21.2: Circulatory systems can be open or closed.

There’s more than one way to build a circulatory system. And, in fact, not all multicellular organisms even need a circulatory system. For example, in spite of their relatively large size and multicellularity, flatworms, as well as cnidarians such as jellyfish, have a body plan that gives every cell easy access to oxygen and nutrients through simple diffusion (FIGURE 21-2). This access is possible because every cell is close to the external surface of the cnidarian’s body or to its internal gastrovascular cavity. Although the gastrovascular cavity is not a true circulatory system, it serves many digestive (“gastro”) and circulatory (“vascular”) functions by directing water into the central mouth and through an elaborate system of channels. Cells lining the mouth and the channels can absorb nutrients and exchange gases by diffusion. The nutrients then diffuse to other cells, none of which are very far away. Once nutrients have been extracted (and waste products picked up), fluid in the gastrovascular cavity is flushed back out of the mouth, and the process is repeated.

Among other multicellular animals, there are two distinct types of circulatory systems: open and closed (FIGURE 21-3). Open circulatory systems are found in insects and most mollusks, and closed circulatory systems are found in all vertebrates. The defining feature of an open circulatory system is that it has one fluid, called hemolymph, which not only circulates to transport nutrients, gases, and waste products but also surrounds each cell in the body; there is no clear distinction between the circulating fluid and the interstitial fluid, the fluid outside the cells that bathes all the body tissues. There is a heart (or sometimes many hearts!) that pumps the hemolymph, but it essentially squirts the fluid throughout the extracellular spaces. Large collecting vessels then channel the hemolymph back to the heart, where it can be pumped throughout the body again. These collecting vessels have little valves that close when the heart pumps, preventing the hemolymph from being pumped back through the same vessel from which it was collected. This one-way collection system gives some order to the circulation of hemolymph throughout the organism’s body.

In closed circulatory systems, the circulating fluid—called blood—is always contained in a vessel as it is pumped throughout the animal’s body, and it is physically and chemically separated from the interstitial fluid that bathes each cell. A muscular heart serves as a pump, and with each contraction it propels blood at high pressure through vessels called arteries. Like a highway system with numerous exits and connecting roads, the arteries branch extensively so that blood can be delivered to all the tissues of the body.

At the organs and tissues, arteries branch into thousands upon thousands of ever-smaller vessels, eventually becoming the narrowest and thinnest-walled: the capillaries. The thin walls of the capillaries allow gases, nutrients, and other molecules to diffuse easily into and out of the tissue, down their concentration gradients (FIGURE 21-4). After passing through the capillaries, blood returns to the heart in vessels called veins, which increase in diameter as they near the heart. We discuss the structure and function of blood vessels in more detail in Section 21-6.