Skip to main content
Back

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

NotesPracticeVideo lessons

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Fluid, Electrolyte, and Acid-Base Balance

Fluid Compartments of the Body

The human body contains various fluid compartments that are essential for physiological processes. Understanding these compartments is crucial for comprehending fluid and electrolyte balance.

  • Intracellular Fluid Compartment (ICF): The fluid within cells, accounting for approximately two-thirds of total body water. It is the site of many 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: The fluid that bathes and surrounds tissue cells, forming the majority of ECF.

    • Plasma: The liquid component of blood, containing dissolved proteins, nutrients, and waste products.

  • Example: In a 70 kg adult, total body water is about 42 liters: ~28 L in ICF, ~14 L in ECF (of which ~11 L is interstitial fluid and ~3 L is plasma).

Relative Fluid Volume and Solute Composition

Each fluid compartment has distinct volumes and solute compositions, which are vital for cellular function and homeostasis.

  • 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.

  • Table: Major Ions in Fluid Compartments

Compartment

Major Cation

Major Anion

Protein Content

ICF

K+, Mg2+

HPO42−

High

ECF (Plasma)

Na+

Cl−

High

ECF (Interstitial)

Na+

Cl−

Low

Additional info: The differences in ion concentrations are maintained by active transport mechanisms such as the sodium-potassium pump.

Control of Water Intake and Elimination

Water balance is regulated by mechanisms that control intake and output, ensuring homeostasis.

  • Intake: Driven primarily by thirst, which is 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 nerve conduction, muscle contraction, and fluid balance. Their concentrations are tightly regulated.

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

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

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

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

  • Calcium (Ca2+): Essential 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+

Fluid balance, nerve impulses

Aldosterone, ADH

K+

Membrane potential, cardiac function

Aldosterone, kidneys

Cl−

Osmotic balance, acid-base

Follows Na+

Mg2+

Enzyme cofactor, neuromuscular

Kidneys

Ca2+

Bone, muscle contraction

PTH, calcitonin, vitamin D

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

The body maintains a narrow pH range (7.35–7.45) through several mechanisms to ensure proper cellular function.

  • Buffer Systems: Immediate, short-term regulation of pH by binding or releasing H+ ions. Major buffers include bicarbonate, phosphate, and proteins.

  • Bicarbonate Buffer Equation:

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

  • Respiratory Regulation: Lungs alter CO2 exhalation; increased respiration removes CO2 (raises pH), decreased respiration retains CO2 (lowers pH).

  • Example: During exercise, increased CO2 production lowers pH, stimulating increased breathing to expel CO2.

Acidosis and Alkalosis: Types and Compensation

Disturbances in acid-base balance are classified as acidosis (pH < 7.35) or alkalosis (pH > 7.45), with metabolic or respiratory origins.

  • Metabolic Acidosis: Caused by increased acid production (e.g., lactic acidosis, diabetic ketoacidosis) or loss of bicarbonate (e.g., diarrhea).

  • Metabolic Alkalosis: Caused by loss of acids (e.g., vomiting) or excess bicarbonate intake.

  • Respiratory Acidosis: Caused by hypoventilation, leading to CO2 retention (e.g., COPD).

  • Respiratory Alkalosis: Caused by hyperventilation, leading to excessive CO2 loss (e.g., anxiety, high altitude).

  • Compensation Mechanisms: The body attempts to restore normal pH via respiratory or renal adjustments.

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

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

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

Additional info: Compensation may not fully correct the pH but helps minimize the deviation until the underlying cause is addressed.

Pearson Logo

Study Prep