1. Is the serum sodium a reflection of total body sodium?
No.
2. Then what is it a reflection of?
It is a reflection of the relative concentration of sodium in water in a liter of plasma. Disorders
of total body sodium reflect disturbances in extracellular fluid (ECF) volume because sodium
is the predominant cation in ECF, as was discussed in Chapter 74. Increases (hypernatremia)
and decreases (hyponatremia) in serum sodium concentration can therefore occur in settings
of low, normal, and high total body sodium. The term dehydration, which strictly speaking
reflects loss of total body water resulting in hypernatremia, is often misused to describe
hypovolemic states. The assessment of total body sodium is an important component of the
approach to the diagnosis and treatment of dysnatremic disorders. Dysnatremias are reflected
in alterations in plasma osmolality.
3. How is plasma osmolality determined?
Plasma osmolality can be either measured by an osmometer or calculated using the following
formula:
Plasma osmolality (mOsm/kg) 5 2[Na](mEq/L) 1
urea (mg/dL)/2.8 1 glucose (mg/dL)/18
Please note the central role of the sodium concentration as the primary determinate of
plasma osmolality based on this equation. Normally, the measured plasma osmolality is no
more than 10 mOsm/kg higher than the calculated plasma osmolality. If the measured osmolality
greatly exceeds 10 mOsm, an osmolar gap reflecting the presence of an osmotically active
substance is not routinely measured in the ECF.
4. Is there a difference between tonicity and osmolality?
Yes. Tonicity is determined by the presence of impermeable solutes such as sodium and
chloride in the ECF. Such solutes set up osmotic gradients that cause water movement across
cell membranes. In contrast, solutes that are permeable to cell membranes (alcohols, urea)
contribute to the measured osmolality of body fluids but do not cause water movement and
therefore are not effective solutes and do not contribute to tonicity.
5. Does hypernatremia always reflect hyperosmolality?
Yes. Not only are patients with hypernatremia hyperosmolar, but they are also hypertonic. In
contrast, not every patient with hyperosmolality is hypertonic. Such is the case with alcohol
ingestion and azotemia.
6. Does hyponatremia always reflect hypo-osmolality?
No. Hyponatremia can also occur with normal or even hyperosmolality.
No.
2. Then what is it a reflection of?
It is a reflection of the relative concentration of sodium in water in a liter of plasma. Disorders
of total body sodium reflect disturbances in extracellular fluid (ECF) volume because sodium
is the predominant cation in ECF, as was discussed in Chapter 74. Increases (hypernatremia)
and decreases (hyponatremia) in serum sodium concentration can therefore occur in settings
of low, normal, and high total body sodium. The term dehydration, which strictly speaking
reflects loss of total body water resulting in hypernatremia, is often misused to describe
hypovolemic states. The assessment of total body sodium is an important component of the
approach to the diagnosis and treatment of dysnatremic disorders. Dysnatremias are reflected
in alterations in plasma osmolality.
3. How is plasma osmolality determined?
Plasma osmolality can be either measured by an osmometer or calculated using the following
formula:
Plasma osmolality (mOsm/kg) 5 2[Na](mEq/L) 1
urea (mg/dL)/2.8 1 glucose (mg/dL)/18
Please note the central role of the sodium concentration as the primary determinate of
plasma osmolality based on this equation. Normally, the measured plasma osmolality is no
more than 10 mOsm/kg higher than the calculated plasma osmolality. If the measured osmolality
greatly exceeds 10 mOsm, an osmolar gap reflecting the presence of an osmotically active
substance is not routinely measured in the ECF.
4. Is there a difference between tonicity and osmolality?
Yes. Tonicity is determined by the presence of impermeable solutes such as sodium and
chloride in the ECF. Such solutes set up osmotic gradients that cause water movement across
cell membranes. In contrast, solutes that are permeable to cell membranes (alcohols, urea)
contribute to the measured osmolality of body fluids but do not cause water movement and
therefore are not effective solutes and do not contribute to tonicity.
5. Does hypernatremia always reflect hyperosmolality?
Yes. Not only are patients with hypernatremia hyperosmolar, but they are also hypertonic. In
contrast, not every patient with hyperosmolality is hypertonic. Such is the case with alcohol
ingestion and azotemia.
6. Does hyponatremia always reflect hypo-osmolality?
No. Hyponatremia can also occur with normal or even hyperosmolality.
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