Human skeletal system

Overview of the Skeletal System

• The skeletal system includes bones and connective tissues
• Connective tissues include ligaments, tendons, and cartilage
• Ligaments connect bone to bone
• Tendons connect muscle to bone
• Earthworms have a hydrostatic skeleton
• Insects have an exoskeleton made of chitin
• Humans have an endoskeleton

Functions of the Skeletal System

• Supports the body
• Protects internal organs
• Stores minerals
• Involved in red and white blood cell production
• Facilitates movement with the help of muscles
• The musculoskeletal system combines the skeletal and muscular systems because of their collaborative work

Bone Composition and Structure

• Adult humans have 206 bones
• Babies have more bones that fuse together by adulthood
• The human skeleton consists of the axial skeleton and the appendicular skeleton
• The axial skeleton includes bones in the skull, ears, throat (hyoid), vertebral column, and ribcage
• The appendicular skeleton includes bones of the arms, shoulder girdle, legs, and pelvic girdle

Classification of Bones by Shape

• Long bones are longer than they are wide and are moved in muscle contraction
• Examples of long bones are the femur, tibia, fibula, humerus, ulna, radius, metacarpals, metatarsals, and phalanges
• Short bones are cube-shaped with length and width nearly equal providing stability
• Examples of short bones are carpals and tarsals
• Sesamoid bones are roundish and handle pressure
• An example of a sesamoid bone is the patella
• Flat bones are thin and often curved
• Examples of flat bones are skull bones and shoulder blade bones (scapulae)
• Irregular bones have shapes that do not fit specific descriptions and protect from various forces
• Vertebrae are examples of irregular bones

Bone Tissue

• Compact bone tissue is a hard outer layer
• Spongy bone tissue contains bone marrow
• Yellow marrow stores fat for long-term energy storage
• Red marrow produces red and white blood cells and platelets
• Red blood cells transport gases
• White blood cells defend the body
• Platelets are cellular fragments critical for blood clotting
• Bones are vascular due to their rich blood supply
• Fluids and medications can be administered into the bone in emergencies

Bone and Cartilage Cells

• Osteoblasts make bone
• Osteoblasts can mature into osteocytes which maintain bone structure
• Osteocytes are the majority of bone cells
• Osteoclasts break down bone structure using lysosomes with enzymes and acids
• Bone remodeling involves old or damaged bone being reabsorbed and replaced
• Remodeling is impacted by injury and exercise
• Bone breakdown releases minerals like calcium
• Hormones regulate calcium release and deposition in bones
• Calcium is essential for muscle contraction
• Chondroblasts make cartilage
• Chondroblasts mature into chondrocytes which maintain the cartilage
• Cartilage supports bone, is important in joints, and serves as a template for bone placement during fetal development
• Bones grow in length with the help of osteoblasts until early adulthood
• Bones can grow in diameter even after they stop growing in length

Bone Fractures and Healing

• After a bone break, a fracture hematoma (collection of blood) forms
• Blood is blocked near the fracture causing bone cell death
• Chondrocytes and osteoblasts create an internal callus and external callus to stabilize the break
• The internal callus is mainly cartilage and the external callus is a mix of cartilage and bone
• Osteoclasts remove damaged bone
• Osteoblasts remodel and make new bone
• Fractures typically heal in 6-8 weeks, but can take longer
• Severe trauma to long bones can cause excessive internal bleeding and release of fat from yellow marrow
• Fat embolism syndrome, though rare, requires emergency care

Bone Development Conditions

• Osteogenesis imperfecta is a genetic disorder affecting collagen production
• Bones become brittle and easily fracture
• Treatments aim to help with mobility and prevent fractures
• Orthopedic research addresses conditions affecting the skeletal system