21.1–21.3: The circulatory system is the chief route of distribution in animals.

A young African elephant runs through the scrubland.

Size matters. When life first arose on earth, the tiny, single-celled organisms were small enough to acquire fuel and raw materials in a straightforward way: those materials could simply diffuse across the cell membrane and be used as needed. Similarly, metabolic waste products could diffuse out of the cell. Today, single-celled organisms and small multicellular organisms in which all cells are in contact with (or just a few cells away from) the external environment acquire raw materials and dispose of metabolic waste in the same way.

The evolution of large body sizes opened up a world of new niches in which organisms could live. Physiologically, however, large body sizes created a host of new challenges. With increasing size, most of an animal’s cells were no longer in direct contact with the outside world, the environment from which the animal obtained the oxygen, nutrients, water, and other substances it needed to survive. Consequently, essential substances could no longer just diffuse in, and waste products could no longer diffuse out adequately. As body size increased, dedicated delivery and removal systems became a necessity.

In animals, the primary distribution system is the circulatory system, which consists of tens of thousands of miles of hollow, tube-like vessels that reach all tissues of the body. In vertebrates, circulatory systems have three principal functions: transport, body temperature regulation, and protection FIGURE 21-1.

Figure 21.1: Like a set of highways for the body. Circulatory system functions: transport, temperature regulation, and protection.

1. Transport. Like a system of highways for delivering important goods and removing garbage, the circulatory system transports oxygen, nutrients, waste products, hormones, and immune system cells in the blood throughout the body.

• In vertebrates, blood vessels take oxygen from the lungs or gills and deliver it to the tissues for energy-releasing cellular respiration. Blood arriving from the lungs or gills is loaded with oxygen and is said to be oxygenated or oxygen-rich.
• Simultaneously, blood vessels whisk away carbon dioxide and other metabolic wastes that are produced by cellular respiration and other cell processes and must be removed from the body. Blood that carries a lot of carbon dioxide is said to be deoxygenated or oxygen-poor.
• Once food particles are digested, the nutrients must be absorbed and delivered to all the tissues of the body, for activity, growth, and reproduction.
• The circulatory system delivers hormones (chemicals produced throughout the body by glands and other tissues; see Chapter 24) to target tissues to regulate growth, development, and reproduction.

2. Body temperature regulation. By expanding or contracting the blood vessels closest to the exterior of the body, animals can release or absorb (or at least minimize the loss of) heat, a process that helps maintain body temperature within the optimum range for metabolism.

3. Protection. A variety of cells and chemicals contribute to an individual’s defenses against infection by pathogens. White blood cells, or leukocytes, can engulf and destroy many disease-causing microorganisms. Platelets and certain chemicals in the blood provide protection by limiting blood loss and infection when the skin or other tissues are damaged.