Osteoporosis - Calcium, Vitamin D and bone density


Calcium, Vitamin D and bone density


  • 99% in bone, 1% in blood
  • Obtained through diet (mainly dairy products), absorption controlled by vitamin D.

Calcium Regulation

  • Controlled by Parathyroid hormone, secreted from the parathyroid gland.
  • An increase in PTH, will cause an increase in calcium levels in the blood. This occurs my two main mechanisms:
    • Decreasing calcium secretion by the kidneys - This results in a increased secretion of phosphate ions
    • Increasing calcium absorption from bone, by action on osteocytes
Found on the surface of bone. Constantly active, and depositing new bone tissue.
Produce osteoid – which is a collagen based material, which makes up the raw material of bone. These cells also help mineralise the bone to give it its strength.normal bone is about 30% osteoid, and 70% mineral salts (mainly calcium and phosphate).  There is a reduction in number of osteoblasts with age
Found within the bone matrix. Constantly active and reabsorbing bone tissue
Remove bone tissue, and allow bone remodelling. Usually, at any one time, 1% of bone is being reabsorbed by osteoclasts. When when the bone is reabsorbed, the products of the reabsorption do not go straight into the ECF, instead they are trapped behind the osteocyctic membrane system.
Essentially an osteoblast that has become trapped in the bone matrix. Found throughout bone, but most importantly, help form the osteocytic membrane system. This is a thin membrane that covers the bone, and separates bone from extracellular fluid.
Through the osteocytic membrane system, helps to regulate the concentration of solutes in the fluid around bone. PTH acts on these cells to increase the amount of calcium pumpedout of the bone fluid system.

Normal bone formation

  1. Osteoid is made by osteoblasts. Consists of 95% collagen, and hyaluronic acid, and peptidoglycans. Collagen fibres tend to run lengthways in bone. Osteoid is very similar to cartilage, however, its structure allows salts to rapidly precipitate within it.
  2. Mineralisation – the main salt that forms contain both calcium and phosphate, and is called hydroxyapatite. Initially the crystals that form are not hydrozyapatitie, but over weeks and months, they will gradually be converted to this form. The precursor forms are knwon as amourphous crystals. These crystals can be rapidly reabsorbed if calcium is needed in the circulation.
    1. Calcium and phosphate will naturally form crystals once their concentrations are over a certain level. In normal ECF and other fluids, there are inhibitors of crystal formation that prevent calcium and phosphate forming crystals, despite their high concentration. In the fluid around bone behind the oesteocytic membrane system, these inhibitors are not present.

Normal Bone reabsoprtion

This is constantly occurring, and happens on about 1% of bone at any one time. Osteocytes have a ruffled border which is in direct contact with the bone matrix. They secrete:
  • Enzymes – which break down the osteoid
  • Acid – including lactic acid, which breaks down the mineral crystals
Osteoclasts also phagocytose fragments of bone crystal to help break them down, before releasing the products.
Ongoing process
Normally (except when we are growing) bone deposition and bone absorption occur at the same rate. Osetoclasts will absorb bone in little cylindrical ‘tunnels’ a few mm in length. This process takes about 3 weeks. Osteoblasts will the deposit bone matrix in concentric layers around the central lamina, until they encroach on a blood vessel, that is often at the centre of one of these holes. The areas around the blood vessel is known as the Haversian canal. The new area of bone that has been deposited is known as an osteon, and this process takes several months.

Why does bone remodelling occur?

  • It allows the alteration of bone strength in relation to the weight bearing the bone has to perform
  • It allows reshaping of the bone in relation to stress patterns
  • Old organic material in bone degenerates, making old bone weaker than new bone
    • In children, rates of remodelling are fast – and they have strong bones
    • In the elderly, rates are much slower, and bones are more brittle


At the site of a fractureosteoprogenitor cells will rapidly differentiate to form large numbers of osteoblasts, to aid the rapid reformation of bone. An organic matrix between the two bone ends will quickly be formed. This is known as a callus. This will then gradually be mineralised over the following weeks and months.
Stress on a bone site increases the rate of new bone formation. Surgeons can manipulate this effect, and in some fractures, the bones are held together strongly to create the impression of stress, so the  $(function(){ var pertama = $('#herb-dict .herblist li:first-child').attr('class'); $('#herb-dict .herblist li.'+pertama).show(); $('#herb-dict .alphlist li#'+pertama+' a').addClass('active'); $('#herb-dict .alphlist li a').click(function(){ var target = $(this).attr('rel') $('#herb-dict .herblist li').hide(); $('#herb-dict .herblist li.'+target).show(); $('#herb-dict .alphlist li a').removeClass('active'); $(this).addClass('active'); }); });