lecture 2 fluids Flashcards
Total body water can be divided into:
Intracellular Compartment: 2/3 of Total Body Water
Extracellular Compartment: 1/3 of Total Body Water
➢ Plasma 1/4 of the ECF
➢ Interstitial 3/4 of the ECF
Major Electrolytes
- Sodium is the major extracellular cation
➢ECF [135 – 145 mmol/L], ICF [4 – 10 mmol/L] - Potassium is the major intracellular cation
➢ECF [3.5 – 5.5 mmol/L], ICF [~ 110 mmol/L]
Different compartments have different concentrations of ions: - Semipermeable membranes separate the different compartments
- Water and small molecules (i.e. Na, K) can move around but larger molecules (i.e.
proteins) cannot
Sodium distribution
Typically, most cell membranes tend to be impermeable to sodium,
and gradient is maintained by the sodium-potassium pump
* Sodium input and output are normally balanced thanks to a great
turnover by the gut and kidneys
Potassium distribution
Potassium is the predominant intracellular cation
* Potassium levels are important to consider because of its effect on
membrane excitability
Water distribution
Water is not actively transported in the body but moves
between the ICF and ECF compartments
* Water distribution in the ICF and ECF is determined by
their osmotic contents
* Typically, the osmotic concentrations (osmolalities) of ICF
and ECF are equal (isotonic)
plasma osmolality
Plasma osmolality measures the different solutes in plasma
Osmolality is primarily determined by sodium concentration
Plasma osmolality can be helpful in evaluating the etiology of sodium imbalances
(i.e. hyponatremia)
Aldosterone
Aldosterone primarily controls sodium excretion
* Secreted in response to a decrease in ECF volume
* Causes sodium retention and loss of potassium
vasopressin
Vasopressin (ADH) primarily controls water excretion
* Secreted in response to an increase in the ECF osmolality and a decrease in
ECF volume
* Promotes water retention→ concentrated urine
Changes in ECF osmolality: loss of water
Decrease of osmolality→ increase of water in the ECF (i.e. drinking
lots of water)
➢Hypothalamic thirst centre is NOT stimulated → no thirst sensation
➢Vasopressin (ADH) release is inhibited → diluted urine
The main goal here is to excrete the excess of water and restore of
normal ECF osmolality
Changes in ECF osmolality: increase of water
Decrease of osmolality→ increase of water in the ECF (i.e. drinking
lots of water)
➢Hypothalamic thirst centre is NOT stimulated → no thirst sensation
➢Vasopressin (ADH) release is inhibited → diluted urine
The main goal here is to excrete the excess of water and restore of
normal ECF osmolality
pure water loss
loss of pure water from ECF,
➢increased ECF osmolality causes water
redistribution between ICF and ECF
isotonic fluid loss
Loss of isotonic saline from ECF
➢No change in osmolality
➢No fluid shift from ICF
➢Volume loss is confined to ECF only
Reference interval for plasma sodium concentration:
133-146 mmol/L
Indirect measure
measure of sodium concentration by diluting the
plasma before the analysis
Direct measure
measure of sodium concentration directly in
undiluted plasma
Indirect measure vs Direct measure
Both measurements provide similar results except when there is
severe hyperlipidemia (pseudohyponatremia)
Pseudohyponatremia
Pseudohyponatremia is an encountered laboratory abnormality defined by a sodium concentration of less than 133 mmol/L in the setting of an osmolality within range
Causes of pseudohyponatremia: hyperlipidemia
In severe hyperlipidemia the plasma water content is significantly decreased
* If you suspect pseudohyponatremia due to severe hyperlipidemia you can measure plasma osmolality
When hyperlipidemia is present, the
actual plasma water fraction is
decreased, and therefore an indirect
measurement will result in falsely low Na
concentration
A direct measurement provides a more
accurate evaluation of sodium (no
pseudohyponatremia)
Causes of pseudohyponatremia: hyperglycemia
Excess of glucose in the plasma increases osmolality
* Hyperosmolarity determines movement of water from the ICF to the ECF diluting sodium concentration
* Decline in plasma sodium is typical as a response to hyperglycemia
Always investigate plasma glucose concentration in patients with unexplained hyponatremia
Pseudohyponatremia should be considered in patients
with high serum lipids,
particularly if serum osmolality is normal.
ex. Lab Results:
Sodium (Na+): 125 mmol/L [133-146 mmol/L]
Serum Glucose: 10 mmol/L (elevated)
Total Cholesterol: 18 mmol/L (very high)
Serum Osmolality: 294 mmol/kg [275-295 mmol/kg]
Hyponatremia
Hyponatremia: plasma sodium concentration is below 133 mmol/L
Causes:
* Sodium depletion
* Water excess
Hypernatremia
Plasma sodium concentrations above 146 mmol/L
Causes
* Water depletion (Frequent finding in elderly people: they usually don’t drink
enough water)
* Combined water and sodium depletion (water loss predominant)
* Sodium excess
