MLAB 2401: Clinical Chemistry
Keri Brophy-Martinez
Overview:
Mineral and Bone Metabolism
Calcium Homeostasis

Organs involved
◦ Small intestine
◦ Skeleton
 Hydroxyapatite
 ( calcium+ phosphorus+ hydroxide)
◦ Kidneys

Origin
◦ Diet

Reservoir
◦ Bone
Forms of Calcium

Bound to albumin–45%
◦ Reversible process
◦ Binding is pH dependent
Free “ionized”–50%
 Complexes with anions -5%

Ionized Calcium
Free calcium in blood
 Tightly regulated
 Biologically active form
 Advantage

◦ Not associated with proteins or anions
Functions of Calcium
Nerve impulse transmission
 Cofactor in certain enzymes
 Coagulation of blood
 Skeletal mineralization
 Preservation of cell membrane integrity
and permeability

Regulation of Calcium
Homeostasis
Parathyroid hormone- PTH
 Vitamin D3
 Calcitonin

Parathyroid Hormone- PTH
Originates from the parathyroid gland
 Secretion based on levels of free calcium


Activities/Roles
◦ Stimulation of osteoclastic bone reabsorption and release of
calcium and phosphate from bone
◦ Stimulation of calcium reabsorption and inhibition of phosphate
reabsorption from the renal tubules
◦ Stimulation of renal production of 1,25(OH)2 vitamin D3, which
increase intestinal absorption of calcium and phosphate
Regulation of Calcium Homeostasis
Vitamin D3/ Cholecalciferol
Synthesis occurs in skin, liver, kidneys
 Affects gut, bone, and parathyroid
 Actions/ Roles
◦ Increases plasma calcium and phosphate
concentration by increasing the absorption of calcium
and phosphate from the gastrointestinal tract
◦ Increases bone resorption and enhances the effects of
PTH in the nephron to promote renal tubular calcium
reabsorption
◦ Stimulates osteoclasts to metabolize bone calcium

Vitamin D: Where Does it Come
From?

Sources of:
◦ Diet
 Animal tissues and products ( liver)
 Irradiated milk
◦ Sunlight
Calcitonin
Secreted by the thyroid gland
 Secreted when blood calcium rises
 Inhibits PTH and Vitamin D
 Inhibits bone resorption, promotes bone
formation

Phosphorous

Origin
◦ Diet
◦ Bone
◦ Phosphate ion is distributed equally.
 Intracellular
 Component of macromolecules
 Extracellular

Inverse relationship with Calcium
Functions of Phosphorous
Component of bones & teeth
 Essential part of cell membranes
 Contributes to enzyme function
 Storage and transfer of energy
 Component of many compounds

Phosphorous Regulation

Kidneys
◦ Excrete or reabsorb

Other factors
◦ PTH
 Increases renal excretion
◦ Vitamin D increases levels
 Phosphate absorption in intestine
 Phosphate reabsorption in kidneys
◦ Growth Hormone, calcitonin, acid-base status
Magnesium
 2nd
most abundant intracellular cation
 Location
◦ 50-60% found in bone
◦ 40-50% muscle & soft tissue
◦ 1% in the RBC

Origin
◦ Diet
 Nuts, hard water, meat, green vegetables, fish, dry
cereal
Forms of Magnesium
Free or ionized (~55%)
 Bound to proteins (~30%)
 Complexed with phosphate, citrate and
other ions (~15%)

Functions of Magnesium

Important co-factor in reactions using
ATP
◦ Carbohydrate metabolism
◦ Muscle contraction
◦ Blood coagulation
Membrane stabilization
 Nerve conduction
 Maintenance of potassium

Magnesium Regulation
Primary regulatory factor
◦ Amount of magnesium in the plasma
 Regulatory control
◦ Kidney
 Excrete or reabsorb magnesium depending
on condition
◦ Parathyroid hormone
 Increases renal reabsorption
 Enhances absorption in intestines

References




Bishop, M., Fody, E., & Schoeff, l. (2010). Clinical Chemistry:
Techniques, principles, Correlations. Baltimore: Wolters
Kluwer Lippincott Williams & Wilkins.
http://rickets.stanford.edu/
http://scrink.com/blog/wellness/2009/02/vitamin-d-coldflu-correlation.html
Sunheimer, R., & Graves, L. (2010). Clinical Laboratory
Chemistry. Upper Saddle River: Pearson .