Chapter 4 The Skeletal System 123 Storage Bones also serve as a storage repository for minerals, notably phosphorus and calcium. Phosphorus plays a vital role in the development and maintenance of healthy bones and teeth, and it also plays a role in the chemical reactions that release energy from stored fat in the body. Calcium is essential for normal functioning of the neuromuscular system, as well as for blood clotting. Through a chemical balancing procedure known as homeostasis (hoh-mee-oh-STAY-sis), discussed in Chapter 1, the body can draw upon the stored phosphorus or calcium in bone if the levels of these minerals in the bloodstream fall below normal. This process is under hormonal control. See Chapter 8 for more information about hormonal control of calcium. Another storage site in the skeletal system is the medullary (MEHD-yoo-lair-ee) cavity, a central hollow space inside most of the long bones, such as those of the arms and legs. The medullary cavity is also known as the marrow cavity because it stores bone marrow, a flexible tissue found inside bones. There are two types of bone marrow—yellow and red. Both types contain a rich blood supply. Yellow marrow, found within the medullary cavity, is a major storehouse for fat in the body. Red marrow is found in the cavities of many bones, including flat and short bones, bodies of the vertebrae, sternum, ribs, and 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. As you will discover in Chapters 10 and 11, blood cells deliver oxygen to tissues throughout the body and also transport waste in the form of carbon dioxide to the lungs, where it can be breathed out. Check Your Understanding 1. List the five functions of the skeletal system. 2. What are two functions of bone marrow? Structures and Classification of Bones The composition and structure of bone make it remarkably strong and resilient, given its relatively light weight. Beyond this, bones assume specialized shapes in accordance with their specific functions. Composition of Bones Cells are the structural building blocks of bone, as they are in other kinds of tissues in the body. Osteocytes are mature bone cells. Bones have both organic and inorganic content. One factor that distinguishes bone from other tissues is that 60%–70% of a bone’s mass comes from its mineral, or inorganic, content—primarily calcium carbonate and calcium phosphate. The remaining 30%–40% of bone mass comes from water and collagen, a protein that provides the bone with flexibility. Collagens and other proteins in bone are considered organic content. Both the mineral and 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. Organization of Bones Bone is structurally organized into two different types of tissue—cortical (compact) and trabecular (cancellous) bone. Whereas cortical (KOR-ti-kal) bone tissue is relatively dense, trabecular (tra-BEHK-yoo-lar) bone tissue, also known as spongy bone, is relatively porous, with a honeycomb structure (Figure 4.1). Cortical 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 include both cortical and trabecular tissue. The function of a given bone determines whether it is composed mostly of cortical or trabecular bone. The outer layer of a bone is always composed of hard, protective cortical bone, with spongy trabecular bone present to varying degrees in the interior. Copyright Goodheart-Willcox Co., Inc.