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

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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 (ICF) Compartment: The fluid within cells, accounting for about two-thirds of total body water. It is the site of most metabolic processes.
Extracellular Fluid (ECF) Compartment: The fluid outside cells, making up about one-third of total body water. It is subdivided into:
- Interstitial Fluid (IF): The fluid that bathes and surrounds tissue cells, comprising about 80% of ECF.
- Plasma: The liquid component of blood, comprising about 20% of ECF.
Other ECF Subdivisions: Includes lymph, cerebrospinal fluid, synovial fluid, etc. (minor volumes).

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 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 (Plasma and Interstitial Fluid): High in sodium (Na+) and chloride (Cl−); low in potassium (K+), magnesium (Mg2+), and phosphate.
Plasma: Contains more proteins (mainly albumin) than interstitial fluid.

Compartment	Main Cations	Main Anions	Proteins
ICF	K+, Mg2+	HPO42−, proteins	High
Plasma (ECF)	Na+	Cl−, HCO3−	High
Interstitial Fluid (ECF)	Na+	Cl−, HCO3−	Low

Additional info: The selective permeability of cell membranes and active transport mechanisms maintain these differences.

Control of Water Intake and Elimination

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

Intake: Mainly through drinking fluids, eating moist foods, and metabolic water production.
Regulation of Intake: The thirst mechanism, controlled by the hypothalamus, responds to increased plasma osmolality or decreased blood volume.
Elimination: Primarily via urine (regulated by kidneys), but also through feces, sweat, and respiration.
Hormonal Control: Antidiuretic hormone (ADH) increases water reabsorption in kidneys; aldosterone and atrial natriuretic peptide (ANP) also influence water and sodium balance.

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

Electrolytes: Structure, Function, and Regulation

Electrolytes are ions that conduct electricity in solution and are vital for physiological processes.

Sodium (Na+): Main ECF cation; essential for fluid balance, nerve impulse transmission, and muscle function. Regulated by aldosterone, ADH, and ANP.
Potassium (K+): Main ICF cation; critical for resting membrane potential and cardiac function. Regulated by aldosterone (promotes excretion in kidneys).
Chloride (Cl−): Main ECF anion; helps maintain osmotic pressure and acid-base balance. Follows Na+ passively.
Magnesium (Mg2+): Important for enzyme function, neuromuscular activity, and ATP production. Regulated by kidneys.
Calcium (Ca2+): Essential for bone structure, muscle contraction, blood clotting, and nerve function. Regulated by parathyroid hormone (PTH), calcitonin, and vitamin D.

Additional info: Imbalances can cause serious effects, such as arrhythmias (K+), tetany (Ca2+), or neurological symptoms (Na+).

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, short-term regulation. Major buffers include:
- Bicarbonate Buffer System:
- Phosphate Buffer System: Important in ICF and kidneys.
- Protein Buffer System: Hemoglobin and plasma proteins act as buffers.
Respiratory Regulation: Alters blood pH by changing CO2 exhalation. Increased respiration removes CO2 (raises pH); decreased respiration retains CO2 (lowers pH).
Renal Regulation: Kidneys excrete or reabsorb H+ and HCO3− to adjust pH. Slowest but most powerful mechanism.

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

Acidosis and Alkalosis: Types and Compensation

Acid-base imbalances are classified by their origin (metabolic or respiratory) and direction (acidosis or alkalosis).

Acidosis: Blood pH below 7.35.
Alkalosis: Blood pH above 7.45.
Metabolic Causes: Due to changes in HCO3− (e.g., kidney failure, diarrhea, vomiting).
Respiratory Causes: Due to changes in CO2 (e.g., hypoventilation, hyperventilation).
Compensation Mechanisms:
- Respiratory Compensation: Lungs adjust CO2 exhalation to correct metabolic imbalances.
- Renal Compensation: Kidneys adjust H+ and HCO3− handling to correct respiratory imbalances.

Disorder	Primary Disturbance	Compensation
Metabolic Acidosis	↓ HCO3−	↑ Ventilation (↓ CO2)
Metabolic Alkalosis	↑ HCO3−	↓ Ventilation (↑ CO2)
Respiratory Acidosis	↑ CO2	↑ Renal HCO3− reabsorption
Respiratory Alkalosis	↓ CO2	↓ Renal HCO3− reabsorption

Additional info: Compensation may not fully restore pH to normal but helps minimize the disturbance.