Fluid, Electrolyte, and Acid-Base Balance: Study Notes

Fluid, Electrolyte, and Acid-Base Balance

Fluid Compartments of the Body

The human body contains water distributed in distinct compartments, each with unique characteristics and functions. Understanding these compartments is essential for grasping fluid and electrolyte balance.

• Intracellular Fluid Compartment (ICF): The fluid within cells, accounting for about two-thirds of total body water. It is the site of most metabolic processes.

• Extracellular Fluid Compartment (ECF): The fluid outside cells, making up about one-third of total body water. It is subdivided into:

  • Interstitial Fluid (IF): The fluid between cells, bathing and surrounding tissue cells.

  • Plasma: The liquid component of blood, responsible for transporting nutrients, hormones, and waste products.

• Example: In a 70 kg adult, approximately 28 L of water is intracellular, while 14 L is extracellular (with about 11 L as interstitial fluid and 3 L as plasma).

Relative Fluid Volume and Solute Composition

Each fluid compartment has a characteristic volume and solute composition, which is crucial for physiological function.

• ICF: High in potassium (K+), magnesium (Mg2+), and phosphate (HPO42−); low in sodium (Na+) and chloride (Cl−).

• ECF: High in sodium (Na+) and chloride (Cl−); low in potassium (K+).

• Plasma: Contains more proteins than interstitial fluid, contributing to osmotic pressure.

• Table: Major Ions in Fluid Compartments

Compartment	Major Cation	Major Anion
ICF	K+	HPO42−
ECF (Plasma & IF)	Na+	Cl−

• Example: The high K+ concentration inside cells is essential for nerve impulse transmission and muscle contraction.

Control of Water Intake and Elimination

Water balance is maintained by regulating intake and output, ensuring homeostasis.

• Intake: Driven primarily by thirst, regulated by the hypothalamus in response to increased plasma osmolality or decreased blood volume.

• Elimination: Occurs mainly via the kidneys (urine), but also through skin (sweat), lungs (water vapor), and feces.

• Hormonal Regulation: Antidiuretic hormone (ADH) increases water reabsorption in the kidneys; aldosterone promotes sodium (and thus water) retention.

• Example: Dehydration triggers thirst and ADH release, reducing urine output to conserve water.

Electrolyte Structure, Function, and Regulation

Electrolytes are ions that play critical roles in physiological processes. Their concentrations are tightly regulated.

• Sodium (Na+): Main ECF cation; essential for fluid balance, nerve function, and muscle contraction. Regulated by aldosterone, ADH, and natriuretic peptides.

• Potassium (K+): Main ICF cation; crucial for resting membrane potential and action potentials. Regulated by aldosterone and renal excretion.

• Chloride (Cl−): Main ECF anion; helps maintain osmotic pressure and acid-base balance. Follows Na+ movement.

• Magnesium (Mg2+): Important for enzyme function, neuromuscular activity, and ATP production. Regulated by intestinal absorption and renal excretion.

• Calcium (Ca2+): Vital for bone structure, muscle contraction, and neurotransmitter release. Regulated by parathyroid hormone (PTH), calcitonin, and vitamin D.

• Table: Electrolyte Functions and Regulation

Electrolyte	Main Function	Regulation
Na+	Osmotic balance, nerve function	Aldosterone, ADH
K+	Membrane potential	Aldosterone, kidneys
Cl−	Osmotic balance, acid-base	Follows Na+
Mg2+	Enzyme cofactor	Kidneys, intestines
Ca2+	Bone, muscle, nerves	PTH, calcitonin

Regulation of pH: Buffer Systems, Renal Function, and Respiration

The body maintains a narrow pH range (7.35–7.45) using multiple mechanisms to prevent harmful shifts in acidity or alkalinity.

• Buffer Systems: Immediate, chemical systems that resist pH changes by binding or releasing H+. Major buffers include bicarbonate, phosphate, and proteins.

• Bicarbonate Buffer Equation:

  • $\mathrm{CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-}$

• Renal Regulation: Kidneys excrete or reabsorb H+ and HCO3− to adjust pH over hours to days.

• Respiratory Regulation: Lungs alter CO2 exhalation, affecting blood pH within minutes.

• Example: During exercise, increased CO2 production lowers pH, triggering faster breathing to expel CO2 and restore balance.

Acidosis and Alkalosis: Causes and Compensation

Disturbances in acid-base balance are classified by origin (metabolic or respiratory) and direction (acidosis or alkalosis).

• Acidosis: Blood pH below 7.35. Can be metabolic (e.g., diabetic ketoacidosis) or respiratory (e.g., hypoventilation).

• Alkalosis: Blood pH above 7.45. Can be metabolic (e.g., vomiting loss of acid) or respiratory (e.g., hyperventilation).

• Compensation Mechanisms:

  • Respiratory Compensation: Lungs adjust CO2 exhalation to correct metabolic disturbances.

  • Renal Compensation: Kidneys adjust H+ and HCO3− handling to correct respiratory disturbances.

• Example: In metabolic acidosis, the respiratory system increases ventilation to lower CO2 and raise pH.

Additional info: Compensation may not fully restore pH to normal but helps minimize the deviation until the underlying cause is corrected.