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Physiology

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Basic Concepts

Question
58 out of 95
 

True about calcium



A Intracellular anion
B Binds to proteins

C Depresses myocardium
D None

Ans. B Binds to proteins

Calcium metabolism

  1. The body of a young adult human contain about 1100 gm calcium (=27.5 mol. of calcium)

i. 99% of calcium is present in bones.

ii. 0.1 % of calcium present in ECF

iii. 0.5 to 1% of skeletal calcium is freely exchangeable with that in the ECF.

iv. Plasma calcium level- 9-11 mg/dL : of which

50% ionized (diffusible)

Calcium complexed to anions ( with phosphate, citrate, HC03 - Diffusible) Q

Protein bound calcium - 41 % (non diffusible) with - Albumin (major) - Globulin

It is free, ionized calcium (Ca++, divalent cation) in the body fluid that is vital second messenger and is necessary for blood coagulation, muscle contraction (all types) and nerve function.

  1. Absorption and Excretion:

i. Intestine: the feedback controlled hormonal regulation of intestinal absorptive efficiency result in a relatively constant daily net calcium absorption of approximately 200 to 400 mg/day despite large changes in daily dietary calcium intake.

Daily intake 400 - 1500 mg/day (Av. = 1000 mg/day)

On average 350 mg - absorbed

    • 250 mg/day calcium enters the intestine via secretions and sloughed mucosal cell lost in faces

    Total ingestion = 1000 mg/day

    Total loss = 650 + 250 = 900 mg/day

      • Therefore 90% of daily intake of calcium is excreted in faces. Q
      • Site calcium is absorbed in upper small intestine (Duodenum and proximal jejunum) Q

      Mechanism of intestinal absorption:

      o Active (Transcellular) Mechanism Main mechanism controlled principally by Vit D3

      o Passive (paracellular) mechanism: accounts for 5%

      o Optimal rates of calcium absorption require gastric acid.

      o High calcium intake Serum Ca++ →↑ synthesis of Vit D3→↑ intestinal absorption and vice versa.

      ii. Renal regulation of calcium metabolism

      Approximately 10% (100 mg/day) of ingested calcium is excreted in urine Q

      99% of filtered calcium is reabsorbed

      In PCT 65% reabsorption via passive, paracellular route, coupled to con­ comitant Nacl reabsorption, not specifically regulated.

          • In the cortical thick ascending limb of Henle's Loop (cTAL) 20% reabsorption via passive paracellular mechanism it requires paracellin-l for reabsorption of calcium and is inhibited by plasma Ca++ and Mg++, acting via CaSR.
          • In DCT 10% reabsorbed -7 regulated by PTH (Transcellular active mechanism)

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