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LMP301 (Winter 2022) > L4 - Water and Electrolytes > Flashcards

L4 - Water and Electrolytes Flashcards

1
Q

Estimate of the water content in the human body?

A
  • ~60% total body mass ~ 42 L
  • 2/3 ICF ~ 28 L
  • 1/3 ECF:
  • 75% interstitial (water that baste cells) ~10.5 L
  • 25% plasma (in blood vessels, intravascular compartments) ~ 3.5 L
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2
Q

Routine Biochemical Testing assesses

A

Plasma (ECF)

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3
Q

Imbalance in Total Body Water content initially reflected in…

A

ECF

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4
Q

Adults: Water intake and Urine output daily?

A

Intake of 1.5 – 3.0 L of water daily to maintain fluid balance
Urine Output 0.8 – 2.0 L of water daily

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5
Q

Primary organ for regulating body water?

A

Kidney

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6
Q

How is compartmentalization maintained?

A
  • ICF and ECF: separation by selectively permeable cellular plasma membrane
  • Intravascular fluid and ISF: separation by capillary endothelium cells
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7
Q

Electrolyte Distribution?

A
  • Plasma and interstitial fluid are somewhat similar
  • ICF concentrations of electrolytes differ from ECF
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8
Q

Major ECF (plasma and interstitial) ions?

A
  • Na+
  • Cl-
  • HCO3-

(Salty outside cells)

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9
Q

Major ICF ions?

A
  • K+
  • Mg2+
  • Protein-
  • Minor components not listed (e.g. Mg2+ ,HPO4^2-, SO4^2-, etc)
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10
Q

Electroneutrality?

A

[cations] = [anions]

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11
Q

Osmolytes?

A

Electrolytes contribute to osmotic pressure

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12
Q

What is Osmotic Pressure?

A

Pressure exerted on either side of a membrane due to differences in the total molar concentrations of solutes dissolved in solutions
• Governs the movement of water
• Dependent on the [Osmolytes]

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13
Q

Oncotic Pressure

A

Osmotic pressure induced by Albumin that cannot easily move out of capillary vessel

Albumin attracts ions and electrolytes => water tries to enter blood vessel => pressure from ECF exerted on the vessel

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14
Q

What maintains the high water content in tissue?

A

The osmotic force created by the high [Na+] (remember high Na+ outside vs. inside cells)

  • allowing tissue to withstand pressures that can exceed 20,000 mm Hg during exercise
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15
Q

Osmolality?

A

Concentration of solutes expressed per mass of solvent
• Thermodynamically more appropriate term as mass, unlike volume, is independent of the temperature;
• unit = Osmol/Kg

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16
Q

Hyperosmolality

A

hyperglycemia, uremia, hypernatremia (e.g. dehydration, diabetes insipidus, uncontrolled diabetes mellitus, renal failure)

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17
Q

Osmolarity?

A

Concentration of solutes expressed per volume of solvent
• Unit = Osmol/L

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18
Q

How to measure Osmolality?

A

Measured by Osmometer: Freezing point depression (colligative property)

  • Principle behind using salt to melt ice (lowers freezing point to <0°C)
  • 1 mole of solute depresses freezing point of H2O by 1.86 °C
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19
Q

Estimated Osmolarity?

A

≈ 2 × [Na+] in mM + [Glucose] in mM + [Urea] in mM

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20
Q

Serum Measured Osmolality (mOsmol/ kg H2O)

A

Measured osmolality is looking at other number of metabolites that is not factored into the calculation

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21
Q

Serum Calculated Osmolality (mOsmol/kg H2O)

A

Simply examines: Sodium, Glucose and Urea

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22
Q

Serum Osmolality Gap?

A

Serum Osmolal Gap = Measured Osmolality – Calculated Osmolarity

* assume osmolality ~ osmolarity

• Determines osmotically active solutes that maybe unaccounted by estimation

e.g., Methanol or Anti-freeze (ethylene glycol) ingestion (poisoning)
• Gold standard test: gas chromatography (not routinely available)

Large O.G. = concern

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23
Q

How useful is the Osmol Gap calculation?

A

O.G > 10 to 14 mmol/kg maybe useful to suggest presence of toxic alcohol or other osmolyte that is accumulating.

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24
Q

Toxic Alcohols that elevates OG

A

Ethylene Glycol

Isopropanol

Methanol

Propylene Glycol

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25
Q

Other Scenarios where OG is elevated

A

Ethanol-related

Renal Failure –uremic acid

Shock –lactic acid

Diabetic Ketoacidosis –Beta hydroxybutyric acids

Other

26
Q

Volemias…

A

…are assessments based on observation of the patient, not based on any laboratory measurements

27
Q

Hypervolemia?

A

increased fluid
• Edema

28
Q

Hypovolemia?

A

decreased fluid
• Dry mucous membranes
• Decreased skin turgor (skin tents up upon pinching)

29
Q

Euvolemia?

A

normal fluid status (appearance on physical exam)

30
Q

What are the 2 Disorders of Sodium Homeostasis?

A

Hyponatremia (low sodium state) vs Hypernatremia (high sodium state):
• Excessive loss, gain, or retention of Na+
• Excessive loss, gain, or retention of H2O

• Symptoms are due to changes in osmolality

31
Q

What is Hyponatremia?

A

(low sodium state) decreased plasma Na+, < 135 mmol/L
• Nausea/headache
• Generalized weakness
• Mental confusion (<120 mmol/L)
• Seizures
Symptoms are due to changes in osmolality

32
Q

Hyponatremic Classification; What are the 2 classes of Hyponatremia (include sub-types)?

A

Decreased Plasma Osmolality (Hypo-osmotic)

  1. Hypovolemic hyponatremia (Depletion) - low water volume
  2. Hypervolemic hyponatremia (Dilution) - high water volume / fluid gain → dilute Na+
  3. Euvolemic hyponatremia

Increased Plasma Osmolality

1.Hyperosmotic Hyponatremia

33
Q

What are the 3 Etiologies of Hyponatremia? (Decreased Plasma Osmolality)

A
  • *1. Hypovolemic hyponatremia** – fluid loss → Na+ goes away with the fluid loss
  • Renal loss
  • Extrarenal loss
  • *2. Hypervolemic hyponatremia** – fluid gain → evidence of swelling and high blood pressure
  • Cirrhosis
  • Heart failure
  • Nephrotic Syndrome
  • *3. Euvolemic hyponatremia** - can’t tell; have to base on biochemical measurement
  • Polydipsia
  • SIADH
  • Adrenal Insufficiency
  • Hypothyroidism
34
Q

What are 2 underlying causes of Euvolemic Hyponatremia?

A

Primary polydipsia: Excess water consumption
• Seen in patients with hypothalamic lesions
• Psychogenic polydipsia: excessive drinking in psychiatric patients
• Polyuria (large volumes of urine production) -> maintain euvolemic state

Syndrome of inappropriate anti-diuretic hormone (SIADH)

35
Q

What is SIADH (contributes to Euvolemic Hyponatremia) and what causes it?

A

Syndrome of inappropriate anti-diuretic hormone

Causes: Sustained ADH release

  • CNS Based: Brain Tumor, Infection (e.g.meningitis)
  • Drugs: anticonvulsants, antiparkinsonian, antipsychotics, antipyretics, antidepressants, ACE Inhibitors
36
Q

How does ADH (Arginine Vasopressin) contributes to the Regulation of Water (osmolality)?

A

In the Hypothalamus: Osmoreceptors detect changes in serum osmolality
• When plasma osmolality > 280 mOsm/kg (too high) ⇒ threshold for ADH release

Main regulator of ADH release by stimulation of Pituitary Gland (elevated plasma osmolality activates osmoreceptors)

→ Posterior Pituitary: secretes ADH (AVP)

→ Kidney: ADH acts on collecting ducts (Vasopressin-2 receptors)

→ leading to increased water reabsorption and decreased water excretion

37
Q

What is Hypervolemic Hyponatremia (Decreased Plasma Osmolality) and what are the possible causes?

A

Dilutional Hyponatremia – fluid overload
• Excess water retention → Edema
• Nephrotic Syndrome → hypoalbuminemia (Albuminuria, hyperlipidemia)
• Cirrhosis
• Advanced Kidney Failure: Excess water retention > Sodium retention (decreased filtration & excretion)
• CHF - Congenitive Heart Failure: Increased ECF, Decreased (effective circulating) Blood Volume
• Summary: ECF is increased ⇒ circulating blood volume decreased

38
Q

Why is the Renin-Angiotensin-Aldosterone System – Important in Heart-Failure? (Think what effect does it have on blood flow)

A

Kidneys sense decreased perfusion (not enough blood circulating) & try to increase blood volume

How?
Renin → Angiotensin I → Angiotensin II → has 2 effects:
1) Vasoconstriction – narrowing of blood vessel
2) Aldosterone release
• Increases Na+ reabsorption in kidneys → Increase blood volume
• Decrease in osmolality sensed by baroreceptors triggers ADH release

  • Increased vasoconstriction + increased blood volume raises the blood pressure
  • Rx: Help with ACE inhibitors or ARBs (Angiosstensin Receptor blockers)
39
Q

What is Hypovolemic Hyponatremia (Decreased Plasma Osmolality) and what are the underlying causes?

A
  • State of Low osmolality, Low sodium
  • Depletional Hyponatremia
  • Excessive loss of Na+, often accompanied by loss of ECF water
  • Renal Causes: when [urine Na+] > 20mmol/L (caused by Diuretics, Nephropathies or Polycystic Kidney Disease)
  • Extrarenal causes of volume loss (e.g. sweating, vomiting, diarrhea; if [urine Na+] < 20mmol/L)
40
Q

Intravenous (IV) Solutions for Hypovolemic patients

A

**Check image in lecture**

41
Q

What is Hyperosmotic Hyponatremia (Increased Plasma Osmolality)?

A

Increased osmolyte (glucose, urea) in plasma
• Extracellular shift of water (out of cells into ECF) or Intracellular shift of Na+ to maintain osmotic balance
• E.g., severe hyperglycemia (Na+ decreased ~1.6 -2.4 mmol/L for every 5.6 mmol/L increase in glucose above 5.6 mmol/L) - (Hyperglycemic Hyperosmolar State → can lead to coma or death)
• Patients maybe asymptomatic (until [Na+] < 120 mmol/L)

42
Q

What are the 2 classes of Hypernatremia?

A

Hypervolemic Hypernatremia and Hypovolemic Hypernatremia

43
Q

Hypervolemic Hypernatremia

A

Increase in water volume and Na+
• Typically, from administering hypertonic solutions (saltier than physiological - IV during operations)
• Primary hyperaldosteronism (Conn’s syndrome) from tumor or hyperplasia

  • Aldosterone > increased Na+ absorption > water follows Na+ > retain water AND Na+
44
Q

What is Hypovolemic Hypernatremia?

A

More water loss than Na+ sodium loss

  • Renal losses: renal disease or diuretics (Urine [Na+] > 20 mmol/L)
  • Extrarenal losses: Sweat, stool, fluid shifts after burns or surgery
45
Q

Euvolemic Hypernatremia?

A
  • Hypodipsia: decreased water intake (e.g. hypothalamic lesion)
  • Diabetes Insipidus
46
Q

ADH deficiency – Diabetes Insipidus (DI)

A

Decreased ADH (secretion or loss)

  • Decreased water reabsorption
  • Increased and dilute urine output
  • Concentrated serum (increased osmolality, hypernatremic)
47
Q

Central DI?

A

Intracranial (tumor, post-neurosurgery, head trauma)

48
Q

Nephrogenic DI?

A

Originates in the kidneys (sensitivity to ADH is lost) - loss of receptors
• E.g. polycystic kidney disease, nephrotoxicity from drugs

49
Q

K+ :major ICF cation?

A
  • Neuro-muscular excitability
  • Heart rhythm and contraction
  • Acid – Base Status

*Check figure on slide 33*

50
Q

Potassium Excretion Peaks at Noon

A

Multiple patients receiving four identical meals every 6 hours

  • Normal K+ diet (100 mmol/day) first 2 days
  • High K+ diet(400 mmol/day) next 6 days
  • Normal K+ diet (100 mmol/day) next 2 days

Rapid K+ adaptation by Kidney occurs in response to either an increase or a decrease in K+ intake.
- circadian rhythm occurs despite evenly spaced meals every 6 hours during a 24-hour period.

51
Q

Normal interval of plasma for Potassium (K+)

A

[K+] = 3.5 – 5.0 mmol/L
• Tight regulation

Remember K+ is abundant in ICF; should not have high concentration in plasma (ECF)

52
Q

Hyperkalemia and Decreased Urinary Excretion?

A

High K+ in blood

plasma [K+] > 5.0 mmol/L

Decreased urinary excretion due to:

  1. Renal failure - decreased filtration (sodium NOT reabsorbed > secreted)
  2. Hypoaldosteronism - Decrease [aldosterone] or decreased response to aldosterone

(Aldosterone increases Na+ reabsorption by stimulation of epithelial sodium channels in exchange for K+ and H+ i.e. Na+ into blood and K+ and H+ from blood out to kidney lumen)

  • Without aldosterone Na+ is not reabsorbed and K+ remains in blood

*slide 37*

53
Q

Hypokalemia

A

Low K+ in blood

plasma [K+] < 3.5 mmol/L

  1. Increased urinary (renal) or GI loss
    - Vomiting resulting in acid loss from stomach → alkalosis
    - Renal loss: hyperaldosteronism (e.g.adrenal adenomas) - too much aldosterone → increased Na+ reabsorption → too much K+ exchanged/leaving blood
  2. Shift of K+ from ECF to ICF
    - Insulin excess (excess treatment with insulin or insulinoma)
  3. Decreased Intake (rare)
54
Q

Anion GAP

A

= [Na+] – [HCO3-] – [Cl-] ≈ 3 – 12 mmol/L

55
Q

Abnormally high value of Anion GAP

A

MUDPILES
• Methanol ingestion
• Uremia of renal failure
• Diabetic Ketoacidosis
• Paracetamol (Acetaminophen) Toxicity
• Isoniazid, iron toxicity, ischemia
• Lactic acidosis
• Ethylene glycol poisoning
• Salicylates intoxication

56
Q

What stimulates ADH (Arginine Vasopressin) Release?

A

thirst → + water intake → elevated plasma osmolality (above 280 mOsm/kg)

→ activates Osmoreptor in the Hypothalamus

57
Q

Where is ADH secreted?

A

Posterior Pituitary Gland

58
Q

What organ does ADH act on to regulate water balance? Name the receptors and downstream pathway.

A

Kidney: ADH acts on collecting ducts (Vasopressin-2 receptors)

ADH (Arginine Vasopressin) binds Vasopressin-2 receptor

→ activates G-alpha subunit of AC6

→ increased cAMP production (ATP-dependant)

→ activates protein kinase → activate transcription factors (nucleus) → Increased aquaporin 2 (AQP2) transcription

→ activated protein kinase → AQP2 phosphorylation → exocytic insertion to luminal membrane → increased water reaborption

59
Q

What is the net effect of ADH (Arginine Vasopressin) presence in the principal cell of Kidney’s Collecting Duct?

A
  • increased transcription of AQP2
  • redistribution of aquaporin-2 to the luminal membrane

In the presence of vasopressin, water enters the principal cell from the lumen through aquaporin-2 (inside of the CD) and exits to the interstitium through aquaporin-3 and aquaporin-4 (into interstitial space)

→ increased water reabsorption

60
Q

What happens when we have SIADH and how does lab test help?

A

Excessive free water retention

  • How does lab test help?
  • Diagnosed when hyponatremia (<135 mmol/L) and hypo-osmolality(<275-280 mOsm/kg) present with concentrated urine
  • [Urine sodium] > 40 mmol/L (thisisabnormal)
  • THG: Volume expansion decreases Na+ reabsorption by kidney
61
Q

Describe how Renin is converted into Aldosterone?

A

Renin (kidney detects decreased perfusion) → converts Angiotensinogen (from liver) → Angiotensin I → Angiotensin II (detected by adrenal glands) → Aldosterone (from adrenal glands)

62
Q

Hyperkalemia – Increased movement of K+from ICF to blood stream

A

Increased movement of K+from ICF to blood stream

• Cellular breakdown (Pseudohyperkalemia–RBC lysis during blood draw)

> due to Tumor lysis after chemotherapy; Crush injuries/trauma

  • Metabolic Acidosis
  • Insulin deficiency/resistance (Insulin promotes dietary K+ into cells)
  • Beta-blockers (medications to reduce blood pressure)

LMP301 (Winter 2022) (10 decks)

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