and oxygen, and just as steadily needs its waste and harmful by-products carried away — a demand that never really stops as long as the tissue stays alive. This is exactly why organisms need dedicated, efficient mechanisms for moving such substances to their cells and away from them.

Different groups of animals have arrived at different solutions to this problem. Simple animals such as sponges and coelenterates take the easiest route: they draw water from their surroundings and circulate it through channels and cavities inside the body, letting individual cells exchange substances directly with this circulating water. Animals with more complex body plans, by contrast, rely on a dedicated internal fluid to carry materials from place to place inside the body.

In humans and most other advanced animals, the principal transport fluid for this job is blood. A second fluid, lymph (also called tissue fluid), works alongside blood to help move certain substances around the body. This chapter looks first at the composition and properties of blood and lymph, and then at how blood itself is circulated through the body.