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Endocrine system anatomy and physiology
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Vitamin D and p. 353
vitamin D and p. 336
functions p. 68
hypervitaminosis lab values p. 474
osteoporosis prophylaxis p. 472
PTH and p. 338
signaling pathways for p. 353
solubility of p. 63
calcitriol production p. 613
The majority of the extracellular calcium, the calcium in the blood and interstitium, is split almost equally into calcium that’s diffusible and calcium that’s not diffusible.
Diffusible calcium is small enough to diffuse across cell membranes and there are two subcategories.
The first is free-ionized calcium, which is involved in all sorts of cellular processes like neuronal action potentials, contraction of skeletal, smooth, and cardiac muscle, hormone secretion, and blood coagulation, all of which are tightly regulated by enzymes and hormones.
The second category is complexed calcium, which is where the positively charged calcium is ionically linked to tiny negatively charged molecules like oxalate and phosphate, which are small anions, that are found in our blood.
The complexed calcium forms a molecule that’s electrically neutral but unlike free-ionized calcium it’s not useful for cellular processes.
Finally there’s the non-diffusible calcium which is bound to large negatively charged proteins like albumin.
The resulting protein-calcium complex is too large and charged to cross membranes, so the non-diffusible calcium is also uninvolved in cellular processes.
Either vitamin D2, or ergocalciferol, which comes from plant sources in our diet, and vitamin D3, or cholecalciferol, which can either come from animal products in our diet, but can also be made in skin cells that are exposed to sunlight.
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