Human Skeletal System: Structure and Functions

The human skeletal system is a complex framework that provides support, protection, and movement to the body. It consists of bones, cartilage, ligaments, and tendons, which collectively perform essential functions. 

Introduction to the Skeletal System

The skeletal system serves as the basic framework of the human body, comprising 206 bones in adults. It is divided into two main subdivisions:

• Axial Skeleton: Forms the central axis of the body.
• Appendicular Skeleton: Includes limbs and girdles that attach to the axial skeleton.

Anatomy of the Skeletal System

Axial Skeleton

The axial skeleton consists of 80 bones and includes:

• Skull: Protects the brain and forms facial structures. It includes 8 cranial bones, 14 facial bones, 6 auditory ossicles, and the hyoid bone.
• Vertebral Column: Composed of 24 vertebrae, sacrum, and coccyx; it supports the body’s weight and protects the spinal cord.
• Thoracic Cage: Includes 12 pairs of ribs and the sternum, protecting vital organs like the heart and lungs.

Appendicular Skeleton

The appendicular skeleton consists of 126 bones:

• Shoulder Girdle: Includes clavicles and scapulae.
• Upper Limbs: Composed of humerus, radius, ulna, carpals, metacarpals, and phalanges.
• Pelvic Girdle: Supports lower limbs and protects pelvic organs.
• Lower Limbs: Includes femur (thigh bone), tibia, fibula, tarsals, metatarsals, and phalanges.

Functions of the Skeletal System

Support: The skeleton provides a rigid framework that supports soft tissues and maintains body shape. For example:

• The pelvis supports abdominal organs.
• The rib cage prevents lung collapse.

Protection: Bones safeguard vital organs:

• The skull encases the brain.
• The vertebral column protects the spinal cord.
• The rib cage shields the heart and lungs.

Movement: Bones act as levers for muscles to facilitate movement. Joints allow flexibility:

• Ball-and-socket joints (e.g., hips) enable wide movement ranges.
• Pivot joints (e.g., neck) allow rotational movement.

Production of Blood Cells: Bone marrow produces red blood cells (RBCs), white blood cells (WBCs), and platelets. This process occurs in flat bones like the sternum and pelvis.

Mineral Storage: Bones store essential minerals such as calcium and phosphate. These minerals are released into the bloodstream as needed for physiological processes.

Endocrine Regulation: Bones release hormones like osteocalcin that regulate blood sugar levels and fat deposition. They also influence calcium metabolism through interactions with parathyroid hormone.

Composition of Bones

Each bone has three layers:

1. Compact Bone: Dense outer layer providing strength.
2. Spongy Bone: Lightweight inner layer with spaces for marrow.
3. Bone Marrow: Produces blood cells and stores fat.

Joints in the Skeletal System

Joints are critical for mobility:

• Fixed Joints: Found in areas like the skull where no movement occurs.
• Movable Joints: Include hinge joints (elbows) and ball-and-socket joints (shoulders).

Developmental Aspects

At birth, humans have approximately 270 bones. Some fuse during growth to form 206 adult bones. Maximum bone mass is achieved between ages 25–30.

Sexual Dimorphism in Skeletons

Subtle differences exist between male and female skeletons:

• Female pelvis is broader to facilitate childbirth.
• Male bones tend to be larger and more robust.

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