mildus wrote:
I know for 3 roles of PTH:
1. in kidney:
a. increased reabsorption of Ca
b. decreased reabsorption of phosphates (why? you know that Ca and phosphates make insoluble Ca-phosphate, so we want to get rid of phoshates in order to have our Ca free)
2. in bones:
increases bone resorption (so increases both Ca and phosphates in plasma)
3. in intestines:
indirect effect: increases absorption of Ca and phosphates (by hormone D3 which is activated by PTH)
Now, you asked about HCO3. I am not sure, but I think this is the reason:
You know that extracellular calcium is found in 3 forms.
1. diffusible (filtrable)
a. ion Ca (free) about 40 %
b. Ca in complex with anions like citrate, phosphate... about 10 %
2. undiffusible (not filtrable)
Ca attached to proteins
Calcium which is attached to proteins serves as a reservoir, so when we need more free calcium in plasma, it can be liberated. How? Recall that proteins and amino acids have both positive and negative groups. Calcium is a positive ion so it binds to negative groups of proteins (COO- group). When you have acidosis (too much H+), H+ as positive ion will also bind to COO- of proteins so there will be less groups to bind with calcium. So calcium will be liberated from proteins and there will be more free calcium in plasma. I want to say that H+ substituted calcium in proteins. I hope that we now understood how increased H+ increases plasma level of calcium. And now, what about bicarbonate ions? When there are less HCO3 (due to decreased reabsorption), what will happen to H+? You know for the formula H+ + HCO3- = H2CO3 (HCO3 buffers H+ ions so decreased HCO3 will increase H+).
Hi, Mildus, you are correct, but I think Calcium is 40% combined with the plasma proteins, 10% is combined with anionic substances of the plasma and interstitial fluids, and the remaining 50% of the calcium in the plasma is both diffusible through the capillary membrane. and ionized.
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| mildus
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Sorry. You are right.
And do you know the difference in calcium concentration between interstitium and plasma?
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| ramoK.
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I think is no difference because the plasma and the interstitial fluids have a normal calcium ion concentration of about 1.2mmol/L…..talking about ionized form of calcium…Please complete me or tell me if I am wrong...Ramo<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
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| mildus
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Ionized calcium concentration is the same.
And what about calcium combined with proteins? You know that plasma proteins concentration is 60 to 80 g/l, and in interstitium, protein concentration is about 20 g/l. So, what about calcium?
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| KrazyK
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Can someone explain to me the mechanism of tetany in hypocalcemia? I think it has no effect on RMP since membrane is impermeable to Ca at rest. Is there no effect on RMP but only on excitability?
Thanks.
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| kingsofke
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Interstitial fluid Ca 1.5 mM(.1 % of total) mostly ionized and Plasma 2.5 mM(.5 % of total) half ionized (as u know)
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| mildus
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