Human Skeletal System

Types of Skeletons

  • Hydrostatic 'skeleton' - This is a 'skeleton' where an animal such as a jellyfish is filled with and supported by fluid.
  • Exoskeleton - An exoskeleton or external skeleton exists on the outside of the animal (e.g. the 'shell' of a crab).
  • Endoskeleton - An endoskeleton or internal skeleton is the backbone of an animal. This backbone can be composed of bone (e.g. human) or of more flexible cartilage (e.g. shark).

Functions of Skeletons

  • Provide support for body organs
  • Provide protection for vital organs such as the brain, heart and lungs
  • Provide a place of attachment of muscles for movement
  • Produce red blood cells and some white blood cells

Structure of Bone

  • The human body has about 206 bones. Some are long bones (e.g. thighbone) and others are smaller (e.g. earbones).
  • A long bone such as the thighbone (femur) is composed of outer compact bone and inner softer spongy bone.
  • There are living bone cells in bone.
  • Inside the hollow cavity of long bones, there is bone marrow. It is a jelly-like substance of two types - red marrow which produces red blood cells and some white blood cells, and yellow marrow which stores minerals and some fat.
  • Most bones have a slight twist which allows them to withstand some strong forces such as those produced when running on hard surfaces.

Names of Some Common Bones



skull cranium
jawbone mandible
collarbone clavicle
shoulder blade scapula
breast bone sternum
backbone or spine vertebrae or vertebral column
tail bone coccyx
upper arm bone humerus
lower arm bone (thumb side) radius
lower arm bone (little finger side) ulna
thigh bone femur
kneecap patella
lower leg bone (inside) tibia
lower leg bone (outside) fibula

Structure of a Joint

  • A joint is where two or more bones meet.
  • Joints act as pivots for the bones which lever about a pivot point.
  • The ends of bones are covered with softer and more elastic cartilage to buffer the force of movement.
  • Bones are held in place by ligaments.
  • Some joints (e.g. knee and elbow) have synovial fluid enclosed in a capsule to help buffer the forces of constant movement.

Types of Joints




Ball-and-socket joint Shoulder, hip Wide range of movement, easily dislocated
Hinge joint Elbow Movement in one direction only
Gliding joint Foot Many bones are held together by ligaments and glide across each other
Pivot joint Head and neck Rotation and up-and-down movement
Condylar joint Knee Movement in one direction, but with slight rotation
Saddle joint Thumb Movement in two directions
Ellipsoidal joint Wrist Movement is up-and-down, and side-to-side without rotation

Tendons and Ligaments

  • Ligaments attach bone to bone.
  • Tendons attach muscle to bone.

Human Muscular System

Types of Muscles

  • Voluntary or Skeletal Muscles - Muscles attached to the skeleton allowing body movement are under conscious control by the brain.
  • Involuntary Muscles - These muscles are not under conscious control. Examples are heart muscle, stomach muscles and muscles of the walls of blood vessels.

How Muscles Work

  • Skeletal muscles are made of many smaller muscle fibres that slide across each other. When a muscle is relaxed, the muscle appears long and thin. When a muscle is contracted, the muscle appears short and thick.
  • Many muscles work together in antagonistic pairs. For example, to lift your forearm, the biceps muscle at the front of your upper arm contracts to lift the forearm. At the same time, the triceps muscle at the back of your upper arm relaxes. The opposite occurs when you lower your arm (triceps contracts and biceps relaxes).
  • Flexor - The flexor is the muscle in each pair which contracts to bend the joint (e.g. biceps).
  • Extensor - The extensor is the muscle in each pair which contracts to straighten the joint (triceps).

Sources of Muscle Energy

  • The energy that muscles need for contraction comes from the breakdown of glucose in the process called Respiration. There are 2 types of respiration.
  • Aerobic Respiration - This occurs when the body is exercising slowly, and where the muscles are receiving a continuous flow of oxygen and glucose to the muscles, and of carbon dioxide and water away from the muscles. This form of respiration gives the body the most amount of energy in the form of lots of energy-rich molecules called ATP.
  • Anaerobic Respiration - This form of respiration occurs when the body does rapid strenuous activity. Very little oxygen is available to the muscles for a short time, and they make small amounts of energy-rich ATP, and large amounts of lactic acid which tires the muscles.

Main Elements of Fitness

  • Strength - Muscular force is needed for lifting and carrying heavy objects.
  • Flexibility - Bones and muscles must be able to move freely.
  • Endurance - To work muscles for a long time is of benefit to fitness.

Ways that Regular Exercise Helps the Body

  • Strengthens the heart and lowers blood pressure
  • Improves muscle tone by keeping the muscles slightly contracted
  • Strengthens the blood vessel walls to allow them to withstand greater blood pressure after exertion
  • Increases oxygen flow to all body cells by building new capillaries, and getting rid of wastes such as carbon dioxide and lactic acid after exercise