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Bone Development, Bone Matrix, and Blood Calcium Homeostasis Help (page 2)

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By — McGraw-Hill Professional
Updated on Aug 30, 2011

Bone Remodeling And Maintenance Of Blood Calcium Homeostasis

Just as osteoblasts are “bone-makers,” cells called osteoclasts ( AHS -tee-oh- klasts ) are “bone-breakers” (-clasts). By bone-“breakers,” of course, we don’t literally mean that these osteoclast cells actually break or fracture the bone! Rather, we mean that the osteoclasts release special digestive enzymes that cause a partial resorption (rih- SORP -shun) or “drinking-in again” of the bone matrix, such that some of the calcium phosphate crystals are dissolved back into calcium and phosphate ions. Such a process often occurs whenever the blood [Ca ++ ] falls toward the lower limit of its normal range (Figure 13.8, A). And as a result of this resorption (dissolving or “drinking-in again”) of bone matrix, Ca ++ ions that were formerly stored in the bone matrix, like a bank, are now released back into the blood circulation. This process, quite clearly, acts to temporarily raise the blood [Ca ++ ]. Because the bone gets thinner and weaker, we say it has remodeled – changed its thickness and strength.

Skins and Skeletons Bone development and bone matrix Bone Remodeling And Maintenance Of Blood Calcium Homeostasis

Skins and Skeletons Bone development and bone matrix Bone Remodeling And Maintenance Of Blood Calcium Homeostasis

Fig. 13.8 Bone-makers, bone-breakers, and blood calcium homeostasis.

Conversely, whenever a person eats a calcium-rich meal, the blood [Ca ++ ] rises toward the upper limit of its normal range (Figure 13.8, B). Thousands of osteoblasts are stimulated, then more Ca ++ ions are extracted from the bloodstream and laid down on the bone collagen fibers as calcium phosphate crystals. The bone once again remodels (changes its shape and size), but this time it goes in the opposite direction, by getting thicker and stronger. And as more and more free Ca ++ ions are extracted from the bloodstream and put into calcium phosphate crystals, the blood [Ca ++ ] falls.

By these contrasting processes involving osteoblasts, osteoclasts, bone remodeling, and bone resorption, therefore, homeostasis of blood [Ca ++ ] is usually maintained. Maintaining a normal blood [Ca ++ ] is critical for human health. Why? One reason is that the contractions of all our body muscles (including those of the heart) require an adequate level of calcium ions within the bloodstream.

 

Practice problems for these concepts can be found at:  Skins And Skeletons Test

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