Disorders of Water and Sodium Balance

Flashcards covering fluid compartments, feedback systems, dehydration, hyponatremia, hypernatremia, SIADH, and Diabetes Insipidus from Dr. Mike's lecture.

What is the major determinant of extracellular fluid (ECF) osmolality and volume?

Serum sodium ($Na$) is the major determinant of ECF osmolality and volume.

What is the major intracellular cation in the body?

Potassium ($K$) is the major intracellular cation.

How is total body water divided between the intracellular and extracellular fluid compartments?

Total body water is divided into Intracellular Fluid (ICF), which comprises approximately $\frac{2}{3}$ of total body water inside cells, and Extracellular Fluid (ECF), which comprises approximately $\frac{1}{3}$ outside cells.

Into what two sub-compartments is Extracellular Fluid (ECF) divided?

Extracellular Fluid (ECF) is divided into plasma and interstitial fluid.

How do feedback mechanisms respond to an increase in plasma osmolality to restore homeostasis?

An increase in plasma osmolality stimulates hypothalamic osmoreceptors, triggering thirst (causing water intake) and ADH release (causing the kidneys to retain water), which lowers plasma osmolality back to homeostasis.

What physiological changes occur when plasma osmolality decreases?

Decreased plasma osmolality leads to decreased ADH secretion, resulting in the production of more dilute urine.

What sequence of physiological events occurs during excessive water loss leading to severe shock?

Excessive water loss decreases total body water and blood volume, which causes tachycardia and hypotension, leading to poor tissue perfusion and severe shock.

How does total body sodium differ from plasma sodium concentration in terms of what they regulate or reflect?

Total body sodium mainly influences extracellular fluid volume, whereas plasma sodium concentration mainly reflects the relationship between sodium and water.

How does the Renin-Angiotensin-Aldosterone System (RAAS) restore ECF volume when blood volume or renal perfusion decreases?

Decreased blood volume or renal perfusion stimulates Renin release to produce Angiotensin II, which induces vasoconstriction and triggers Aldosterone secretion. Aldosterone promotes $Na^+$ reabsorption, leading to water retention and increased ECF volume.

How is hyponatremia defined based on serum sodium concentration?

Hyponatremia is defined as a serum sodium concentration of $< 135\text{ mmol/L}$.

What are the three major categories of hyponatremia?

1. Hypovolemic hyponatremia
2. Euvolemic hyponatremia
3. Hypervolemic hyponatremia

What is the underlying mechanism of hypovolemic hyponatremia?

Hypovolemic hyponatremia involves a loss of both sodium and water, where sodium loss is proportionally greater than water loss.

What are the main causes of euvolemic hyponatremia?

The main causes are Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH), hypothyroidism, adrenal insufficiency, and excessive water intake.

What is the characteristic fluid finding in hypervolemic hyponatremia?

Both total body sodium and total body water are increased, but total body water increases more than sodium.

What sequence of events causes brain edema and neurological symptoms in hyponatremia?

Decreased serum sodium lowers plasma osmolality, causing water to move INTO brain cells, which produces cerebral edema and neurological symptoms.

Why must chronic hyponatremia not be corrected too rapidly?

Overly rapid correction of chronic hyponatremia can cause osmotic demyelination syndrome.

How is hypernatremia defined based on serum sodium concentration?

Hypernatremia is defined as a serum sodium concentration of $> 145\text{ mmol/L}$.

What is the primary difference between Central Diabetes Insipidus and Nephrogenic Diabetes Insipidus?

Central Diabetes Insipidus is caused by inadequate ADH production or release, whereas Nephrogenic Diabetes Insipidus occurs when the kidneys do not respond adequately to ADH.

How does a water deficit lead to neurological symptoms like seizures and coma in hypernatremia?

Water deficit increases serum sodium and plasma osmolality, drawing water out of brain cells and causing brain cell shrinkage, which leads to thirst, neurological symptoms, and potentially seizures or coma.

What complication can occur if chronic hypernatremia is corrected too rapidly?

Rapid correction of chronic hypernatremia can cause cerebral edema.

What is the pathophysiological pathway in Diabetes Insipidus that leads to elevated serum sodium?

Decreased ADH action reduces renal water reabsorption, leading to a large urine volume and excessive water loss, which increases serum sodium concentration.

What sequence of events in SIADH leads to decreased plasma osmolality?

Excess ADH increases water reabsorption in the kidneys, causing dilution of $Na^+$ and hyponatremia, which results in decreased plasma osmolality.

What are the typical clinical findings associated with SIADH?

Low serum sodium, low serum osmolality, inappropriately concentrated urine, and clinical euvolemia.

How do brain cell volume changes differ between hyponatremia and hypernatremia?

In hyponatremia, water excess causes brain cells to swell (leading to cerebral edema), whereas in hypernatremia, water deficit causes brain cells to shrink.

Which specific hormones regulate water balance versus sodium reabsorption?

ADH primarily regulates water balance, whereas Aldosterone primarily promotes sodium reabsorption and potassium secretion.