Physiology of the Skeletal System
Physiology of the Skeletal System
Learning Objective: Examine the physiology of the skeletal system.
The skeletal system has several important roles in the body:
• Protecting, supporting, and providing a framework for organ systems of the body
• Helping with movement
• Developing new bone
• Regulating the blood calcium level
• Hematopoiesis, the formation of blood cells and platelets
The last three roles will be addressed in more depth in the following sections.
Development of New Bone
Learning Objective: Describe the remodeling and health process of bones.
Between the periosteum and the endosteum, the osteoclasts and osteoblasts continuously remodel bones. The osteoclasts are bone cells that break down bone. The osteoblasts, bone-forming cells, make the bones strong, durable, and able to heal. Because bones have a good blood supply, they easily heal after trauma or a break (fracture).
When a fracture occurs, a protective blood clot and callus form at the break. During the healing process, new bone cells start to grow on both sides of the break. Eventually, the new bone from both sides meets, closing the fracture, and the callus is absorbed.
Regulation of the Blood Calcium Level
Learning Objective: Describe how the skeletal system is involved with the regulation of the blood calcium level.
About 98% of the body’s calcium is stored in the bones. Calcium is moved between the blood and bones through the work of the osteoblasts and osteoclasts. It is removed from the blood as the osteoblasts form new bone. This lowers the blood calcium level. When the osteoclasts break down bone, calcium is released into the blood, thus increasing the blood calcium level.
Two hormones work to maintain the blood calcium level. The parathyroid hormone from the parathyroid gland increases the activity of the osteoclasts when the blood calcium level is decreased. Calcitonin, which is produced in the thyroid, helps lower the blood calcium level. Calcitonin promotes bone formation by the osteoblasts and inhibits the bone breakdown by the osteoclasts.
Hematopoiesis
Learning Objective: Describe the skeletal system’s role with hematopoiesis.
Spongy bone is found at the center of most bones. It is less dense than compact bone and has a network of open spaces that contain red bone marrow. The red bone marrow is a soft, gelatinous tissue that consists of blood stem cells. These stem cells can become white or red blood cells or platelets.When a fracture occurs, a protective blood clot and callus form at the break. During the healing process, new bone cells start to grow on both sides of the break. Eventually, the new bone from both sides meets, closing the fracture, and the callus is absorbed.