Bone Development, Bone Matrix, and Blood Calcium Homeostasis Help (page 2)
Blood Calcium Homeostasis
Besides hematopoiesis and protection from physical trauma, another critical function of the endoskeleton is blood calcium homeostasis ; that is, the maintenance of a relatively constant blood calcium ion concentration. Symbolically speaking, we use Ca ++ to identify blood calcium ions , and brackets, [ ], to denote concentration. Thus, we have [Ca ++ ] to indicate the blood calcium ion concentration.
The blood calcium ion concentration, [Ca ++ ], within humans, is usually measured in units of mg/dL – milligrams ( MIH -lih- grams ) of calcium ions per deciliter ( DEH -sih- lee -ter) – of blood. A deciliter is one-“tenth” ( deci -) of a “liter.” And milligrams is a unit representing the number of “thousandths” ( milli -) of a “gram” of some substance. Hence, blood [Ca ++ ] in mg/dL denotes the number of milligrams of calcium ions present within one-tenth of a liter of blood. The normal or reference range for blood [Ca ++ ] is from a low of about 8.5 to a high of approximately 10.6 mg Ca ++ /dL of blood (in adults). Taking the same approach we employed for thermoregulation (Figure 13.2), we can use the S-shaped pattern, once again, to represent the homeostasis of blood calcium ion concentration, over time (see Figure 13.6). In naming this particular pattern of chemical concentration, we use the suffix, - emia (“blood condition of”), and the root or main idea, calc (“calcium”). We therefore have some form of calcemia (kal- SEE -me-uh), or “condition of calcium” (ion concentration) within the “blood.”
Bone Development and Bone Matrix
“How is normocalcemia related to bone development and bone matrix?” you might well ask at this time. The answer is provided by a close look at Figure 13.7. During the development of a long bone, such as the femur (“thigh” bone), the process essentially begins with a cartilage model – a miniature version of the bone that is composed of cartilage, rather than bone tissue. Being soft and rubbery, cartilage is more suitable for life within the mother’s uterus (womb). As development progresses, however, blood vessels break into the cartilage model and bring osteoblasts ( AHS -tee-oh- blasts ) along with them. Osteoblasts are literally “bone” (oste) “formers” (-blasts).
The osteoblasts are large, spider-shaped cells that produce bone collagen fibers. After these tough collagen fibers are laid down, the osteoblasts then extract Ca ++ ions, phosphorus, and other chemicals from the bloodstream. Ossification ( ah -sih-fih- KAY -shun), the “process of bone formation,” then begins. During ossification, the osteoblasts supervise the depositing of sharp, needle-shaped crystals onto the surfaces of the bone collagen fibers. These sharp crystals are composed of calcium phosphate ( FAHS -fate), as well as a number of other minerals. Essentially, bone matrix appears within the cartilage model of the long bone, because the newly produced bone collagen fibers become heavily coated with the calcium phosphate crystals. Being snow-white, these crystals eventually hide the underlying collagen fibers. Viewed with the naked eye, the entire bone matrix thus appears snow-white. The bone matrix is white, like cement, of course, because white is the color of the many thousands of calcium crystals covering the collagen fibers in the matrix.
By the time the child becomes an adult, her femur is mostly snow-white. You might think of all the bone matrix in the femur as essentially being a storage bank for calcium ions.
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.
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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