1. Electrolytes
Part 1
Lecture 14

2. Electrolytes
Cathode
Anode
Electrolytes are ions capable of carrying an electricl charge
Anions: (-) → Anode
Cations: (+) → Cathode
Major cations of the body
Na+, K+, Ca2+ & Mg2+
Major anions of the body
Cl-, HCO3-, HPO42- & SO42-
HPO42-: Hydrogen phosphate; HCO3: Hydrogen Carbonate;  Sulfate 

3. Essential Component in Numerous processes
Volume and osmotic pressure (Na+, K+, Cl-)
Myocardial rhythm and contraction (K+, Mg2+, Ca2+)
Cofactors in enzyme activation (Mg2+, Ca2+, Zn2+).
Regulation of ATPase ion pump (Mg2+)
Acid/Base balance (pH) (HCO3-, K+, Cl-)
Coagulation (Mg2+, Ca2+)
Neuromuscular (K+, Mg2+, Ca2+)
The body has complex systems for monitoring and maintaining electrolyte concentrations

4. Water
Maintenance of water homeostasis is vital to life for all organisms
Maintenance of water distribution in various body fluids is a function of electrolytes (Na+, K+, Cl- & HCO3-)

5. Water
Average water content of human body is 40-75% of total body weight.
Solvent for all body processes
Transport nutrients to cells
Regulates cell volume
Removes waste products → urine
Body Coolant → sweating
Water is located in intracellular and extracellular compartments
Intracellular fluid (ICF) is the fluid inside the cells
Extracellular fluid (ECF) and subdivided into the
intravascular extracellular fluid (plasma)
and the interstitial cell fluid that surrounds the cells in the tissue

7. Water
Normal plasma ~ 93 % H2O, the rest is a mixture of Lipids and proteins.
Concentration of ions within the cells and plasma is maintained by:
Energy consumption: Active transport
Diffusion: Passive transport
Maintaining conc. of electrolytes affect distribution of water in compartments.
Most membranes freely permeable to water.
Conc. of ions on one side affect flow of water across the membrane.

8. Osmolality
Physical property of a solution is based on the concentration of solutes per kilograms of solvent (mOsm/Kg)
Sensation of thirst & arginine vasopressin hormone (AVP) [formerly, Antidiuretic hormone (ADH)] are stimulated by hypothalamus in response to increased blood osmolality
Thirst → more water intake
AVP → increase water absorption in kidney
A strict definition of an osmole is the amount of osmotically active particles that when dissolved in 22.4 L of solvent at 0 degrees celsius exerts an osmotic pressure of 1 atmosphere. This is equivalent to the observation that 1 mole of a perfect gas occupying a volume of 22.4 L exerts a pressure of 1 atmosphere.
1 osmole = 1 mole of osmotically active particles

9. Clinical Significance
Osmolality is the parameter to which hypothalamus responds to maintain fluid intake.
The regulation of osmolality also affects the Na+ concentration in plasma
account for ~90% of osmotic activity in plasma
Another process which affects Na+ concentration is regulation of blood volume.

10. Clinical significance
To maintain normal plasma osmolality ( mOsm/Kg) hypothalamus must respond quickly to small changes
1-2% increase in osmolality: 4 fold increase in AVP secretion.
1-2% decrease in osmolality: shuts off AVP secretion.
Renal water regulation by AVP and thirst play important roles in regulating plasma osmolality.
Renal water excretion is more important in controlling water excess,
Whereas thirst is more important in preventing water deficit or dehydration.
Consider what happens in several conditions.

11. Water Load Excess intake of water lower plasma osmolality
Kidney is important in controlling water excess
AVP and thirst are suppressed
Water is not reabsorbed, causing a large volume of dilute urine to be excreted
Hypoosmolality and hyponatremia usually occur in patients with impaired renal excretion of water

12. Water deficit
As a deficit of water, plasma osmolality begins to increase.
Both AVP secretion and thirst are activated.
Although AVP contributes by minimizing renal water loss, thirst is the major defense against hyperosmolality and hypernatremia.
A concern in infants, unconscious patients, or anyone who is unable to either drink or ask for water.

13. Regulation of blood volume
Blood volume is essential in maintaining blood pressure and ensure perfusion to all tissues and organs.
Regulation of both sodium & water are interrelated in controlling blood volume
Renin-angiotensin-aldosterone: system of hormones that respond to decrease in blood volume and help maintain the correct blood volume.

14. Regulation of blood volume
Changes in blood volume is detected by receptors in:
the cardiopulmonary circulation ,
carotid sinus,
aortic arch
and glomerular arterioles
They activate effectors that restore volume by:
appropriately varying vascular resistance,
cardiac output,
and renal Na and H2O retention.

15. Angiotensin converting enzyme (ACE)
ACE: angiotensin converting enzyme

17. M. Zaharna Clin. Chem. 2009

18. Regulation of blood volume
Other Factors effecting blood volume:
Atrial natriuretic Peptide (ANP) → sodium excretion →  blood volume
Volume receptors →  release of AVP → conserve water →  blood volume
Glomerular filtration rate (GFR):  in volume expansion and  in volume depletion
) atrial natriuretic peptide (ANP), released from the myocardial atria in response to volume expansion

19. Increase in Blood Volume

20. Determination of Osmolality
Serum or urine sample (plasma not recommended due to the use of anticoagulants)
Based on properties of a solution related to the number of molecules of solutes per kilogram of solvent such as:
Freezing point ( osmolality freezing point temp.)
Vapor pressure ( osmolality  Vapor pressure)

21. Determination of Osmolality
Freezing Point Osmometer:
Standardized method using NaCl reference solution.
Specimen is supercooled to - 7ºC, to determine freezing point
 osmolality causes depression in the freezing point temp.
More solutes present the longer the specimen will take to freeze.

22. Osmolal Gap= measured osmolality - calculated osmolality
Osmolal gap is the difference between the measured osmolality and the calculated one.
Osmolal Gap= measured osmolality - calculated osmolality
The osmolal gap indirectly indicates the presence of osmotically active substances other than sodium, urea or glucose. (ethanol, methanol or β-hydroxybutyrate)
Plasma osmolality is a measure of the concentration of substances such as sodium, chloride, potassium, urea, glucose, and other ions in blood. It is calculated as the osmoles of solute per kilogram of solvent. (note: the values 20, 3 and 3.7 convert mg/dL into mmol/L ‘mosmol/kg’)
In plasma, the oncotic pressure is only about 0.5% of the total osmotic pressure. This may be a small percent but because colloids cannot cross the capillary membrane easily, oncotic pressure is extremely important in transcapillary fluid dynamics.
The serum sodium is multiplied by two to account for the osmolal contributions of the accompanying anions (chloride and bicarbonate)

23. Case Study
A sixty-seven year old white male was found pulseless and resuscitated; then brought to the emergency room. He had been reported to be drinking in a bar all afternoon, and had then fallen from a ten foot balcony to snow covered ground. He arrived in the emergency room with a fractured occiput and was unresponsive.
Admission Lab. results:
mEq = mg x valence /atomic, molecular or formula weight
the back of the head or skull.
Na=143 mEq/l
( )
BUN=4 mg/dL
(6 – 20)
pH=7.30
(7.35 – 7.45)
Cl=105 mEq/l
(95-105)
GLU=104 mg/dL
Osmolality=356 mOsm/kg
(275 – 295)

24. Osmolal Gap= measured osmolality - calculated osmolality
Case Study
Cal. Osmo. = (2 X 143) + (104/20) + (4/3)
= = 293
Osmolal Gap= measured osmolality - calculated osmolality
= 356 – 293 = 63
An OG value greater than 15 is considered a critical value
The presence of low blood pH, elevated anion gap and greatly elevated OG is a medical emergency that requires prompt treatment

25. Na+
M. Zaharna Clin. Chem. 2009

26. Sodium Most abundant extracellular cation- 90%
Major function is maintaining the normal water distribution & osmotic pressure of plasma
Role in maintaining acid-base balance (Na+, H+ exchange mechanism)
Normal range Serum: mmol/L
ATPase ion pump: the way the body moves sodium and potassium in and out of cells.
3 Na+ out of the cell for every 2 K + in and convert ATP to ADP.

28. Regulation of Sodium Balance
Plasma Na+ concentration depends:
on the intake and excretion of water
and, on the renal regulation of Na+
Three processes are of primary importance:
Intake of water in response to thirst ( p. osmolality)
The excretion of water (AVP release)
The blood volume status, which affects Na+ excretion through aldosterone, angiotensin II, and ANP (atrial natriuretic peptide).

29. Nephron

30. Regulation of Sodium Balance
70 % of sodium that is filtered is reabsorbed in proximal tubules.
Remainder occurs in the ascending loop of Henle (without water absorption) & DCT under regulation of:
Aldosterone
Renin-Angiotensin system
Atrial natriuretic Peptide (ANP) → sodium excretion
electroneutrality is maintained by either Cl- reabsorption or hydrogen ion (H) secretion.

31. Hyponatremia Defined as a serum/plasma level less than 135 mmol/L.
One of the most common electrolyte disorders in hospitalized and non-hospitalized patients
Levels below 130 mmol/L are clinically significant.

32. Causes of Hyponatremia
Accumulation of extracellular glucose induces a shift of free water from the intracellular space to the extracellular space. Serum sodium concentration is diluted by a factor of 1.6 mEq/L for each 100 mg/dL increase above normal serum glucose concentration
continued secretion or action of the antidiuretic hormone
In cirrhosis: vasodilation, increase in AVP and impaired ability to excrete ingested water
Systemic osmolarity is normal or even increased, not decreased, as in true (ie, hyposmolar) hyponatremia. 
traumatic brain injury develop hyponatremia, renal loss of sodium following intracranial disorders
The principal clinical concern of hyponatremia is the encephalopathy associated with acute-onset severe hyponatremia and consequent cerebral edema. 

33. Hypernatremia
Hypernatremia: increased sodium concentration > 145 mmol/l
Result of excess water loss in the presence of sodium excess, or from sodium gain
With severe elevations of sodium, seizures and coma may occur.

34. Sodium determination Methods: Flame emission spectrophotometry
measurement of light emitted when the element is excited by energy in the form of heat.
Flame emission spectrophotometry
determination of the concentration of an element by measurement of light emitted when the element is excited by energy in the form of heat.

35. Atomic absorption spectrophotometry

36. Ion Selective electrode
Reference electrode
selective membrane at the ion selective electrode, allows measured ions to pass, but excludes the passage of the other ions

37. Potassium Major intracellular cationK+
Major intracellular cation
20 X greater concentration in the cell vs. outside.
2% of the bodies potassium circulates within the plasma.
Function:
Regulates neuromuscular excitability
Hydrogen ion concentration
Intracellular fluid volume
An elevated plasma K decreases the resting membrane potential (RMP) of the cell (the RMP is closer to zero), which decreases the net difference between the cell’s resting potential and threshold (action) potential.

38. Effects on Cardiac muscle
Ratio of K+ intracellular & extracellular is important determinant of resting membrane potential across cell membrane
Increase plasma potassium; decreasing the resting membrane potential, increase excitability, muscle weakness
decreases the net difference between the cell’s resting potential and threshold (action) potential
Decrease extracellular potassium; decrease excitability

40. Potassium Role in Hydrogen Concentration
In hypokalemia (low serum K+),
As K+ is lost from the body, Na+ and H+ move into the cell.
The H+ concentration is, therefore, decreased in the ECF, resulting in alkalosis.

41. Regulation of potassium
The kidneys are important in the regulation of K+ balance.
Initially, the proximal tubules reabsorb nearly all the K+.
Then, under the influence of aldosterone, K+ is secreted into the urine in exchange for Na+ in both the distal tubules and the collecting ducts.
Thus, the distal tubule is the principal determinant of urinary K+ excretion.
Most individuals consume far more K+ than needed; the excess is excreted in the urine but may accumulate to toxic levels if renal failure occurs.

42. Hypokalemia Decrease of serum potassium below 3.5 mmol/l
Insulin promotes acute entry of K into skeletal muscle and liver by increasing Na, K-ATPase activity;
RTA as tubular excretion of H decreases, K excretion increases.
An enzyme exists in mineralocorticoid target tissues to prevent overstimulation by glucocorticoids. This enzyme, 11-beta hydroxysteroid dehydrogenase type II (Protein:HSD11B2), catalyzes the deactivation of glucocorticoids
Hypermineralocorticoid-like effects. In the kidney, cortisol activation of mineralocorticoid receptors alters renal tubular exchange of sodium (retained), potassium (excreted)

44. Hyperkalemia Increase potassium serum levels > 5 mmol/l
Associated with diseases such as renal and metabolic acidosis

45. Potassium determination
Assay method:
Ion selective Electrode
a valinomycin membrane is used to selectively bind K+
Synthetic ionophores
 poly(vinylchloride) (PVC) membrane 

46. Chloride Major extracellular anion Cl– is involved in maintaining:
osmolality,
blood volume,
and electric neutrality.
In most processes, Cl– ions shift secondarily to a movement of Na+ or HCO3–.
Cl– ingested in the diet is Completely absorbed by the intestinal tract.

47. Chloride
Cl– ions are filtered out by the glomerulus and passively reabsorbed, in Conjuction with Na, by the proximal tubules.
Excess Cl– is excreted in the urine and sweat
Excessive sweating stimulates aldosterone secretion, which acts on the sweat glands to Conserve Na+ and Cl–

48. Electric Neutrality
Sodium/chloride shift maintains equilibrium within the body.
Na reabsorbed with Cl in proximal tubules.
Chloride shift
In this process, carbon dioxide (CO2) generated by cellular metabolism within the tissue diffuses out into both the plasma and the red cell.
In the red cell, CO2 forms carbonic acid (H2CO3), which splits into H+ and HCO3- (bicarbonate).
Deoxyhemoglobin buffers H+, whereas the HCO3- diffuses out into the plasma and Cl- diffuses into the red cell to maintain the electric balance of the cell
Deoxyhemoglobin: The form of hemoglobin without oxygen

49. Chloride shift

50. Hypochloremia Hypochloremia: < 98 mmol/l
Metabolic alkalosis is a primary increase in serum bicarbonate (HCO3-) concentration
Nasogastric intubation is a medical process involving the insertion of a plastic tube (nasogastric tube or NG tube) through the nose, past the throat, and down into the stomach.

51. Hypercholremia Hypercholremia: > 109 mmol/l
Defect in proximal tubule bicarbonate reabsorption, or a defect in distal tubule hydrogen ion secretion, or both. This results in a hyperchloraemic metabolic acidosis with normal to moderately decreased GFR

52. Assay Coulometric titration (ref. method) Ion selective electrode
measure amount of analyte by measuring amount of current and time required to complete reaction
use Ag electrode to produce Ag+
Ag (s) » Ag+ + e-
Ag+ + Cl- » AgCl
Ion selective electrode