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Fluid, Electrolyte, and Acid-Base Balance: Anatomy & Physiology Study Guide

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Chapter 39: Fluid, Electrolyte, and Acid-Base Balance

Introduction

The human body relies on a delicate balance of fluids, electrolytes, and acids and bases to maintain homeostasis. Disruptions in these balances can lead to significant physiological consequences. This chapter explores the normal structure and function of body fluids, electrolytes, and acid-base balance, as well as common imbalances and their clinical implications.

Normal Structure and Function of Fluids, Electrolytes, Acids, and Bases

Body Fluid Compartments

  • Intracellular Fluid (ICF): Fluid within cells; about 2/3 of total body water.

  • Extracellular Fluid (ECF): Fluid outside cells; includes interstitial fluid (between cells), plasma (in blood vessels), and transcellular fluids (e.g., cerebrospinal, synovial).

  • Water Content: Varies by age, sex, and body composition. Infants have higher water content than adults.

Functions of Body Water

  • Regulates body temperature

  • Transports nutrients and waste

  • Acts as a solvent for electrolytes and other substances

  • Lubricates joints and tissues

Movement of Body Fluids

  • Osmosis: Movement of water across a semipermeable membrane from low to high solute concentration.

  • Diffusion: Movement of solutes from high to low concentration.

  • Filtration: Movement of water and solutes through a membrane due to hydrostatic pressure.

  • Active Transport: Movement of substances against a concentration gradient, requiring energy (e.g., sodium-potassium pump).

Osmotic and Hydrostatic Pressure

  • Osmotic Pressure: Pulls water into a compartment (e.g., plasma proteins pull water into blood vessels).

  • Hydrostatic Pressure: Pushes water out of a compartment (e.g., blood pressure pushes water out of capillaries).

Regulation of Body Fluids

Fluid Intake and Output

  • Average adult intake and output: ~2500 mL/day

  • Intake sources: Oral fluids, food, metabolism

  • Output routes: Urine, feces, sweat, insensible loss (lungs, skin)

Hormonal Regulation

  • Antidiuretic Hormone (ADH): Increases water reabsorption in kidneys, reducing urine output.

  • Aldosterone: Promotes sodium and water reabsorption, potassium excretion.

  • Atrial Natriuretic Peptide (ANP): Increases sodium and water excretion.

Table: Average Daily Fluid Intake and Output

Source

Intake (mL)

Output (mL)

Oral fluids

1200

Urine: 1500

Food

1000

Feces: 200

Metabolism

300

Insensible loss: 800

Total

2500

2500

Electrolyte Balance

Major Electrolytes and Their Functions

Electrolyte

Normal Range

Main Functions

Sources

Sodium (Na+)

135–145 mEq/L

Regulates ECF volume, nerve/muscle function

Table salt, processed foods

Potassium (K+)

3.5–5.0 mEq/L

Cell metabolism, cardiac/neuromuscular function

Fruits, vegetables, meats

Calcium (Ca2+)

9–10.5 mg/dL

Bone/teeth structure, muscle contraction, blood clotting

Dairy, leafy greens

Magnesium (Mg2+)

1.3–2.1 mEq/L

Enzyme activity, neuromuscular function

Whole grains, nuts

Chloride (Cl-)

98–106 mEq/L

Maintains osmotic pressure, acid-base balance

Table salt

Bicarbonate (HCO3-)

22–26 mEq/L

Major buffer in acid-base balance

Produced by body

Phosphate (PO43-)

2.5–4.5 mg/dL

Bone/teeth structure, energy metabolism

Meat, dairy, nuts

Electrolyte Imbalances

  • Hyponatremia: Low sodium; causes include vomiting, diarrhea, diuretics. Symptoms: confusion, seizures.

  • Hypernatremia: High sodium; causes include dehydration, excess salt intake. Symptoms: thirst, restlessness.

  • Hypokalemia: Low potassium; causes include diuretics, vomiting. Symptoms: muscle weakness, arrhythmias.

  • Hyperkalemia: High potassium; causes include renal failure, tissue damage. Symptoms: cardiac arrhythmias.

  • Hypocalcemia: Low calcium; causes include hypoparathyroidism, vitamin D deficiency. Symptoms: tetany, muscle cramps.

  • Hypercalcemia: High calcium; causes include malignancy, hyperparathyroidism. Symptoms: weakness, confusion.

  • Hypomagnesemia: Low magnesium; causes include malnutrition, alcoholism. Symptoms: neuromuscular irritability.

  • Hypermagnesemia: High magnesium; causes include renal failure. Symptoms: lethargy, decreased reflexes.

Acid-Base Balance

pH and Buffer Systems

  • Normal blood pH: 7.35–7.45

  • Acidosis: pH < 7.35

  • Alkalosis: pH > 7.45

  • Buffer systems: Bicarbonate, phosphate, and protein buffers help maintain pH.

Bicarbonate-Carbonic Acid Buffer System

  • Major buffer in ECF; maintains acid-base balance by regulating carbonic acid (H2CO3) and bicarbonate (HCO3-).

  • Equation:

Acid-Base Imbalances

Disorder

pH

PaCO2

HCO3-

Primary Cause

Respiratory Acidosis

<7.35

>45 mmHg

Normal or >26

Hypoventilation, CO2 retention

Respiratory Alkalosis

>7.45

<35 mmHg

Normal or <22

Hyperventilation, CO2 loss

Metabolic Acidosis

<7.35

Normal or <35

<22

Increased acid, loss of HCO3-

Metabolic Alkalosis

>7.45

Normal or >45

>26

Loss of acid, gain of HCO3-

Compensation Mechanisms

  • Respiratory compensation: Lungs alter CO2 exhalation to adjust pH.

  • Renal compensation: Kidneys excrete or retain H+ and HCO3- to adjust pH.

Fluid and Electrolyte Imbalances

Fluid Volume Deficit (Dehydration)

  • Causes: Vomiting, diarrhea, hemorrhage, diuretics, burns.

  • Symptoms: Thirst, dry mucous membranes, decreased skin turgor, hypotension, tachycardia.

  • Lab findings: Increased hematocrit, BUN, urine specific gravity.

Fluid Volume Excess (Overhydration)

  • Causes: Renal failure, heart failure, excessive IV fluids.

  • Symptoms: Edema, weight gain, hypertension, crackles in lungs, jugular venous distension.

  • Lab findings: Decreased hematocrit, BUN, urine specific gravity.

Assessment and Nursing Considerations

Assessment of Fluid and Electrolyte Status

  • History: Fluid intake/output, medications, medical conditions.

  • Physical exam: Skin turgor, mucous membranes, edema, vital signs, neurological status.

  • Laboratory tests: Serum electrolytes, BUN, creatinine, hematocrit, urine specific gravity, arterial blood gases (ABGs).

Arterial Blood Gas (ABG) Interpretation

  • Key values: pH, PaCO2, HCO3-, PaO2, O2 saturation.

  • Normal ranges:

    • pH: 7.35–7.45

    • PaCO2: 35–45 mmHg

    • HCO3-: 22–26 mEq/L

    • PaO2: 80–100 mmHg

    • O2 saturation: 95–100%

Blood Types and Transfusion Considerations

Blood Groups

  • ABO system: Four main types: A, B, AB, O; determined by antigens on red blood cells.

  • Rh system: Rh-positive or Rh-negative based on presence of D antigen.

  • Compatibility is crucial for safe transfusions.

Blood Transfusion Components

  • Whole blood: Rarely used; contains all blood components.

  • Packed red blood cells (PRBCs): Used for anemia, blood loss.

  • Plasma: Contains clotting factors, proteins.

  • Platelets: Used for bleeding disorders, thrombocytopenia.

Summary Table: Common Electrolyte Imbalances

Disorder

Underlying Causes

Clinical Manifestations

Interventions

Hyponatremia

Vomiting, diarrhea, diuretics

Confusion, seizures

Monitor sodium, restrict fluids, administer saline

Hypernatremia

Dehydration, excess salt

Thirst, restlessness

Monitor sodium, encourage fluids

Hypokalemia

Diuretics, vomiting

Muscle weakness, arrhythmias

Monitor potassium, administer supplements

Hyperkalemia

Renal failure, tissue damage

Cardiac arrhythmias

Monitor potassium, restrict intake, administer medications

Hypocalcemia

Hypoparathyroidism, vitamin D deficiency

Tetany, muscle cramps

Monitor calcium, administer supplements

Hypercalcemia

Malignancy, hyperparathyroidism

Weakness, confusion

Monitor calcium, increase fluids

Key Equations and Concepts

  • Osmolarity:

  • Bicarbonate Buffer Equation:

Clinical Application Example

Case Study: A 65-year-old male with heart failure and pneumonia presents with confusion, edema, and shortness of breath. Lab results show hyponatremia and low serum osmolality. The nurse should monitor fluid intake/output, assess neurological status, and administer IV saline as ordered.

Additional info:

  • Some tables and case studies were summarized for clarity and brevity.

  • Expanded explanations and definitions were added for self-contained study.

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