The human skeleton: structure, types of bone and functions

What you'll learn

This revision guide covers the human skeletal system as tested in the CXC CSEC Human and Social Biology examination. You will learn to identify the major bones of the skeleton, classify bones according to their shape, explain the structure of long bones, and describe the multiple functions of the skeletal system. This topic connects directly to movement, support, and protection—essential concepts for Paper 1 multiple choice and Paper 2 structured questions.

Key terms and definitions

Axial skeleton — The central core of the skeleton comprising the skull, vertebral column, ribs and sternum; protects vital organs and provides attachment points for muscles.

Appendicular skeleton — The bones of the limbs and the girdles (shoulder and pelvic) that attach them to the axial skeleton; primarily involved in movement and locomotion.

Compact bone — Dense, hard outer layer of bone tissue with a solid structure; provides strength and protection (also called cortical bone).

Spongy bone — Lighter, porous inner bone tissue with a honeycomb-like structure; reduces bone weight while maintaining strength (also called cancellous or trabecular bone).

Periosteum — Tough membrane covering the outer surface of bones; contains blood vessels, nerves and bone-forming cells necessary for growth and repair.

Bone marrow — Soft tissue found in the cavities of bones; red marrow produces blood cells while yellow marrow stores fat.

Cartilage — Smooth, flexible connective tissue found at joints and in the skeleton; reduces friction and absorbs shock during movement.

Ossification — The process of bone formation where cartilage is gradually replaced by hard bone tissue; begins before birth and continues through adolescence.

Core concepts

The structure of the human skeleton

The adult human skeleton contains approximately 206 bones, though this number varies slightly between individuals. The skeleton is divided into two main parts:

The axial skeleton (80 bones):

• Skull (cranium and facial bones) — 22 bones
• Vertebral column (spine) — 33 vertebrae (7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, 4 fused coccygeal)
• Rib cage — 12 pairs of ribs plus the sternum (breastbone)
• Hyoid bone in the neck

The appendicular skeleton (126 bones):

• Pectoral (shoulder) girdle — 2 clavicles (collar bones) and 2 scapulae (shoulder blades)
• Upper limbs — humerus, radius, ulna, carpals (wrist), metacarpals (hand), phalanges (fingers)
• Pelvic (hip) girdle — 2 hip bones (each formed from ilium, ischium and pubis)
• Lower limbs — femur, patella (kneecap), tibia, fibula, tarsals (ankle), metatarsals (foot), phalanges (toes)

You must be able to identify major bones on a diagram for CSEC examinations. Focus particularly on the skull, vertebral column, ribs, sternum, clavicle, scapula, humerus, radius, ulna, carpals, metacarpals, phalanges, pelvis, femur, patella, tibia, fibula, tarsals and metatarsals.

Types of bones classified by shape

Bones are classified into five categories based on their shape and structure. Each type is adapted for specific functions:

Long bones:

• Longer than they are wide with a shaft and two ends
• Examples: femur (thighbone), humerus (upper arm), tibia, fibula, radius, ulna, metacarpals, metatarsals, phalanges
• Function: act as levers for movement, support body weight, produce blood cells
• Caribbean relevance: Athletes like Usain Bolt from Jamaica have long bones optimized for powerful leg movements in sprinting

Short bones:

• Roughly cube-shaped with similar length and width
• Examples: carpals (wrist bones), tarsals (ankle bones)
• Function: provide stability with limited movement, absorb shock
• Important in cricket players' wrists for controlled batting movements

Flat bones:

• Thin, flattened and usually curved
• Examples: skull bones (cranium), ribs, sternum, scapulae, pelvis
• Function: protect internal organs, provide large surface areas for muscle attachment, produce blood cells

Irregular bones:

• Complex shapes that don't fit other categories
• Examples: vertebrae, sacrum, coccyx, some facial bones
• Function: protect nervous tissue (spinal cord), provide attachment for muscles

Sesamoid bones:

• Small, round bones embedded in tendons
• Example: patella (kneecap), small bones in hands and feet
• Function: protect tendons from wear, improve mechanical advantage of muscles

The structure of a long bone

Long bones have a specific internal structure that you must know in detail for CSEC examinations:

External features:

• Diaphysis — the long central shaft made primarily of compact bone with a hollow medullary cavity
• Epiphysis (plural: epiphyses) — the rounded ends of the bone covered with articular cartilage where joints form
• Periosteum — the outer membrane covering the bone surface (except at joints)
• Articular cartilage — smooth cartilage covering the epiphyses to reduce friction at joints

Internal features:

• Compact bone — the dense outer layer of the diaphysis forming a hard cylinder
• Spongy bone — the porous bone tissue found in the epiphyses and lining the medullary cavity
• Medullary cavity — the hollow space inside the diaphysis containing bone marrow
• Red bone marrow — fills the spaces in spongy bone; produces red blood cells, white blood cells and platelets
• Yellow bone marrow — fills the medullary cavity in adults; consists mainly of fat cells acting as an energy reserve
• Blood vessels — enter through the periosteum to supply nutrients and oxygen

This structure provides maximum strength with minimum weight. The compact bone resists bending forces, while the spongy bone distributes stress and reduces overall mass.

Functions of the skeleton

The human skeleton performs five major functions, all equally important for CSEC examination purposes:

1. Support:

• Provides a rigid framework that supports soft tissues and organs
• Holds the body upright against gravity
• Example: The vertebral column supports the trunk and head; the femur supports body weight during standing and walking
• Caribbean relevance: Construction workers and cane cutters in regions like Barbados and Trinidad rely on strong skeletal support for physically demanding work

2. Protection:

• Encloses and shields delicate organs from injury
• Specific examples:
  • Skull protects the brain
  • Vertebral column protects the spinal cord
  • Rib cage protects the heart and lungs
  • Pelvis protects reproductive organs and bladder

3. Movement:

• Bones act as levers that muscles pull on to produce movement
• Joints between bones allow different types of movement
• Bones provide attachment points for skeletal muscles via tendons
• Example: The elbow joint allows the arm to bend when the biceps muscle contracts, pulling on the radius
• Important for activities like playing cricket, football, or performing calypso dance movements

4. Production of blood cells:

• Red bone marrow in spongy bone produces blood cells through a process called haematopoiesis
• Produces:
  • Red blood cells (erythrocytes) — transport oxygen
  • White blood cells (leucocytes) — fight infection
  • Platelets (thrombocytes) — enable blood clotting
• Main sites: vertebrae, ribs, sternum, pelvis, and ends of long bones (epiphyses)

5. Storage of minerals:

• Bones store calcium and phosphorus
• Calcium is released into the bloodstream when needed for:
  • Muscle contraction
  • Nerve impulse transmission
  • Blood clotting
  • Maintaining strong bones and teeth
• Acts as a mineral reservoir maintaining homeostasis
• Caribbean relevance: Adequate calcium intake from dairy products, fish with bones (like those in fish broth), or fortified foods is essential for maintaining bone strength in the tropical climate

Bone composition and properties

Bones are living organs composed of:

Organic components (approximately 30%):

• Collagen fibres — provide flexibility and tensile strength
• Bone cells — osteoblasts (build bone), osteocytes (mature bone cells), osteoclasts (break down bone)
• Blood vessels and nerves

Inorganic components (approximately 70%):

• Calcium phosphate and calcium carbonate — provide hardness and compressive strength
• Other minerals — magnesium, fluoride, sodium

This combination of organic and inorganic materials makes bone:

• Strong enough to support body weight and resist breaking
• Flexible enough to absorb shock without shattering
• Light enough to allow movement
• Capable of self-repair after injury

Poor nutrition, particularly calcium or vitamin D deficiency, can lead to weak bones. In the Caribbean, where many people have naturally darker skin, vitamin D production from sunlight may still be insufficient without dietary supplementation, affecting bone health.

Growth and development of bones

Understanding bone development is essential for CSEC:

Foetal development:

• The skeleton initially forms as cartilage models
• Ossification begins before birth as calcium salts are deposited
• Some bones (flat bones of skull) form directly from membrane

Childhood and adolescence:

• Long bones grow in length at the epiphyseal plates (growth plates) — areas of cartilage between the diaphysis and epiphysis
• Bones grow in thickness as new layers are added beneath the periosteum
• Complete ossification of growth plates occurs between ages 18-25, ending height increase

Factors affecting bone growth:

• Adequate calcium and vitamin D intake
• Protein for collagen formation
• Growth hormone and sex hormones
• Weight-bearing exercise stimulates bone strengthening
• Caribbean diet considerations: fish, leafy green vegetables, and fortified cereals provide essential nutrients

Worked examples

Example 1: Structured question on bone classification (6 marks)

The diagram shows four different bones from the human skeleton.

(a) Identify which bone is classified as a long bone and give TWO features that identify it as such. (3 marks)

Answer:

• The femur is a long bone (1 mark)
• It has a shaft (diaphysis) that is longer than it is wide (1 mark)
• It has two distinct ends (epiphyses) and a hollow medullary cavity (1 mark)

Mark scheme notes: Accept any correct long bone. Two appropriate structural features needed for full marks.

(b) State THREE functions of the skeleton and give one specific example of each. (3 marks)

Answer:

• Protection — the ribs protect the heart and lungs (1 mark)
• Movement — bones act as levers for muscles to produce movement, e.g., the humerus acts as a lever when the biceps contracts (1 mark)
• Blood cell production — red bone marrow in the sternum produces red blood cells (1 mark)

Mark scheme notes: Function AND specific example required for each mark. Accept support or mineral storage as alternatives.

Example 2: Short answer on bone structure (4 marks)

Describe the structure and function of compact bone and spongy bone in a long bone.

Answer:

• Compact bone forms the hard outer layer of the bone shaft/diaphysis (1 mark)
• It is dense and solid, providing strength and protection (1 mark)
• Spongy bone is found at the ends of the bone/epiphyses with a honeycomb structure (1 mark)
• It is lighter than compact bone, reduces overall weight, and contains red bone marrow for blood cell production (1 mark)

Mark scheme notes: Must distinguish between structure AND function for both types of bone. Location can substitute for structural description.

Example 3: Extended response on bone functions (8 marks)

Explain how the structure of the human skeleton enables it to carry out FOUR of its functions.

Answer: Support: The vertebral column consists of 33 strong vertebrae stacked on top of each other, providing a rigid central support for the trunk and allowing us to stand upright (2 marks).

Protection: Flat bones like those of the skull are hard and curved, forming a protective case around the brain. Similarly, the rib cage surrounds the heart and lungs (2 marks).

Movement: Long bones such as the femur and humerus act as rigid levers. Muscles attach to these bones via tendons, and when muscles contract, they pull on the bones to produce movement at joints (2 marks).

Blood cell production: Spongy bone in flat bones (ribs, sternum) and at the ends of long bones contains red bone marrow which produces red blood cells, white blood cells and platelets (2 marks).

Mark scheme notes: Each function requires structural reference and clear explanation of how structure relates to function. Award 2 marks per function when both elements present.

Common mistakes and how to avoid them

• Confusing compact and spongy bone locations: Remember compact bone forms the outer layer of the diaphysis (shaft), while spongy bone is found in the epiphyses (ends) and lining the medullary cavity. Don't reverse these locations.

• Mixing up axial and appendicular skeleton components: The vertebral column, ribs and skull are axial (central); limbs and girdles are appendicular (attached). The pelvis connects them but is considered appendicular.

• Stating functions without examples: When asked to "describe" or "explain" functions, always provide specific examples. Don't just write "protection" — specify which bones protect which organs.

• Forgetting that bone is living tissue: Many students describe bone as dead or inactive. Emphasize that bones contain living cells, blood vessels, and nerves; they grow, repair, and respond to stress.

• Incomplete answers on bone structure: When describing long bone structure, include both external features (periosteum, articular cartilage) and internal components (compact bone, spongy bone, medullary cavity, bone marrow). Diagrams often show cross-sections.

• Confusing yellow and red bone marrow functions: Red marrow produces blood cells; yellow marrow stores fat. Red marrow is found in spongy bone; yellow marrow fills the medullary cavity in adults.

Exam technique for "The human skeleton: structure, types of bone and functions"

• Command word awareness: "State" requires simple facts (1 mark each); "Describe" needs characteristics or features (2+ marks); "Explain" demands reasoning with "because" or "so that" (higher marks). For skeleton questions, "Identify" means name the bone from a diagram.

• Diagram questions: Practice labelling skeletal diagrams accurately. Use a ruler for label lines, ensure they touch the correct structure, and write clearly. Major bones appear frequently: skull, vertebral column, ribs, sternum, humerus, radius, ulna, femur, tibia, fibula.

• Structure your extended answers: For 6-8 mark questions on functions, write one clear paragraph per function. Always link structure to function using connecting phrases like "this enables" or "which allows." Four well-developed points score better than six superficial ones.

• Use correct anatomical terminology: Write "vertebral column" not "backbone"; "cranium" not "head bone"; "clavicle" not "collar bone" (unless in brackets). Examiners expect precise scientific terminology at CSEC level. Underline or bold key terms if instructed.

Quick revision summary

The human skeleton comprises 206 bones divided into axial (skull, vertebral column, ribs, sternum) and appendicular (limbs and girdles) sections. Bones are classified by shape: long, short, flat, irregular, and sesamoid. Long bones have a shaft (diaphysis) with compact bone surrounding a medullary cavity, and ends (epiphyses) containing spongy bone. The five functions are support, protection, movement, blood cell production, and mineral storage. Bones are living organs composed of organic collagen and inorganic calcium salts, making them strong yet flexible. Growth occurs at epiphyseal plates until early adulthood.