Functions of Bones

Functions of Bones

Bone is a connective tissue that performs several basic functions: hemopoiesis, movement, support and protection, and storage of minerals and lipids.

■■ Hemopoiesis: This is the process in which blood cell production begins in the yolk sac of the developing embryo, which is also referred to as ­hematopoiesis. Later, this process occurs in the red bone marrow, which contains stem cells that form all blood cell types. Its color is derived from hemoglobin, which is the oxygen-carrying pigment of the red blood cells. The locations of red bone marrow differ between children and adults. In children, red bone marrow is in the spongy bone inside most of the bones of the body. As children mature into adults, much of the red bone marrow degenerates into a fatty tissue called yellow bone marrow. As a result, adults have red bone marrow only in certain portions of the axial skeleton such as the flat bones of the skull, the vertebrae, the sternum, the ribs, and the hip bones. Adults also have red bone marrow in the proximal epiphyses of the femur and humerus.

■■ Movement: The bones assist in body movement by providing attachments for skeletal muscles that pull on the bones, causing the bones to act as levers (FIGURE 7-9). For example, when the arm is bent and then straightened at the elbow, the bones and muscles are functioning together in a lever like way. When the arm bends, the hand is moved against resistance provided by the weight of the arm, and the muscles on the anterior side of the arm (including the biceps brachii) supply force. When the arm is straightened, the triceps brachii muscle on the posterior side of the arm supplies the force. The types of movements that are possible depend on the type of joints involved.

■■ Support and protection: The bones support and protect vital organs of the body such as the brain, spinal cord, lungs, and heart. They also protect other soft tissues of the body. The bones of the lower limbs support the trunk of the body during standing, whereas the rib cage supports the thoracic wall.

■■ Storage of minerals and growth factor: The bones store more than 90% of the minerals—calcium and phosphorus—which are essential for many body processes. When these minerals are needed, some connective bone tissue is broken down, so they can be released into the bloodstream. Phosphorus is important for the creation of organic phosphate, which provides adenosine triphosphate. Important growth factors are stored in the mineralized bone matrix.

■■ Fat storage: In fat, triglyceride is an energy source stored in bone cavities.

■■ Hormone production: The hormone osteocalcin is produced by bones. It helps to regulate bone formation and also protects against glucose intolerance, diabetes mellitus, and obesity.

1. What is the function of red bone marrow?

2. What chemical substances are stored in the bones?