Fluid, Electrolytes, and Acid-Base Balance: Structured Study Notes for Anatomy & Physiology

Fluid, Electrolytes, and Acid-Base Balance

Normal Structure and Function of Body Fluids

Body fluids are essential for maintaining homeostasis, transporting nutrients, and facilitating cellular processes. Understanding their composition and movement is fundamental in anatomy and physiology.

• Total Body Water (TBW): The sum of all water in the body, distributed between intracellular and extracellular compartments.

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

• Extracellular Fluid (ECF): Fluid outside cells, including interstitial (between cells), intravascular (within blood vessels), and transcellular (e.g., cerebrospinal, synovial) fluids.

• Solutes: Substances dissolved in body fluids, classified as:

  • Crystalloids: Small molecules (e.g., electrolytes) that easily pass through membranes.

  • Colloids: Larger molecules (e.g., proteins) that do not pass easily through membranes.

  • Electrolytes: Ions such as Na+, K+, Ca2+, Mg2+, Cl-, HCO3-, and PO43-.

  • Nonelectrolytes: Substances like glucose and urea that do not dissociate into ions.

Movement of Body Fluids

Fluid movement between compartments is governed by physical principles and membrane properties.

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

• Osmotic Pressure: The force exerted by solutes drawing water across a membrane.

• Osmolality: Number of solute particles per kilogram of solvent.

• Osmolarity: Number of solute particles per liter of solution.

• Tonicity: The effect of a solution on cell volume:

  • Isotonic: No net movement of water; cell size remains unchanged.

  • Hypertonic: Water moves out of cells; cells shrink.

  • Hypotonic: Water moves into cells; cells swell.

• Oncotic Pressure: Osmotic pressure exerted by plasma proteins, important for maintaining fluid balance in blood vessels.

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

Regulation of Body Fluids

Fluid balance is tightly regulated by several physiological mechanisms.

• Renin-Angiotensin System: Hormonal system that regulates blood pressure and fluid balance by controlling sodium and water retention.

• Osmoreceptors: Specialized cells in the hypothalamus that detect changes in plasma osmolality and stimulate thirst or release of antidiuretic hormone (ADH).

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

• Thirst Mechanism: Activated by increased plasma osmolality or decreased blood volume.

• Atrial Natriuretic Peptide (ANP): Released by the heart in response to increased blood volume; promotes sodium and water excretion.

Movement and Regulation of Electrolytes

Electrolytes are vital for nerve conduction, muscle contraction, and cellular function.

• Diffusion: Passive movement of ions from high to low concentration.

• Filtration: Movement of water and solutes due to pressure gradients.

• Active Transport: Energy-dependent movement of ions against their concentration gradient (e.g., Na+/K+ pump).

• Major Cations: Sodium (Na+), magnesium (Mg2+), potassium (K+), calcium (Ca2+), hydrogen (H+).

• Major Anions: Chloride (Cl-), bicarbonate (HCO3-), phosphate (PO43-).

Acid-Base Balance

Maintaining a stable pH is crucial for enzymatic and metabolic processes.

• Normal Body pH: 7.35 to 7.45

• Buffering Systems: Chemical systems (e.g., bicarbonate, phosphate, proteins) that resist changes in pH.

• Respiratory Regulation: Lungs control CO2 exhalation, affecting acid-base status.

• Renal Regulation: Kidneys excrete or retain H+ and HCO3- to maintain pH.

Blood Groups and Components

Blood typing and components are essential for transfusion and immune compatibility.

• Antigens: Surface markers on red blood cells; main groups are ABO and Rh.

• Blood Testing: Type and screen procedures ensure compatibility.

• Blood Components:

  • Red blood cells (erythrocytes)

  • White blood cells (leukocytes)

  • Platelets (thrombocytes)

  • Plasma

Altered Structure and Function of Fluids and Electrolytes

Fluid Imbalances

Imbalances can result from various pathological or physiological conditions.

• Fluid Volume Deficit:

  • Isotonic: Loss of water and electrolytes in equal proportions.

  • Hypertonic: Greater loss of water than electrolytes.

• Fluid Volume Excess:

  • Isotonic: Excess water and electrolytes.

  • Hypotonic: Excess water relative to electrolytes.

  • Edema: Accumulation of fluid in interstitial spaces.

• Simultaneous Fluid Volume Excess and Deficit: Seen in conditions like cirrhosis.

Mechanisms of Edema Formation

Edema results from altered capillary dynamics and protein concentrations.

• Increased capillary permeability (e.g., inflammation)

• Decreased plasma oncotic pressure (e.g., hypoalbuminemia)

• Increased capillary hydrostatic pressure (e.g., venous obstruction)

• Lymphatic obstruction

Electrolyte Imbalances

Electrolyte disturbances can have significant clinical consequences.

Imbalance	Serum Level	Types/Notes
Hyponatremia	< 135 mEq/L	Hypovolemic, Hypervolemic
Hypernatremia	> 145 mEq/L
Hypokalemia	< 3.5 mEq/L
Hyperkalemia	> 5.0 mEq/L
Hypocalcemia	< 9 mg/dL
Hypercalcemia	> 10.5 mg/dL
Hypomagnesemia	< 1.3 mEq/L
Hypermagnesemia	> 2.1 mEq/L
Hypochloremia	< 98 mEq/L
Hyperchloremia	> 106 mEq/L
Hypophosphatemia	< 3 mg/dL
Hyperphosphatemia	> 4.5 mg/dL

Acid-Base Imbalances

Disorders of acid-base balance are classified by their origin and effect on pH.

• Respiratory Acidosis: Increased CO2 due to hypoventilation (acute or chronic).

• Respiratory Alkalosis: Decreased CO2 due to hyperventilation.

• Metabolic Acidosis: Decreased HCO3- or increased acid; may present with Kussmaul respirations.

• Metabolic Alkalosis: Increased HCO3- or loss of acid.

Assessment of Fluid, Electrolyte, and Acid-Base Balance

Health History and Vital Signs

Assessment begins with a thorough history and monitoring of vital signs.

• Recent changes in fluid intake, diet, and lifestyle habits.

• Attention to fever, tachycardia, respiratory changes, and blood pressure alterations.

Intake and Output

• Oral Intake: Includes all fluids and foods that become liquid at room temperature.

• Output: Measurable body fluids and drainage.

• 24-Hour Fluid Balance: Documented intake and output per shift, summed for daily totals and trend analysis.

Weight and Edema

• Weight Change: 1 kg (2.2 lb) = 1 L (1000 mL) of fluid.

• Pitting Edema: Graded on a 4+ scale (1+ to 4+ based on indentation depth and duration).

• Brawny Edema: Tissue is swollen and hard, not easily indented.

Skin Turgor and Mucous Membranes

• Skin Turgor: Pinched skin returns quickly to normal in healthy fluid balance; remains tented in deficit.

• Mucous Membranes: Moist and pink when normal; dry, sticky, and cracked in severe deficit.

Diagnostic Tests

• Arterial Blood Gases (ABG):

  1. Assess oxygenation (PaO2, O2 saturation).

  2. Check pH for normal range (7.35–7.45).

  3. Evaluate PaCO2 and HCO3- for metabolic or respiratory origin.

  4. Determine compensation status.

Factors Affecting Balance

• Age (infants and elderly at higher risk)

• Stress (increases fluid retention, decreases renal excretion)

• Weight (obesity affects fluid distribution)

• Surgery (NPO status, blood loss, stress, drainage, vomiting)

• Medical conditions (cardiac, hepatic, renal, respiratory disorders)

Nursing Diagnosis, Planning, Implementation, and Evaluation

Nursing Diagnosis

• Fluid Imbalance: Nausea, vomiting, output > intake, dry mucous membranes, high urine specific gravity and osmolarity.

• Fluid Retention: Bounding pulse, labored respirations, pitting edema, crackles in lungs, weight gain.

• Dehydration: Fluid loss, weak pulse, tachycardia, thirst.

Planning

• Goals focus on restoring or maintaining normal balance:

  • Moist mucous membranes

  • No pitting edema within 48 hours

  • Normal pulse rate by discharge

Implementation and Evaluation

Monitoring Fluid Balance

• Ongoing evaluation of disease impact and treatment effectiveness

• Vital signs, intake/output, daily weights, and blood tests

Maintaining Fluid and Electrolyte Balance

• Restricting Fluid Intake: Distribute intake throughout the day, adjust for meals and medications.

• Restricting Electrolyte Intake: Sodium restrictions (mild, moderate, severe); patient education on diet.

Oral Replacement

• Increase fluid intake to offset losses; avoid caffeine.

• Potassium and calcium supplements as needed.

Intravenous Therapy

• IV fluids as medication; follow administration protocols.

• Advantages: rapid effect, nutrition, transfusion, and volume restoration.

• Types of solutions:

  • Crystalloids: Hypotonic, isotonic, hypertonic (e.g., normal saline, lactated Ringer's).

  • Colloids: Protein or starch solutions for oncotic pressure.

• Site selection based on location, vein condition, therapy purpose/duration.

• Peripheral and central venous catheters (ONC, butterfly, midline, PICC, CVC, implanted port).

• Equipment: solution bag, tubing, needle/catheter, flow-monitoring device.

• Flow rate: affected by distance, position, catheter size, obstruction; use control devices for safety.

Complications and Care

• Risks: infection, occlusion, phlebitis, infiltration, extravasation, fluid overload, speed shock, embolism.

• Peripheral IV care: verify solution, rate, bag volume, drip chamber, tubing, site, and dressing.

• Central venous access: frequent tubing/dressing changes, aseptic technique, routine checks, flushing protocols.

Blood Administration

• Transfusion for oxygen transport, clotting, oncotic pressure, infection protection.

• Complications: hemolytic, allergic, febrile reactions; fluid overload; bacterial contamination.

Total Parenteral Nutrition (TPN)

• Hypertonic IV solution for complete nutritional support via central access.

• Complications: infection, air embolism, fluid overload, hyperglycemia, rebound hypoglycemia.

• Home IV therapy: continuous or intermittent infusions.

Evaluation

• Continuous assessment and care plan revision based on lab results and clinical status.

• Early intervention prevents severe complications.

Key Equations

• Osmolality: $\text{Osmolality} = \frac{\text{Number of solute particles}}{\text{Kilogram of solvent}}$

• Osmolarity: $\text{Osmolarity} = \frac{\text{Number of solute particles}}{\text{Liter of solution}}$

• Body pH: $\text{pH} = -\log_{10}[\text{H}^+]$

Example

• Edema Formation: A patient with liver cirrhosis may develop edema due to decreased plasma protein synthesis, leading to reduced oncotic pressure and fluid accumulation in tissues.

• Metabolic Acidosis: Diabetic ketoacidosis presents with low pH and Kussmaul respirations as the body attempts to compensate by increasing respiratory rate.

Additional info: These notes expand on the original slides with definitions, mechanisms, and clinical examples to provide a comprehensive study guide for Anatomy & Physiology students.