Fluid-Balance Hormone Interactions
Fluid-balance hormone interactions explain how ADH, aldosterone, and atrial natriuretic peptide help regulate water balance, sodium balance, plasma osmolality, blood pressure, and blood volume. This calculator focuses on measurable physiology rather than anatomy. It estimates hormone drive from serum sodium, plasma osmolality, urine osmolality, urine sodium, blood pressure, and volume status.
Core formulas and relationships
When calculated osmolality mode is selected, the calculator estimates plasma osmolality from sodium, glucose, and BUN:
\[
\begin{aligned}
Osm_{\mathrm{calc}}
&= 2\cdot Na^+ + \frac{\mathrm{glucose}}{18} + \frac{BUN}{2.8}
\end{aligned}
\]
Mean arterial pressure is estimated from systolic and diastolic blood pressure:
\[
\begin{aligned}
MAP
&= DBP + \frac{SBP - DBP}{3}
\end{aligned}
\]
ADH mainly increases water reabsorption. Aldosterone mainly supports sodium retention and potassium secretion. ANP is usually associated with volume expansion and promotes sodium and water excretion.
How to interpret the result
High plasma osmolality or low volume usually increases ADH drive. Low volume and low pressure usually increase aldosterone drive. High volume and high pressure usually increase ANP drive. The calculator separates water-retention tendency from sodium-retention tendency because water balance and sodium balance are related, but they are not identical.
A conflict flag appears when the hormone pattern does not match the osmolality, sodium, pressure, or volume context. For example, high ADH despite low plasma osmolality is a teaching conflict because low osmolality would usually suppress ADH unless another signal is overriding it.
Common pitfalls
- Assuming serum sodium alone fully explains water balance.
- Forgetting that ADH controls water more directly than sodium.
- Interpreting aldosterone without considering volume and blood pressure.
- Ignoring urine osmolality and urine sodium when checking hormone consistency.
Micro example
If serum sodium is \(150\ \mathrm{mmol/L}\), glucose is \(95\ \mathrm{mg/dL}\), and BUN is \(18\ \mathrm{mg/dL}\), then:
\[
\begin{aligned}
Osm_{\mathrm{calc}}
&= 2\cdot 150 + \frac{95}{18} + \frac{18}{2.8} \\
&= 311.7\ \mathrm{mOsm/kg}
\end{aligned}
\]
This high osmolality supports increased ADH drive, especially if volume status is low or urine is concentrated.
When to use this tool
This tool is useful for learning fluid physiology, sodium-water balance, ADH response, aldosterone response, ANP response, dehydration patterns, water excess, SIADH-like teaching patterns, and volume expansion. It is an educational model and should not be used for diagnosis or treatment decisions.
