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

Fluid, Electrolyte, and Acid-Base Balance

Introduction

Fluid, electrolyte, and acid-base balance are essential for maintaining homeostasis in the human body. These processes ensure that the internal environment remains stable, supporting cellular function and overall health.

Body Fluid Compartments

Major Compartments and Their Composition

• Intracellular Fluid (ICF): Fluid within cells, comprising about two-thirds of total body water.

• Extracellular Fluid (ECF): Fluid outside cells, including interstitial fluid, plasma, and transcellular fluids. Makes up about one-third of total body water.

• Dominant Ions: Sodium (Na+) is dominant in ECF, while Potassium (K+) is dominant in ICF.

Composition of Body Fluids

• Water: The main component of body fluids.

• Electrolytes: Charged particles such as Na+, K+, Ca2+, Cl-, HCO3-.

• Non-electrolyte solutes: Glucose, urea, proteins, etc.

Approximate Concentration of Major Solutes in Body Fluids

Solute	Plasma	Interstitial Fluid	Intracellular Fluid
Sodium (Na+)	153.2	145.1	12.0
Potassium (K+)	4.3	4.1	150.0
Calcium (Ca2+)	3.8	3.4	1.4
Magnesium (Mg2+)	1.4	1.3	34.0
Chloride (Cl-)	115.5	118.0	4.0
Bicarbonate (HCO3-)	25.7	27.0	8.0
Phosphate (PO43-)	2.0	2.0	95.0
Protein	61.0	6.0	54.0
Other	6.7	6.9	94.0

Regulation of Water Balance

Water Gain and Loss

• Water Intake: Mainly from ingestion (90%) and cellular metabolism (10%).

• Water Loss: Occurs via urine (61%), evaporation (35%), and feces (4%).

Sources of Water	Routes by Which Water Is Lost
Ingestion (90%)	Urine (61%)
Cellular metabolism (10%)	Evaporation: perspiration and respiratory passages (35%)
	Feces (4%)

Thirst Regulation

Thirst is the primary mechanism for regulating water intake. It is controlled by:

• Hypothalamic osmoreceptors (detect increased osmolality)

• Arterial and juxtaglomerular apparatus baroreceptors (detect blood pressure changes)

• Dryness of the mouth

• Distension of the stomach

Regulation of Water Loss

• Kidneys: Main route for water excretion.

• Skin and Respiratory Passages: Water lost by evaporation (insensible and sensible perspiration).

• Digestive Tract: Minimal water loss under normal conditions; increases with vomiting or diarrhea.

Regulation of Intracellular Fluid (ICF) Composition

Factors Affecting ICF Composition

• Large proteins cannot exit the cell.

• Ion transport creates concentration gradients (e.g., Na+, K+, Ca2+).

• Electrical charge differences across the membrane.

• Osmosis regulates water movement.

Electrolyte Regulation

Sodium (Na+)

• Dominant extracellular ion; contributes to 90–95% of ECF osmotic pressure.

• Regulated by ADH (osmolality), RAAS, and ANH (blood pressure).

• Normal value: 135–145 mEq/L.

• Major excretion via kidneys; minor via sweat.

Hyponatremia and Hypernatremia

Condition	Causes	Symptoms
Hyponatremia	Inadequate intake, losses, excess water, hyperglycemia	Lethargy, confusion, seizures, coma, changes in blood volume
Hypernatremia	High intake, saline solutions, aldosterone, water loss	Thirst, fever, restlessness, convulsions, edema

Potassium (K+)

• Dominant intracellular ion; crucial for resting membrane potential (RMP).

• Regulated by aldosterone; excess K+ secreted in distal convoluted tubule and collecting ducts.

• Normal value: 3.5–5 mg/dL.

Hypokalemia and Hyperkalemia

Condition	Causes	Symptoms
Hypokalemia	Alkalosis, insulin, reduced intake, renal loss	Decreased excitability, bradycardia, AV block
Hyperkalemia	Cell trauma, reduced excretion	Neuromuscular irritability, muscle weakness, paralysis

Calcium (Ca2+)

• Extracellular concentration tightly regulated (8.6–10.3 mg/dL).

• Regulated by kidneys, digestive tract, and bones.

• Hypocalcemia: below normal; Hypercalcemia: above normal.

Chloride, Magnesium, and Phosphate Ions

• Chloride (Cl-): Main extracellular anion; follows sodium regulation.

• Magnesium (Mg2+): Mostly in bones; cofactor for enzymes; regulated by kidneys.

• Phosphate (PO43-): Component of DNA, RNA, ATP; acts as buffer; regulated by PTH.

Magnesium and Phosphate Imbalances

Condition	Causes	Symptoms
Hypomagnesemia	Malnutrition, alcoholism, renal dysfunction	Irritability, muscle weakness, tetany
Hypermagnesemia	Renal failure, antacids	Nausea, muscle weakness, bradycardia
Hypophosphatemia	Reduced absorption, hyperparathyroidism	Reduced metabolic rate, blood clotting
Hyperphosphatemia	Renal failure, tissue destruction	Calcium phosphate deposits, symptoms of low Ca2+

Acid-Base Balance

Importance of pH Regulation

• Hydrogen ion concentration affects enzyme activity and cellular function.

• Normal blood pH: 7.35–7.45.

• Acidosis: pH < 7.35; Alkalosis: pH > 7.45.

Buffer Systems

• Carbonic Acid/Bicarbonate Buffer System: $\mathrm{H_2CO_3 \rightleftharpoons HCO_3^- + H^+}$

• Protein Buffer System: Hemoglobin and histone proteins act as buffers.

• Phosphate Buffer System: Important for intracellular buffering.

Respiratory Regulation of Acid-Base Balance

• Respiratory system adjusts pH by altering CO2 exhalation.

• Increased H+ stimulates respiratory center to increase breathing rate (hyperventilation), reducing CO2 and raising pH.

Renal Regulation of Acid-Base Balance

• Kidneys excrete H+ and reabsorb HCO3- to regulate pH.

• Renal compensation is slower but more powerful than respiratory compensation.

Acid-Base Imbalances

• Acidosis: Depression of the central nervous system.

• Alkalosis: Hyperexcitability of the nervous system.

• Classified as respiratory or metabolic based on the underlying cause.

Respiratory Acidosis and Alkalosis

• Respiratory Acidosis: Increased CO2 (hypoventilation); kidneys compensate by excreting H+ and reabsorbing HCO3-.

• Respiratory Alkalosis: Decreased CO2 (hyperventilation); kidneys compensate by reducing H+ excretion and HCO3- reabsorption.

Metabolic Acidosis and Alkalosis

• Metabolic Acidosis: Accumulation of H+ or loss of HCO3-; respiratory compensation by hyperventilation.

• Metabolic Alkalosis: Loss of H+ (e.g., excessive antacid use); respiratory compensation by hypoventilation.