2. Chapter 4. Macroelements
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143 Terms
What are macroelements?
Elements present in the body in higher concentration, such as calcium, phosphorus, potassium, chloride, and sodium.
What are microelements?
Elements present in very small concentration, such as iron, zinc, copper, iodine, manganese, and cobalt.
What are electrolytes?
Charged particles essential for physiological functions.
Which electrolytes are cations?
Na⁺, K⁺, Ca²⁺, and Mg²⁺.
Which electrolytes are anions?
Cl⁻, HCO₃⁻, H₂PO₄⁻, HPO₄²⁻, and SO₄²⁻.
Chloride (Cl⁻)
Bicarbonate (HCO₃⁻)
Dihydrogen phosphate (H₂PO₄⁻)
Hydrogen phosphate (HPO₄²⁻)
Sulfate (SO₄²⁻)
Main functions of electrolytes
Maintain osmotic pressure, maintain pH, support heart and muscle function, participate in redox reactions, and act as enzyme cofactors.
What is sodium?
The main extracellular cation.
Normal serum sodium value
135–145 mmol/L.
Main function of sodium
Regulation of osmotic pressure and water distribution.
Which organ mainly regulates sodium?
The kidney.
How is sodium commonly measured?
By ion-selective electrode, direct or indirect.
What is hyponatremia?
Low sodium concentration in blood.
Symptoms of hyponatremia
Nausea, weakness, confusion, and neurological symptoms.
Why does hyponatremia cause neurological symptoms?
Water enters CNS cells, causing cell swelling.
Important first step in hyponatremia assessment
Determine plasma osmolality.
Most common type of hyponatremia
Hypoosmotic hyponatremia.
What is hypernatremia?
High sodium concentration in blood.
Hypernatremia is always what?
Hyperosmotic.
Why does hypernatremia cause neurological symptoms?
Water leaves CNS cells, causing cell shrinkage.
Symptoms of hypernatremia
Tremor, irritability, ataxia, confusion, and coma.
What is potassium?
The main intracellular cation.
Normal serum potassium value
3.5–5.0 mmol/L.
Potassium reference value in newborns
3.7–5.9 mmol/L.
What maintains high intracellular potassium?
Na⁺/K⁺-ATPase pump.
Why is the Na⁺/K⁺-ATPase pump important?
It maintains ionic gradients for nerve impulses and muscle contraction.
What is hypokalemia?
Low potassium concentration in blood.
Symptoms of hypokalemia
Muscle weakness, tachycardia, ECG changes, and irritability.
What is hyperkalemia?
High potassium concentration in blood.
Symptoms of hyperkalemia
Bradycardia, weakness, mental confusion, and dangerous cardiac effects.
Why should hemolyzed samples not be used for potassium?
Erythrocytes release potassium and cause falsely high results.
Why can cold storage falsely increase potassium?
Cooling reduces glycolysis and Na⁺/K⁺-ATPase activity, so potassium leaves cells.
Why can delayed sample separation affect potassium?
Potassium diffuses from blood cells into plasma over time.
Why can fist pumping increase potassium falsely?
It releases potassium from muscle cells.
Best handling for accurate potassium
Use heparinized tubes, keep at room temperature, and separate plasma from cells within about 1 hour.
Methods for sodium and potassium determination
Electrochemical methods, atomic absorption spectrophotometry, flame emission spectrophotometry, and spectrophotometric methods.
Most common method for sodium and potassium
Ion-selective electrode method.
What is an ion-selective electrode?
A potentiometric electrode with a membrane selective for a specific ion.
What is indirect ISE?
Method where the sample is diluted before measurement.
Where is indirect ISE usually used?
Large automated biochemical analyzers.
Disadvantage of indirect ISE
High proteins or lipids can cause falsely low results because of dilution effects.
What is direct ISE?
Method where the sample is measured directly without dilution.
Where is direct ISE usually used?
Point-of-care devices and blood gas analyzers.
Advantage of direct ISE
It is not affected by dilution-related errors.
Preferred method for electrolyte determination
Direct ISE.
Which blood is often used in emergency electrolyte analysis?
Heparinized whole blood.
Which heparin salts should be used for electrolyte analysis?
Lithium or ammonium heparin.
What is chloride?
The most abundant extracellular anion.
Normal serum chloride value
100–108 mmol/L.
Main functions of chloride
Osmotic pressure, water balance, and acid-base balance.
Chloride usually follows which electrolyte?
Sodium.
How does chloride help acid-base balance?
It exchanges with bicarbonate to maintain electroneutrality.
Sweat chloride is used to diagnose what?
Cystic fibrosis.
What is osmosis?
Movement of water through a semipermeable membrane toward the more concentrated solution.
What is osmometry?
Measurement of dissolved particles contributing to osmotic pressure.
Why is plasma osmolarity measured?
To assess electrolyte and acid-base disorders.
Formula for calculated plasma osmolarity
mOsm/kg = 1.86 × Na⁺ + glucose + urea + 9.
Normal plasma osmolarity
275–300 mOsm/kg.
What is the osmotic gap?
Difference between measured and calculated osmolarity.
What can an increased osmotic gap indicate?
Unmeasured osmotically active substances.
What does urine osmolarity show?
Renal concentrating ability.
Normal usual urine osmolarity
300–900 mOsm/kg H₂O.
Urine osmolarity with high fluid intake
About 50 mOsm/kg H₂O.
Urine osmolarity with severe fluid restriction
Up to 1400 mOsm/kg H₂O.
What happens to urine osmolarity in progressive kidney disease?
It is reduced because concentrating ability decreases.
What is bicarbonate?
An extracellular anion formed from CO₂.
Bicarbonate buffer equation
CO₂ + H₂O ⇄ H₂CO₃ ⇄ H⁺ + HCO₃⁻.
Main function of bicarbonate
Maintaining acid-base balance.
Normal arterial blood pH
7.35–7.45.
Normal venous blood pH
7.32–7.38.
Life-threatening blood pH
Below 7.0 or above 7.8.
Main regulators of acid-base balance
Buffers, respiratory system, and kidneys.
Main buffer systems
Bicarbonate, phosphate, protein, and hemoglobin buffer.
What is the alkaline component of bicarbonate buffer?
HCO₃⁻.
What is the acidic component of bicarbonate buffer?
H₂CO₃.
Normal plasma bicarbonate concentration
21–32 mmol/L.
Normal bicarbonate to carbonic acid ratio
20:1.
How do kidneys regulate acid-base balance?
Secrete H⁺, produce NH₄⁺, and reabsorb bicarbonate.
What is acidosis?
Blood pH below 7.35.
What is alkalosis?
Blood pH above 7.45.
What is metabolic acidosis?
Acidosis caused by decreased HCO₃⁻.
What is respiratory acidosis?
Acidosis caused by CO₂ retention and increased H₂CO₃.
What is metabolic alkalosis?
Alkalosis caused by increased HCO₃⁻.
What is respiratory alkalosis?
Alkalosis caused by CO₂ loss during hyperventilation.
What does blood gas analysis measure?
pH, PCO₂, and PO₂.
Which sample is needed for blood gas analysis?
Heparinized arterial blood.
Why must blood gas samples be anaerobic?
Air contact changes PCO₂ and PO₂.
When should blood gas analysis be performed?
Within 15 minutes.
Normal arterial PCO₂
35–45 mmHg.
Normal arterial PO₂
80–100 mmHg.
Where is most calcium found?
In the skeleton.
How much calcium is in the skeleton?
About 99%.
Normal total plasma calcium
2.25–2.75 mmol/L.
Forms of plasma calcium
Diffusible and non-diffusible calcium.
What is non-diffusible calcium?
Calcium bound mainly to albumin.
What is diffusible calcium?
Calcium that can cross semipermeable membranes.
What is ionized calcium?
Free calcium, the physiologically active form.
Normal ionized calcium
1.05–1.40 mmol/L.
Functions of extracellular calcium
Bone mineralization, blood coagulation, membrane permeability, and membrane excitability.
Which hormones regulate calcium?
Parathyroid hormone and 1,25-dihydroxy vitamin D.
How does PTH increase calcium?
Mobilizes calcium from bone, increases renal calcium reabsorption, and stimulates vitamin D activation.