Calcium Basics

Question 1

calcium - overview & functions

Answer 1

*functions: bone formation, muscle contraction, blood clotting
*main source of calcium = diet (milk, cheese, veggies, etc)
*intake balanced through feces & urine loss
*stored primarily in bone, some in ICF, some in ECF

Question 2

calcium states

Answer 2

*calcium is stored in bones, intracellular fluid, and extracellular fluid
*calcium in the ECF (aka serum calcium) is what carries out body functions
*serum calcium exists in 3 forms:
1. free calcium (ACTIVE form; about 50%) → ionized serum calcium
2. bound to proteins (esp. albumin) = ~45% (NOT active)
3. bound to anions ~5%

Question 3

effects of albumin of serum calcium

Answer 3

*normal albumin state (4g/dL) = normal total calcium, normal free (ionized) calcium
*low albumin state = normal total calcium, high free calcium
*high albumin state = normal total calcium, low free calcium

Question 4

corrected calcium equation & explanation (based on albumin)

Answer 4

*remember that free (aka ionized) serum calcium is what actually carries out the functions of calcium
*either too much or too little free calcium causes problems
*whenever a pt’s albumin is higher or lower than 4, we correct the calcium to make sure we have an accurate assessment of the calcium levels

equation: corrected Ca = (0.8 x [4.0 - patient’s albumin] + serum Ca level)

Question 5

corrected calcium equation

Answer 5

corrected Ca = (0.8 x [4.0 - patient’s albumin] + serum Ca level)

Question 6

effects of pH on calcium

Answer 6

*in acidemia, extra hydrogen ions bind to negatively charged albumin binding sites; albumin binds less positively charged Ca2+ → higher free (ionized) calcium
*in alkalemia, fewer hydrogen ions to bind negatively charged albumin binding sites; albumin binds more positively charged Ca2+ → lower free (ionized) calcium
*therefore, in acidotic/alkalotic states, check an ionized (free) calcium level

Question 7

phosphorous - overview

Answer 7

*functions: muscle contraction, cell metabolism (the “P” in ATP), oxygen release from RBCs
*stored primarily in bone, some in ICF, tiny bit in ECF
*NOT protein bound (no need to calculate a correction for albumin levels)
*dietary source
*intake balanced by urine loss alone

Question 8

hormones that play a role in calcium regulation

Answer 8

1. PTH (parathyroid hormone) → results in increased serum Ca2+
2. calcitriol (Vitamin D) → results in increased serum Ca2+
3. calcitonin → “tones down” serum calcium (decreases Ca2+)

Question 9

parathyroid glands - function

Answer 9

*trigger: LOW free/ionized Ca2+
*mechanism: secrete PTH → increased bone resorption to release Ca2+ and increased Ca2+ reabsorption from urine
*effect: INCREASE SERUM CALCIUM and decreased serum phosphorous

Question 10

parathyroid hormone - effects

Answer 10

1. direct effect on bone: stimulates osteoclasts → increased bone resorption to release Ca2+; also releases phosphorous
2. direct effect on kidney: increased calcium reabsorption from urine; promotes phosphate excretion in urine (phosphate-trashing hormone)
3. indirect effect on GI tract: increased Ca2+ absorption from gut but increasing calcitriol formation
4. stimulates conversion of 25-OH(D) to 1,25(OH)2D by the kidneys → increased calcitriol

recall: PTH is released in response to low serum calcium, and it results in increased serum calcium and decreased serum phosphorous

Question 11

calcitriol / vitamin D - overview

Answer 11

*calcitriol (1,25-dihydroxyvitamin D) is the metabolically active form of vitamin D
*liver stores vitamin D or converts it to 25-hydroxyvitamin D
KIDNEYS convert 25-hydroxyvitamin D to calcitriol (1,25-dihydroxyvitamin D) via 1-alpha-hydroxylase; *PTH stimulates this conversion

Question 12

conversion to calcitriol by the kidneys

Answer 12

*KIDNEYS convert 25-hydroxyvitamin D to calcitriol (1,25-dihydroxyvitamin D) via 1-alpha-hydroxylase
*PTH stimulates this conversion

Question 13

calcitriol - effects

Answer 13

1. small intestine (most critical effect): increased Ca2+ and phosphorous absorption
2. bone: increases bone osteoclast activity to release Ca2+ and phosphorous
3. kidneys: increases Ca2+ and phosphorous reabsorption in proximal tubules

overall: increases serum calcium and increases serum phosphorous

Question 14

mechanism: low vitamin D → secondary hyperparathyroidism

Answer 14

*low vitamin D = low calcitriol = low calcium and low phosphorous due to decreased gut reabsorption
*body compensates by increasing PTH → urinary excretion of phosphorus
*result: secondary hyperparathyroidism (high PTH, normal/low calcium, low phosphorous)

Question 15

calcitonin - effects

Answer 15

*calcitonin TONES DOWN calcium (results in decreased serum calcium)
*released from parafollicular cells of the thyroid (aka C-cells)
*opposes PTH resorptive effects on bone osteoclasts → PROMOTES BONE FORMATION
*inhibits renal reabsorption of calcium