CH+6+Bones+and+Bone+Tissue

Functions of the Skeletal System

• System Overview: The skeletal system includes bones, joints, and their associated supporting tissues.

• Tissue Composition: Bones are complex organs composed of multiple tissue types, including dense regular collagenous connective tissue, dense irregular collagenous connective tissue, bone tissue (osseous tissue), and bone marrow.

• Protection: Certain flat and broad bones shield underlying vital organs from mechanical trauma:

  • The skull encloses and protects the brain.

  • The sternum (breastbone) and ribs protect the heart, lungs, and major thoracic blood vessels.

  • The pelvis shields lower abdominal and pelvic organs.

• Mineral Storage and Acid-Base Homeostasis:

  • Bone serves as the primary reservoir in the body for essential mineral salts, notably calcium ($Ca^{2+}$), phosphorus ($PO_4^{3-}$), and magnesium ($Mg^{2+}$).

  • Mobilization and deposition of these mineral salts are critical for maintaining systemic electrolyte balance and acid-base homeostasis in blood plasma.

• Blood Cell Formation (Hematopoiesis):

  • Bones house red bone marrow, a specialized, highly vascularized connective tissue.

  • Red bone marrow contains hematopoietic stem cells responsible for producing all blood cell lineages, including red blood cells, white blood cells, and blood platelets.

• Fat Storage:

  • Bones contain yellow bone marrow, which consists predominantly of adipocytes (fat cells).

  • Yellow bone marrow stores triglycerides; when energy is required, triglyceride breakdown liberates fatty acids that cells can utilize for fuel.

• Movement:

  • Bones serve as attachment sites for skeletal muscles via tendons.

  • Muscle contraction exerts pulling forces on bones, generating mechanical leverage and movement at joint articulations.

• Support:

  • The skeleton provides the structural framework for the human body, supporting soft tissues and bearing total body weight against gravity.

Classification of Bones by Shape

• General Classification: All $206$ bones in the adult human body are classified into five distinct categories based on their overall shape, regardless of their actual size:

• Long Bones:

  • Defined as being longer than they are wide.

  • Examples include most bones of the upper and lower limbs, such as the humerus, radius, ulna, femur, tibia, fibula, metacarpals, metatarsals, and phalanges.

• Short Bones:

  • Roughly cube-shaped, possessing dimensions where length is approximately equal to width.

  • Examples include the bones of the wrist (carpals) and the ankle (tarsals).

• Flat Bones:

  • Characterized by a thin, broad, and flattened structure.

  • Examples include the ribs, pelvis, sternum (breastbone), and most bones of the cranial vault in the skull.

• Irregular Bones:

  • Possess complex, irregular geometries that do not fit into any other anatomical shape class.

  • Examples include the vertebrae and certain complex cranial and facial bones (e.g., sphenoid, ethmoid).

• Sesamoid Bones:

  • Specialized, small, flat, oval