electrolytes 3( Mg+,Ca+,HPO,lactate) Flashcards
Magnesium
- 4th most abundant cation in the body (after Na+, K+, Ca2+)
- 2nd most abundant intracellular cation (after K+)
- Mg2+ in the body can be found in:
- Bone (53%) mostly**
- Muscle, organs, soft tissue (46%)
- Serum and RBCs (<1%)
- Magnesium Distribution
- of Mg2+ is bound to protein ( mostly albumin )
- 61% exists in the free or ionized state**
- 5% complexed with other ions (phosphate, citrate)
• Only the free ion is physiologically active** IN A FORM THAT CAN BE USED BY THE BODY
role of magnesium
- Cofactor for more than 300 enzymes
- Transcellular ion transport
- Neuromuscular transmission
- Synthesis of carbohydrates, proteins, lipids and nucleic acids
- Release of and response to certain hormones
dietary magnesium
• Raw nuts • Dry cereal • Vegetables • Meats • Fish • Fruit • The small intestine can absorb 20-65% of dietary Mg2+. Regulation
regulation of magnesium
• Regulation of Mg2+ is largely controlled by the kidney.
- Reabsorbed when there is a deficit
- Excreted when in excess
• Reabsorption of Mg2+:
- 25-30% of non-protein bound Mg2+ is reabsorbed by the proximal convoluted tubule (less than Na+: 60-75%).
- 50-60% is reabsorbed in the ascending Loop of Henle*****
- 2-5% reabsorbed in the distal convoluted tubule.
hormone regulation of magnesium
• Parathyroid hormone (PTH)
- Increases renal reabsorption of Mg2+
- Enhances absorption of Mg2+ in the intestine
- Aldosterone and Thyroxine
- Increases renal excretion of Mg2+
specimen for magnesium
- Serum
- Lithium heparin plasma
• Avoid hemolysis and separate cells from serum ASAP
- [Mg2+] inside RBCs are 10X higher than in the ECF
• Unacceptable anticoagulants:
- Oxalate, citrate, EDTA
- They will bind with Mg2+
• Urine -24-hour specimen preferred
- Must be acidified with HCl to prevent precipitation
magnesium - methods of deteminants
The most common methods involve colorimetry:
** just know that there are 3 main types for colorimetry
- Calmagite
• Mg2+ combines with calmagite to form a reddish-violet complex
• Read @ 532 nm - Formazan Dye
• Mg2+ binds with the dye to form a colored complex
• Read @ 660 nm - Methylthymol Blue
• Mg2+ binds with the chromogen to form a colored complex
• Reference method: Atomic Absorption Spectrophotometry (AAS)**
when is magnesium most frequently observed
in hospitalized patients in ICUs; rarely seen in non hospitalized patients
Clinical significance of magnesium
Hypomagnesemia
causes: reduced intake, decreased absorption, increased excretion
symptoms: tetany *** irregular muscle spasms
Hypermagnesemia
causes: renal failure is most common cause **, increased intake ( renal failure plus taking meds that increase Mg2+)
symptoms: GI. decreased thrombin, neurological
Reference range of magnesium
0.63-1.0 mmol/L
distribution of calcium in the blood
~ 99% of calcium in the body is part of bone <1% is in the extracellular fluid (ECF)
• Calcium exists in plasma in three states:
1) 40% is protein bound
- 80% of which is bound to albumin
- 20% of which is bound to globulins
2) 15% is bound to anions (HCO3-, citrate, lactate)
3) 45% is ionized (Ca2+)
- Physiologically active form
- Protein-bound calcium is non-diffusible and cannot pass through capillary
walls.
- Anion-bound and ionized forms ARE diffusible and can pass through capillary walls.
Functions of Calcium
1) Structural component of bones and teeth
2) Necessary for blood coagulation
3) Required for transmission of nerve pulses } K+ also needed
4) Required for muscle contraction and normal heartbeat } K+ also needed
5) Necessary for activation of some enzymes
6) Role in the transfer of inorganic ions across cell membranes
- i.e. membrane permeability
Calcium balance in blood ( 3 processes)
- Maintained by 3 processes:• Absorption (intestine)
- Excretion (kidney)
- Movement in/out bone
calcium balance in blood 3 hormones
- These processes are regulated by 3 hormones:• Parathyroid Hormone (PTH)
- Calcitonin (CT)
- Vitamin D • 1,25-dihydroxycholecalciferol (1,25-(OH)2D) is the active form of vitamin D
- Converted to the active form by hydroxylation in the kidneys
calcium absorption in the intestines is increased & decreased by
Increased by:
• Increased protein intake (40% Ca bound)
• Low gut pH increases solubility• Vitamin D (needed for Ca absorption)
• PTH (makes active Vitamin D)
Decreased by: • Low protein • High pH decreases solubility • Excess Fatty Acids (complex with Ca) • Phytic acid (insoluble complex) • Increased serum phosphate • Calcitonin
• Serum calcium is regulated by: PTH, vitamin D, and calcitonin
calcium and parathyroid hormone
- PTH secretion is stimulated by a ↓ in ionized Ca2+
- Secretion is stopped by an ↑ in ionized Ca2+
• Exerts three effects on bone and kidney:
In the bone:
1. Activates bone resorption
• Activated osteoclasts break down bone and release Ca2+ into ECF
In the kidney:
- Increases tubular reabsorption of Ca2+
- Stimulates renal production of active vitamin D
calcium & vitamin D
• Vitamin D3 is obtained from the diet or exposure of skin to sunlight.
- In the liver:
- Vitamin D3 is converted to 25-hydroxycholecalciferol (25-OH-D3)
- Inactive form of vitamin D
- In the kidney:
- 25-OH-D3 is hydroxylated to 1,25-dihydroxycholecalciferol (1,25-[OH]2-D3)
- Biologically active form
- The active form:
- ↑ Ca2+ absorption in the intestine
- Enhances effect of PTH on bone resorption
calcium & calcitonin
- Produced in thyroid gland
- Secreted when [Ca2+] in blood ↑
- i.e. secreted in response to a hypercalcemic state
- Ca2+ is ↓ by inhibiting the actions of PTH and vitamin D
Total Calcium Specimen Collection and Stability
- Heparinized Plasma or serum collected without venous stasis
- (i.e. without a tourniquet or leave tourniquet on for no longer than 1 minute)
- Unacceptable anticoagulants:(Will bind with calcium ions and alter the calcium concentration)
- EDTA
- Oxalate
• 24 hour urine
• acidified with HCl to keep pH low and prevent calcium precipitation
Will bind with calcium ions and alter the calcium concentration
Methods of Analysis - Total Calcium
Most common method:
• Spectrophotometry of calcium-dye complexes
- Orthocresolphthalein complexone (CPC)
- Uses 8-hydroxyquinoline to prevent Mg2+ interference - Arsenazo III dye
- Reference Method:
- Atomic absorption spectrophotometry (AAS)
- Rarely used in the clinical setting
Spectrophotometric Method - Total Calcium
• Uses calcium-dye complex (dye binding method)
• O-cresolphthalein Complexone method (CPC)
- Calcium + o-cresolphthalein ——-> calcium-cresolphthalein complex
(purple)
- Read at 575nm
- The color is proportional to calcium
- 8-hydroxyquinoline is used to prevent Mg2+ interference
• Vitros uses Arsenazo III dye
Ionized Calcium( % of total calcium )
- 45% of total serum calcium
- Physiologically active form - free calcium ion (Ca2+)
- More sensitive marker for calcium disorders
Ionized Calcium Specimen Collection
• Heparinized Whole Blood in syringe (recommended)
- Easier to keep anaerobic
- If not analyzed in 30min, collect on ice slurry
• Serum: anaerobic in filled SST vacutainer tube
- Keep capped
- Clotting & centrifugation should be done quickly (<30 mins) at room temperature
• Heparinized plasma and heparinized whole blood in vacutainers can also
be used.
• Unaffected by slight hemolysis
• If sample is grossly hemolyzed - reject
- Hemoglobin is released which binds with Ca2+, causing falsely low results
pH and Ionized Calcium
- Increase in pH = ↓ ionized calcium
- Proteins will release H+ = more protein with negative charge
- Ca2+ will bind with the negatively charged protein → less ionized Calcium
- So keep specimen anaerobic
- Exposure to air = ↓CO2 = ↑pH = ↓Ca2+
- Decrease in pH = ↑ ionized calcium
- Opposite effect → more ionized Calcium
Ionized Calcium Analysis
Calcium Ion-Selective Electrode
- Measures activity of Ca2+ ions
- Solid State – uses a membrane impregnated with special molecules that selectively bind the Ca2+ ions
- Change in potential is proportional to ionized calcium concentration
calcium reference ranges
- total & critical
- ionized
Total Calcium
Adult 2.15 - 2.50 mmol/L
Critical Values < 1.65 or > 3.20 mmol/L
Ionized Serum Calcium
Adult 1.16 - 1.32 mmol/L
Calcium & phosphorus
- Found in the mineral phase of hard tissues
- Bone, dentin, enamel
- Co-exist as extracellular crystalline hydroxyapatite
distribution of phosphate
• Main intracellular fluid (ICF) anion
- Body pool:
- 80% contained in bone
- 20% in soft tissues
- 1% active in serum/plasma (inorganic phosphate)
Present at pH 7.4 as:
HPO42-80
H2PO4-20
Buffer pair:
↓pH = ↑H+, so HPO42- + H+ = ↑H2PO4- Ratio ↓
↑pH = ↓H+, so H2PO4- → HPO42- + H+ Ratio ↑
function of phosphate
- Structure of cell membranes (phospholipid)
- Structural component of bones/ teeth
- Required for the formation of nucleic acids (RNA, DNA)
- Formation of high energy storage compounds (ATP, Creatine Phosphate)
- Buffer in ECF
- HPO42- + H+ H2PO4-
regulation of phosphate
- Phosphate can be:
- Absorbed in the intestine
- Released from cells into the blood
- Lost from bone
- Most important factor in phosphate regulation is PTH.
- Lowers concentration by ↑ renal excretion
- Vitamin D ↑ phosphate
- Increases phosphate absorption in intestine
- Increases phosphate reabsorption in kidney
• Excessive secretion of growth hormone may ↑ phosphate due to
decreased renal excretion of phosphate
Inorganic Phosphate Specimen Collection
Serum/Lithium Heparin Plasma
• No hemolysis (Phosphate is the main ICF anion)
• Separate from cells ASAP
Unacceptable anticoagulants: • Oxalates • Citrate • EDTA • will interfere with the analytic method
24 hour urine • Phosphate levels are highest in morning and lowest in evening (diurnal variation)
Inorganic Phosphate Analysis
- Direct analysis
- Phosphate + molybdate compound → phosphomolybdate compound (340nm)
- Kinetic measurement
- Reduction Method
- Extra step
- Phosphomolybdate compound (above) is reduced to molybdenum blue (600-700nm)
- Fiske and Subbarow
Inorganic Phosphorus Reference Range
Serum/ Plasma 0.78 – 1.42 mmol/L
Neonates and children will have higher values.
Clinical Significance of Inorganic Phosphate
Hypophosphatemia: Hyperparathyroidism Vitamin D deficiency Diabetic ketoacidosis (DKA) Chronic obstructive pulmonary disease (COPD) Asthma Inflammatory bowel disease Alcoholism
Hyperphosphatemia: Acute or chronic renal failure Severe infections Neoplastic disorders Intravascular hemolysis Lymphoblastic leukemia
Lactate
• By-product of the emergency mechanism that produces ATP when O2
delivery is severely decreased
excess lactate in the blood
• Pyruvate is normal end product of glucose metabolism.
- Pyruvate is converted to Acetyl-CoA
- Acetyl-CoA enters citric acid cycle and produces 38 mol of ATP/mol of glucose oxidized
- When there is a deficiency of O2:
- Acetyl-CoA cannot be produced
- NADH accumulates
- Pyruvate is converted to lactate through anaerobic metabolism
- Only 2 mol of ATP/mol of oxidized glucose is produced
- Excess lactate is released into the blood
Excess lactate in blood is an
early indicator of the severity
of O2 deprivation ( hypoxia)
Lactate - Specimen Handling
- Tourniquet should not be used for collection
- Increased venous stasis increases lactate levels
- Iodoacetate or fluoride can be used (grey top)
- Inhibit glycolysis
• Specimen should be collected on ice and separated from cells ASAP
Lactate - Analysis
enzymatic method ( used on clinical analyzers)
Lactate - Clinical Significance
Used for metabolic monitoring of critically ill patients.
• Severity of illness and prognosis
Two types of lactic acidosis: 1. Type A • Associated with hypoxic conditions • Shock, myocardial infarction, severe CHF, pulmonary edema, severe blood loss
- Type B
• Metabolic origin
• Diabetes mellitus, severe infection, leukemia, liver or renal disease,
toxins (ethanol, methanol, salicylate poisoning)
lactate reference range
venous 0.5-2.2 mmol/L
Common Problems When Evaluating Serum Electrolytes
Hemolysis (false ↑ K+)
• Small veins
• Moisture
• Blood sample in syringe expelled through needle
• Specimen not clotted before centrifugation
• Important for serum samples (i.e. SST tube)
• Vigorous ringing of clot
Hemoconcentration
• H2O and small molecules diffuse out of vessel into tissues leaving relative ↑ in non-diffusible elements (larger molecules) such as protein & total calcium.
• Caused by tourniquet in place for too long or on too tightly.
Too much hand pumping (false ↑ K+)
Serum/plasma on cells too long (false ↑ K+ and PO4)
