Physiology - Skeletal System

The branches of science that will help you understand the body parts and functions are anatomy and physiology. Anatomy deals with the study of the human body (the component parts, structure and position) and physiology the study of how the body functions.

Body Systems

Select the appropriate body system link, in the table below, to view the available information.

Skeletal	Muscular	Neurological	Endocrine
Cardiovascular	Respiratory	Digestive

Skeletal System

The skeletal system provides four basic functions:

Support for tissues and muscle
Protection for vital organs
Movement through bones and attached muscles
Storage for minerals and immature blood cells

Growth

Ossification is the process by which bone is formed. Some bones (e.g. the flat bones of the skull) are formed in one stage from the connective tissue. This process is known as intramembranous or direct ossification.

Other bones (e.g. short bones) are formed from the cartilaginous model of the future bone developed in the embryo, being dissolved and replaced by bone cells. This process is known as endochondral or indirect ossification - most bones are formed this way.

Support

Bones and cartilage that make up the skeleton are the only rigid materials in the body. The 206 bones of the skeleton provide a framework and points of attachment for many of the soft tissues of the body. The five main classifications of bones are : Long (e.g. femur), Short (e.g. tarsal bones of the foot), Flat (e.g. frontal bone of the skull), Irregular (e.g. vertebrae) and Sesamoid (e.g. knee cap)

Cross Section of a Long Bone

Protection

These structures protect some of the vital tissues and functional organs of the body. Typical examples are:

Skull - protects the brain
Vertebrae - protects the spinal cord
Thoracic cage - protects the heart and lungs

Movement

Bones act as levers during movement and provide solid structures to which muscles are attached. The joints allow movement between bones and these movements are directly related to the type of joint and range of motion. Joints fall into one of three categories: Fixed fibrous or Synarthroses (e.g. bones of the skull), Slightly moveable or Amphiarthroses (e.g. symphysis pubis) and Freely movable or Diarthroses.

Freely Movable joints comprises of four main groups: Ball and Socket (e.g. hip), Hinge (e.g. elbow), Pivot (e.g. radius and ulna) and Gliding (e.g. carpal joint of the wrist)

Component parts of a synovial joint

A fibrous capsule surrounds the joint and is strengthened by ligaments. The stability of these joints is dictated by the shape of articulating surfaces, their surrounding ligaments and muscles. For example, the knee is given great strength from 2 cruciate and 2 collateral ligaments. Whilst one of the hardest joints to dislocate is the hip. It is formed with the head of femur fitting neatly into the socket or acetabulum in the pelvis.

Articular or hyaline cartilage covers and protects the ends of bones which meet to form a joint and therefore allows freedom of movement. It is a very hard, smooth material which does not repair itself when damaged.

Tendons connect muscle tissue to bone and although more elastic than ligaments, have a far greater tensile strength than muscle.

Synovial membrane lines the joint cavity and covers the tendons and ligaments which pass through it. The membrane produces synovial fluid which lubricates the joints

Shoulder Joint

Ligaments are tough fibrous bands of tissue which connect bone to bone and help stabilise a joint, the strongest ligament in the body being situated at the front of the hip capsule, preventing excessive backward movement of the legs. Ligaments, although stronger than muscle tissue, have fewer nerve endings and less blood supply , and therefore take longer to repair when damaged. Whilst these strong fibrous bands offer great stability to a joint in preventing excessive movement, if they are stretched or torn through injury, they do not necessarily return to their former length and therefore may remain stretched, therefore offering reduced stability to that particular joint.

A bursa is a small sac formed in connective tissue lined by a synovial membrane and containing a small amount of synovial fluid. It is situated between moving parts, often between tendon and bone, to prevent rubbing.

Storage

In some bones, there is red marrow which produces red blood cells, some white cells and platelets. Minerals, especially calcium and phosphorous are also stored in bones and can be distributed to other parts of the body.

Effect of exercise on the skeletal system

The condition of bone may be improved by exercise as it responds to mechanical stresses. These mechanical stresses usually take the form of skeletal muscle pulling at their points of attachment being their origins and insertions. Where these mechanical stresses are applied, most it has been shown that more mineral salts are deposited and more collagenous fibres are produced. Therefore, both the density and size of bone in these areas may be increased and these changes in bone structure are stimulated by increased loads being placed on the skeleton. This has been borne out by greater bone mass being observed in weight lifters than in other lighter endurance athletes such as joggers. Other examples include racquet players who have been shown to have greater bone density in their playing arms. It has even been shown that if a leg is immobilised by being placed in plaster, due to a fracture, that even after a few weeks the bone becomes decalcified from lack of mechanical stress.

Whilst it may therefore be considered beneficial to utilise exercise to maintain healthy bones, great care must be taken with children whose bones and muscles are still developing. They should not be subjected to forms of sport involving high degrees of mechanical stress, partly because of the weaknesses that still exist within the bones, and also because of adverse effects on the development of these bones before maturity.

There are two main effects on bones as we grow older. Bones begin to lose calcium and this is one of the factors contributing to the condition called osteoporosis. Secondly, with age less protein is produced which alters the make-up of bone and sometimes creates brittle bones.

Associated Pages

The following Sports Coach pages should be read in conjunction with this page:

Associated Books

The following books provide more information related to this topic:

Principles of Anatomy and Physiology, G.J. Tortora et al., ISBN 0 06 046704 5
Strength Training Anatomy, F. Delavier, ISBN 0 7360 4185 0
Atlas of Skeletal Muscles, R.J. Stone et al., ISBN 0 697 13790 2
The Muscle Book, P. Blakey, ISBN 1 873017 00 6
Advanced Studies in Physical Education and Sport, P Beashel et al., ISBN 0 17 4482345
Physical Education and the Study of Sport, B. Davis et al., ISBN 0 7234 31752
Essentials of Exercise Physiology, W.D. McArdle et al., ISBN 0 683 30507 7
Physical Education and Sport Studies, D. Roscoe et al., ISBN 1 901424 20 0
The World of Sport Examined, P. Beashel et al., ISBN 0 17 438719 9
Advanced PE for Edexcel, F. Galligan et al., ISBN 0 435 50643 9
Examining Physical Education, K. Bizley, ISBN 0 435 50660 9
Sport and PE, K Wesson et al., ISBN 0 340 683821
PE for you, J. Honeybourne, ISBN 0 7487 3277 2