Ca Balance and Reproduction

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Ca Balance and Reproduction

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University of Southern California
Bisc 307l - General Physiology
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BISC 307L 2nd Edition Lecture 20 Current Lecture Calcium in the Body Roles: As Ca2+: ~ 1% • Intracellular signal for enzymes, contraction, exocytosis, etc. • Necessary for adhesion at tight junctions • Cofactor for blood coagulation • Affects neuronal excitability (hypocalcemia  hyperexcitability) (side note: hypercalcemia leads to hypoexcitability.) All those functions of Ca2+ really only amount to 1% of the calcium in the body. Most of the rest of it is in bones. Not as free Ca2+ ions, but as a type of Ca2+ phosphate called hydroxyapatite, which forms the mineral portion of bones. As calcium phosphate (hydroxyapatite): ~ 99% • forms the mineral portion of bones So calcium exists in the body not only as the most important structural component of bone, but as a storage pool where excess Ca2+ can be deposited or liberated to maintain the reservoir of Ca2+ in the body at appropriate levels. Hormones that Regulate Body Calcium The hormones that regulate plasma Ca2+ come from 2 main places – the thyroid and the parathyroid . Pictured to the right is a thyroid gland, seen from the back. Calcitonin is synthesized by the C cells in between the thyroid follicles. And embedded within the thyroid gland are the PT glands(pink nodules). There are 3 main hormones that regulate plasma Ca2+: 1. Parathyroid hormone released from the PT glands, embedded in the posterior surface of the thyroid gland(pink nodules). PT hormone is essential for life – it raises plasma calcium. 2. The second hormone has 3 names – calcitriol, vitamin D3, or 1,25 dihydroxy- cholecalciferol. It is a steroid hormone made from vitamin D. Vitamin D can be made in the skin or can be taken in through diet. 3. The liver cells take that circulating vitamin D and change it into an intermediate, which is processed in the kidney. PTH will stimulate the conversion of the intermediate into calcitriol, which also raises plasma calcium. 4. Calcitonin – secreted by C cells in the thyroid gland(in between the thyroid follicles in the thyroid gland) Ca 2+ Balance: The small intestine is the only source of dietary Ca2+, and is the source of new Ca2+ being added to the body (new Ca2+ comes from absorption across the wall of the SI). Dietary Ca2+ in the SI has two fates: stay and be passed onto the larger intestine to be excreted out in feces, or be absorbed into the ECF. The ECF is the box in the middle, and it shows the normal level of free Ca2+ concentration. And the circle at the bottom is the intracellular fluid in cells where the free calcium is much less (a thousandth or lower relative to the ECF) – we’ve talked about how active transport creates a strong inward gradient for Ca to enter cells(active transport pumps it out). Bone is a very important reservoir of Ca2+ - Ca2+ can be taken from the ECF and deposited into the bones – shown in the upper left corner of the ...

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