122 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. found in the cavities of many bones, including flat and short bones bodies of the vertebrae, sternum, and ribs and the articulating ends of long bones. Blood Cell Formation It is in the red marrow that the critically important function of hematopoiesis (hee-ma-toh-poy-EE-sis), or blood cell formation, occurs. Red blood cells deliver oxygen to tissues throughout the body and transport waste in the form of carbon dioxide to the lungs, where it can be exhaled. Because red blood cells have a life span of only about 120 days, it is important for the red bone marrow to constantly produce new red blood cells. SELF CHECK 1. List the five functions of the skeletal system. 2. What are two functions of bone marrow? Bone Classification and Structure The composition and structure of bone make it remarkably strong and resilient given its relatively light weight. The shapes of bones vary according to their specific functions. Composition of Bones Cells are the structural building blocks of bone, as they are in other body tissues. Mature bone cells are called osteocytes. One factor that distinguishes bone from other tissues is that 60% to 70% of a bone’s weight comes from its mineral content—primarily calcium carbonate and calcium phosphate. These mineral salts make bone hard and resistant to compression. The remaining 30% to 40% of a bone’s weight comes from collagen—a protein that provides bone’s flexibility and resistance to tension (being stretched)—and water. Both the minerals and the water content contribute to bone strength. The bones of children tend to be more flexible than the bones of adults due to higher collagen and water content. Types of Bone Tissue Bone includes two different types of tissue: cortical bone and trabecular bone. Whereas cortical bone tissue is relatively dense, trabecular bone tissue, also known as spongy bone or cancellous bone, is relatively porous, with a honeycomb structure (Figure 5.1). Cortical (compact) bone is stiffer due to its higher mineral content, so it is generally stronger than trabecular bone. Trabecular bone, with its spongy structure, is more flexible than cortical bone. Most bones contain both cortical and trabecular tissue. The function of a given bone determines whether it is composed mostly of cortical or trabecular bone. For example, the long bones in the arms and legs are primarily composed of strong cortical bone tissue, although there is trabecular bone inside the ends of these bones. The bones in the spinal column contain a large amount of trabecular bone inside their cortical shells, giving them a certain amount of shock-absorbing capability. However, in all bones, the outer layer is always composed of hard, protective cortical bone, with spongy trabecular bone present to varying degrees inside. Figure 5.2 compares the properties of these two types of bone tissue. Understanding Medical Terminology The word cortical (coming from cortex) pertains to the outer layer of something. For example, the outer layer of the brain is known as the cerebral cortex. The outer layer of many structures and objects, including a plant stem or even a rock, is also known as its cortex. The type of bone tissue forming the outer layer of bone is therefore called cortical bone. Steve Gschmeissner/Science Source Figure 5.1 A scanning electron micrograph of trabecular bone tissue.