By serving as a structural frame, the skeletal system interacts with the muscular system to enable movement. But this is just one of many important functions of a skeleton (FIGURE 23-36). A skeletal system can also provide support for the organism, affording shape and structure and a means of securing the organs and other structures within the animal’s body. And in addition to enabling movement and providing support, a skeletal system offers protection—the brain, for example, is protected from harm within the skull, and the heart is protected within the rib cage. In some organisms, including humans, the skeletal system is also the site for the production of important blood and immune cells, a process that occurs in bone marrow (see Section 26-3). Bones also serve as a reservoir of some minerals, such as calcium, absorbing excess quantities from the bloodstream or releasing minerals when their concentration in the blood is low (see Section 24-5).

Figure 23-36The skeletal system serves multiple purposes.

Figure 23-37There are three distinct types of skeletal systems in animals. Shown (top to bottom): bell jellyfish, lobster, and toad.

There are three distinct types of skeletal systems (FIGURE 23-37). In a hydrostatic skeleton—such as is found in earthworms, jellyfish, and squid—pressure created within a fluid-filled cavity creates sufficient rigidity that the muscles surrounding it can generate movement, including, in earthworms, the ability to dig. An exoskeleton is a rigid outer covering that supports and protects an animal’s body. An exoskeleton may be the shell of a turtle, the rigid, calcium-based covering of some mollusks, or the polysaccharide-based, chitinous exoskeleton of insects, crabs and lobsters, and spiders. Exoskeletons can also be valuable in reducing some animals’ vulnerability to drying out. The third type of skeleton—an endoskeleton—is a support structure of hard mineralized tissue, as is found in echinoderms (such as sea stars and sand dollars) and in all vertebrates, including humans.

The composition of the endoskeleton differs between echinoderms and vertebrates, and among the vertebrates. Echinoderm endoskeletons consist of a rigid, calcium-based structure, similar to bone. Among the vertebrates, the jawless fishes and sharks have a flexible but strong skeleton made from cartilage. All other vertebrates have a skeletal system made primarily from bone, a harder, but still resilient connective tissue material (described in Section 20-2).

At the locations in the vertebrate skeleton where bones meet, several types of joints occur. These may allow movement while maintaining support, such as in the shoulder, hip, knee, and elbow joints. Or they may enable little movement (such as the joints between vertebrae) or no movement at all (such as the “joints” that are the sutured connections between the bones of the skull). Arthritis is a condition in which joints have become damaged. There are different forms of arthritis. In the most common type, osteoarthritis, a breakdown of connective tissue, usually cartilage, reduces the cushioning of the bones and can lead to inflammation of tissue around the joint, and accompanying pain.

Figure 23-38Osteoporosis weakens bone.

Another type of breakdown in the skeletal system is osteoporosis, a disease that occurs as the density of bone is reduced and the chemical composition of the bones changes (FIGURE 23-38). Occurring most commonly in women, particularly following menopause, osteoporosis increases the risk of bone fractures and falls. Dietary modifications, including increased intake of calcium and vitamin D that can strengthen bones, along with lifestyle changes such as increased exercise that can build bone mass and strengthen muscles, can reduce osteoporosis and the risks associated with it.