More information on the bones and joints

The activities of the cells is under the influence of a number of different hormones: calcitonin, from cells situated in the thyroid gland, inhibits the reabsorption of bone and lowers the level of calcium and phosphate in the blood; parathyroid hormone, produced by the parathyroid glands which sit on either side of the thyroid, increases bone reabsorption, and so also increases the level of calcium and phosphate in the blood. Besides adequate dietary intake of calcium and other essential minerals effective bone formation requires the presence and activation of vitamin D. This is one of the fat-soluble vitamins, found in some plants and in liver and fish oils. It has a fairly complicated metabolism, requiring the action of sunlight on the skin as well as passage through both liver and kidney before becoming fully active, after which it works to increase the absorption of calcium and phosphate from the gut and their subsequent deposition on the bony matrix.

The amount of bone we have, our so-called bony mass, depends on a number of factors. It is increased by high dietary intake of calcium in our youth, enhanced by the effects of exercise, since more bone is laid down in regions where the bone is stressed, which it is during exercise. It is decreased by cigarette smoking, by excess alcohol intake, by a deficiency of vitamin D or a lack of sunlight or diseases affecting the liver or kidney, and by excessive bone reabsorption - particularly in women after the menopause. There are congenital disorders affecting bone formation, described in the section covering infancy and childhood.

Bone marrow exists in the central cavity of all bones, but in the adult the red marrow responsible for producing blood cells - a process called haemopoeisis - is only present in the bones of the skull, the vertebrae, the ribs, the breastbone and the upper bones of the arms and legs; elsewhere it is replaced by fat. The marrow contains cells which are called pleni-potential, because they have the potential to divide and produce different cells such as red cells, white cells and platelets - the cells responsible for helping our blood to clot (see blood and circulation in the chest cavity section).

Joints are places where two or more bones meet. The bones are held together by ligaments, consisting of tough white tissue which does not stretch but which is flexible, so that it may limit the direction of movement of the joint - in the joint at the end of the finger for example there is no side to side movement, the shoulder joint, in contrast, lets you move your arm forwards, backwards, outwards and up. Some joints, such as the one at the front of the pubis, allow very little movement - the bones here are separated by a substance called fibro cartilage, which is relatively dense and acts as a sort of shock absorber. We have the same sort of substance in between the vertebrae of the spine, where it's called an intra-vertebral disc. Joints that are freely moveable, like the hip or the knee, are constructed differently. They are enclosed within a capsule made up of ligaments lined by a membrane - called a synovial membrane - which secretes a fluid called synovial fluid which lubricates the movement of the joint. The surface of the bones is covered by a thin layer of more elastic cartilage called hyaline cartilage. Such joints are moved by the action of muscles which are attached to the bones by tendons. These are tough white cord like structures which focus the power of the muscle on to a particular area of the bone where they are bound to the periosteum. Some of these tendons, particularly those which are relatively lengthy such as the ones which move the wrist joint, run through a sheath - a tendon sheath - lined with synovium which produces synovial fluid in order to reduce friction. Over joints where there is also pressure, such as the knee or the elbow, small sacs form filled with synovial fluid. Each of these is called a bursa.