Fluid and Electrolyte Balance

Approximately 60% of body weight is fluid/water.

Fluid is divided into two compartments:

  • Intracellular Fluid (ICF): Fluid within cells, making up the majority of body fluid.

  • Extracellular Fluid (ECF): Fluid outside cells, consisting of:

    • Plasma: The liquid component of blood.

    • Interstitial Fluid: Fluid that surrounds cells and tissues.

Ion Distribution

The distribution of ions differs between ICF and ECF:

Intracellular Fluid:

  • Main cation: Potassium (K)

  • Other cations: Magnesium (Mg) and Sodium (Na) (lesser degree)

  • Main anion: Phosphate (PO₄³⁻)

Extracellular Fluid:

  • Main cation: Sodium (Na)

  • Main anion: Chloride (Cl)

Hormonal Regulation of Water Levels

Water balance relies on hormonal control. Key hormones involved include:

  • Antidiuretic Hormone (ADH):

    • Released when sodium levels increase.

    • Stimulates thirst and reduces urine output to retain water, thereby lowering ECF sodium concentrations.

  • Renin-Angiotensin-Aldosterone System (RAAS):

    • Renin converts angiotensinogen into angiotensin I, which is converted into angiotensin II.

    • Stimulates release of Aldosterone, promoting sodium retention and water reabsorption.

  • Natriuretic Peptides (ANP & BNP):

    • Released when blood pressure is high, leading to decreased ADH and aldosterone secretion.

    • Promotes sodium loss, resulting in increased water loss and reduced thirst sensation.

Fluid Balance

Average intake and loss of fluid is about 2500 mL/day.

Intake:

  • Consuming food and beverages.

  • Water produced from metabolic processes (10% of total).

Loss:

  • Mostly through urine, but also:

    • Insensible Loss: Approx. 30% lost through exhalation and skin (not consciously detected).

    • Sensible Loss: Notable sweating.

    • Obligatory Loss: About 100 mL/day lost in feces (cannot be controlled).

Osmotic Balance Between ICF and ECF

Fluid shifts between compartments balance osmotic pressure:

  • ECF changes lead to water movement from ICF to ECF.

  • Example: Decrease in ECF concentration leads to movement from ICF to maintain equilibrium.

Dehydration and Hypotonic Hydration

  • Dehydration:

    • Losing more water than sodium, leading to decreased ECF volume and cell crenation.

    • Can impair cellular functions.

  • Hypotonic Hydration (Water Intoxication):

    • More water intake than sodium, causing cells to swell (expand).

    • Symptoms similar to alcohol intoxication and can be life-threatening.

Sodium Imbalance

  • Hypernatremia: Too much sodium, leading to water retention, hypertension, and edema.

  • Hyponatremia: Too little sodium, can result in water intoxication and cellular swelling.

Potassium Regulation

  • Hyperkalemia: High potassium levels, causing cardiac arrhythmias and muscle weakness.

  • Hypokalemia: Low potassium levels leading to hyperpolarization of excitable cells, muscle weakness, and loss of tone.

Calcium Regulation

Calcium levels controlled by:

  • Parathyroid Hormone (PTH): Increases calcium in the blood.

  • Calcitonin: Lowers calcium levels in the blood.

  • Hypercalcemia: High calcium leading to muscle weakness and arrhythmias.

  • Hypocalcemia: Low calcium causing excessive excitability and potential tetany.

Chloride Balance

Chloride levels maintain acid-base balance, closely following sodium levels.
Hyperchloremia or Hypochloremia can lead to acid-base disturbances.